wvrivuenveis Objective When you have gone through this chapter, you should be able to explain the importance of the correct selection of materials have an understanding of the most common construction metals explain the various methods used for prevention of corrosion Introduction In the previous chapter various types of corrosion and influence from the environ- ment have been discussed. We will now have a closer look at other methods of corrosion prevention: correct selection of materials appropriate construction design use of paints and coatings cathodic and anodic protection conditioning of the environment Correct selection of materials A number of handbooks contain tables showing whether a metal/alloy is suitable for a given corrosive environment. Information on the corrosion rates of a number of metallic materials in connection with various concentrations of chemicals and temperatures may be listed. Based on information in the handbooks, experiences from projects etc., suitable metals/alloys are selected. Conditions such as lifetime requirements and accessibility for replacement are also of great importance. Many factors play a part in the selection of materials. The final selection is often made on the basis of considerations related to corrosion engineering and the economy. Further considerations may concer various combinations of materials and the risk of special types of corrosion under the given environmental conditions. Various metals and their areas of use will be given a short description below. Steel Pure iron finds insignificant technical use. On the other hand, iron alloyed with carbon and smaller amounts of the elements manganese and silicon constitutes the 36 Corrosion PROTECTIONmost widely used metallic construction material. Types of steel where the tensile strength properties are mainly determined by the carbon contents are called carbon steel or unalloyed steel. Steel (carbon stee}) has a carbon content of up to 1.3%. while in the case of cast iron it is even higher, be- tween 2-4%. Apart from carbon, such steels also contain minor quantities of sul- phur, phosphorus, silicon and manganese from the manufacturing process. Other elements can also influence the properties of the steel. For example, the silicon content is very important for the thickness of the zinc coating produced by means of hot dip galvanizing. By alloying steel (carbon steel) with small quantities of the alloying elements chromium. nickel. molybdenum. phosphorus and vanadium a large variety of steel qualities is obtained. Such stee] qualities are usually called low alloy steels. The total contents of the alloying elements rarely exceed a few per cent. The corrosion resistance of unalloyed steel and low alloy steel is more or less the same. Steel used for building constructions have excellent mechanical properties, but will corrode in the atmosphere at humidities above 60-70%. The rate of corrosion is influenced by factors such as time of wetness, pH value and air pollution. During milling (rolling) of the steel mill scale is created. Mill scale will act as a cathode on the steel, if not completely removed. At first mill scale acts as a barrier and reduces the corrosion of the steel, but when the mill scale cracks and flakes off, the corrosion rate of such exposed localized areas will increase due to unfavourable cathode-anode ratios. The rust formed provides only slight protection of the steel. Structural steel and plates of large thicknesses are usually supplied hot rolled, whereas thinner sheets (below 4 mm) are cold rolled. The thin sheets do not have mill scale. Due to corrosion, but also for cosmetic reasons, it is common practice to pro- vide the steel with’some sort of protective treatment. In most cases this is accom- plished by painting the steel. Mill scale Outdoor exposure Corrosion Figure 2.1 At outdoor exposure mill scale cracks and steel corrodes Weathering steels Weathering steel is a special type of low alloy steels which, when exposed out- doors, has better resistance against corrosion than ordinary structural steels. The improved resistance is a result of the weathering steels having been alloyed with slightly higher amounts of chromium, phosphorus and copper.Rust (porous) Rust (dense) ‘Weathering steel Figure 2.2. After a few years a dense rust layer (patina) is formed on weathering steels ° 2 Due to the decorative rust layer and lower raté of corrosion it is not common prac+ tice to paint weathering steel. The construction design is of the utmost importance. Discoloration of light-coloured materials such as concrete is a problem if rain wa- ter is not drained effectively from the weathering steel. Corrosion (um) 44 um/year Reference steel ’ a 5 Years Figure 2.3 Corrosion of Atmofix 52A and an unalloyed reference steel exposed outdoors in highly polluted atmosphere [1] f i t 38 Corrosion PROTECTIONOn surfaces which are constantly damp or exposed to a marine environment no patina will be formed. Weathering steels are consequently more suitable for struc- tures which are exposed to alternate periods of wetness and dryness. Weathering steels are probably more widely known by their trade names, such as Cor-Ten A and B, Patinax 37, Atmofix 52, etc. In the USA approximately 2000 bridgés built from weathering steel during the period around 1960 to 1980 have not been painted. After some time extensive cor- rosion problems have been observed which will result in a considerable reduction of the calculated lifetime of the structure if nothing is done about it. Blast cleaning to Sa 2¥4 and subsequent application of epoxy mastic systems is now being consid- ered. [2} The term Sa is one of four degrees of cleanliness defined in connection with blast cleaning. A degree of cleanliness of Sa 2% corresponds to, very careful blast cleaning of the steel to the extent that almost all rust is removed. Stainless steels As opposed to low alloy steels. stainless steels contain substantial amounts of al- loying elements in the range of 15-30%. Generally, the various types of stainless steels are grouped after their crystalline structure, such as ferritic, austenitic, and ferritic-austenitic. Differences in the struc- ture result in differences in their corrosion properties. In chloride-containing envi- ronment with a risk of pitting and crevice corrosion, steels of an austenitic structure are often preferred over ferritic or ferritic-austenitic structures. The table below gives a summary of some of the most common stainless alloys, their crystalline strictures and grades according to various standards. Table 2.1 Common terms for various high alloy steels in accordance with EN, “ASTM and UNS [en ASTM uns [Structure [%c [%cr [%NI[%Mo | | | doverate) | coverage) | average) | (overage) 14016 | 430 $43000_| Ferritic 0.04 | 16.5 1.4301 | 304 $30400 | Austenitic | 0.04] 18.1 | 8.3 0.02; 18.1 | 8.3 1.4436 | 316 $31600 | Austenitic | 0.04) 16.9 [10.7] 2.6 1.4432 | 316L $31603 | Austenitic | 0.02} 16.9 | 10.7] 2.6 1.4539 | 904L NO8904 | Austenitic | 0.01} 20 25 43 0.01} 20 18 6.1 0.01} 24 22 7.3 Duplex 0.02} 22 5.7 3.4 Duplex 0.02 | 25 7 4 1.4547 | S31254 1.4652 | S32654 1.4462 | $32205 1.4410 | $32750 $31254 | Austenitic $32654 | Austenitic $31803 $32750 | | | 1.4307 | 304L Be Austenitic The main alloying element in stainless alloys is chromium. When chromium is used for alloys, a thin and protective chromium-oxide film is formed on the surface ofthe metal. The alloy must as a minimum have a chromium content of approximately 11% to prevent rust from appearing on the steel. Stainless steel must have access to oxygen in order to maintain the oxide film. Besides alloying stainless steel with chromium, also nickel and molybdenum are added. An example of a stainless steel of this type is steel with approximately 17% chromium, 11% nickel and 2.6% molybdenum, see Table 2.1 Many countries have standards for stainless steels. The terms ASTM 304 and ASTM 316 from American Society for Testing and Materials (ASTM) designate examples of stainless steels. It is quite common to describe stainless steel as 18/8 steel. The figures indicate the contents of chromium and nickel, respectively, in the steel. Molybdenum can also be added. An alloy with 17% chromium, 11% nickel and 2.6% molybdenum corresponds to ASTM 316. Other steel qualities have better corrosion resistance to sea water compared to the types mentioned above. The material ASTM $31254, often referred to as 254 SMO or 6 Mo, is a high quality steel alloy containing 20% chromium, 18% nickel and 6.1% molybdenum. When this material was first introduced on the market, tests showed good corrosion resistance against crevice corrosion at temperatures up to approximately 40°C. However, subsequent practical experience has shown that the material has been subject to pitting/crevice corrosion even at lower tem- peratures. The corrosion resistance of ASTM S31254 in for example sea water is much better than that of an “ordinary” stainless steel, for example ASTM 304. This is due to the high alloy contents. Under particularly corrosive conditions also ASTM 31254 may have too little corrosion resistance, and other materials must be cho- sen. Formability and weldability are greatly improved when nickel and molybdenum are added. It is a general misconception that stainless steels do not corrode. In sea water a number of these-alloys will suffer extensive corrosion. Despite the fact that alloys with a chromium content of minimum 12% result in steel qualities which do not rust, these types will nevertheless be subject to corrosion. Defects in the oxide film may result in Passive condition pitting corrosion a Active condition Figure 2.4 Active and passive condition 40 — CORROSION PROTECTIONIn chloride-containing environments localized corrosion attacks will occur. Pitting and crevice corrosion may result in complete perforation of pipe walls or severe attacks on seal faces on flanges. Intercrystalline corrosion is a type of corrosion which may appear on stainless steels with a high carbon content (0.05-0.08% C). Welding of stainless steels with a high carbon content may cause precipitation of chromium carbides, leading to depletion of chromium to below 12% in the metal adjacent to the precipitates, which is thus no longer “stainless”. If the steel is exposed to an aggressive environment, such sites may become anodic to the rest of the metal, and may corrode. This type of corrosion attack can be reduced by selecting stainless alloys with low carbon contents (< 0.03%) or possibly adding small quantities of titanium or niobium in order to prevent the formation of chromium carbide. Alternatively, the steel can be heat treated at high temperature and cooled very rapidly so that no chromium carbides can be formed. There are many areas of use for stainless steels, from household articles (cut- lery, saucepans, etc.) to use for the transportation and storage of aggressive liquids, including chloride-containing substances. Water, salts Pitting corrosion / ? : a Stainless steel Oxide film coor Figure 2.5. Pitting corrosion on stainless steel Titanium and nickel alloys Titanium alloys have excellent corrosion resistance when exposed in both marine “and industrial atmospheres. They are also suitable for use in fresh water and in sea water. Due to high initial costs the use of titanium alloys has been limited. But under demanding conditions the selection of titanium alloys may be advantageous be- cause they have a number of excellent properties. This has been the case in the process industry which has had good experience with the use of titanium for pumps, valves, etc. Also the oil industry has started to use titanium, for example for heat exchangers. Nickel alloys are considered among the most corrosion-resistant alloys in exist- ence. Such alloys are used in particularly aggressive media, and when long life- times are required. They are extremely resistant towards sea water with high veloc- ity of flow. In stagnant and slow sea water they may be subject to pitting and crevice corrosion. [3] You may have heard mention of materials such as Monel, Chlorimet, Hastelloy B and C, and Inconel, which are all examples of nickel alloys. 2 PROTECTION AGAINST CORROSION = 41. |Copper and copper alloys Copper is a relatively noble, but soft metal. By alloying copper a large number of alloys with many different areas of use are produced. Well-known alloys are brass (copper and zinc) and bronze (copper and tin), just to mention a few. The rate of corrosion of copper in air is low, approximately 0.5-2.5 ym/year. This is due to the fact that the corrosion product formed on copper is very dense and durable. Most people have probably at some point in time seen the green patina, verdigris, on objects made of copper. The use of copper for construction has a Jong tradition. Copper alloys are often used for roofs, mountings, statues, etc. Copper and copper alloys are also widely used for water-bearing pipes, con- densers and heat exchangers as well as for components like valves, pumps, etc. When selecting materials, attention must be paid to the medium which the alloy will get into contact with. This is perhaps especially important in connection with the transport of fresh water and sea water in the pipes. Copper pipes used for hot and cold water piping and hot water appliances are well-known. For transport of sea water more resistant copper alloys should be chosen. Alloys of copper-nickel or aluminium-brass are often used for this purpose. Copper alloys are generally sensitive to the velocity of flow in the pipes. Ata too high velocity, erosion corrosion may arise. In order to avoid such damage on “ordi nary” copper pipes, the flow velocity of the sea water should not exceed approxi- mately 1 m/s. In the case of copper-nickel the upper limit is considered to be ap- proximately 3.5 m/s. Besides erosion corrosion, copper alloys are also attacked by pitting corrosion. Selective corrosion may occur on various brass alloys. The tendency of fouling is smaller on copper than on other metals. Consequently, copper alloys are extensively used in water-bearing pipes, and for the same reason copper-containing antifouling paints are used on ships’ hulls. Zinc When we consider the areas of use of hop dip galvanized steel, this is utilized for a number of purposes where long lifetime without extensive maintenance is required. Apart from the advantage of a low corrosion rate of hot dip galvanized steel in a number of atmosphere types, the zine coating also provides corrosion protection of the steel in case of damage. A major part of the steel used for power masts, railings, lamp posts, crash barriers, etc. is hot dip galvanized. It has been estimated that approximately 40% of all zinc produced on a world-wide basis is used for corro- sion protection of steel. [4] Zinc coatings can be applied to steel by various methods that will be discussed later. Depending on the method of application and the thickness, zinc coatings may Provide excellent protection to steel for many years, depending on the exposure environment. Corrosion on zine in air is generally slow. In urban and industrial atmospheres it takes place at 1-10 um/year. In rural and marine atmospheres it is even lower and only approximately 0.5-2 nvyear. Sacrificial anodes made of zinc are used for cathodic protection of steel. 42 CORROSION PROTECTIONAluminium When aluminium is exposed to the air, a thin oxide film will quickly form on the surface. The oxide film is only approximately 0.01 um thick, but protects the metal effectively against corrosion. In the case of aluminium alloys, improved properties in respect of ultimate strength, ductility, corrosion performance, etc. can be obtained. ‘Aluminium alloys have become popular for many applications, including roof. panels and facade cladding. The average rate of corrosion over a 20-year period will even in industrial atmosphere rarely exceed | pm/year for such alloys. Often minor pitting corrosion attacks occur. Such attacks can be difficult to spot with the naked eye. The corrosion rate of such attacks usually decreases with time. In the case of pitting corrosion the following maximum depths of the pittings after 20 years in different atmospheres have been measured [5]: rural atmosphere 10-55 pm urban atmosphere 100-190 ym marine atmosphere 85-260 im As aluminium is relatively ignoble, there may be a risk of galvanic corrosion in contact with more noble metals such as steel, stainless steel and copper. The metals should therefore be insulated from each other. Naturally, the prerequisite for gal- vanic corrosion is the presence of an electrolyte at the point of contact. In spite of the metals being insulated from each other, corrosion may occur on the aluminium. ‘This may happen if the aluminium is exposed to copper-containing water. In this case, the copper is deposited on the surface. The copper particles form small cath- ‘odes and the result may be serious corrosion damage. Aluminium alloyed with 4.5% magnesium and 1% manganese, among other elements, is caHéd sea water-resistant aluminium. The alloy has high resistance to corrosion in a marine environment. For many years sea water-resistant aluminium has been used for the hulls of high-speed vessels. For protection of flare booms on offshore platforms sea water-resistant aluminium is applied by means of thermal spraying. When an aluminium hull is painted at and below the water line, antifouling paints should not be applied directly on top of the aluminium because they are usually based on copper pigments. Damage to the paint results in an unfavourable difference in area between the copper-containing paint (the cathode) and aluminium. This may cause corrosion of the hull. Aluminium is not suitable for processing and storage of inorganic acids, with the exception of concentrated nitric acid and sulphuric acid. However, it may be sufficiently resistant to withstand several organic acids. Alkalis also attack alu- minium, but inhibitors, for example silicates, added to diluted alkaline solutions reduce such attacks. An inhibitor is a substance which retards the reaction speed in a process 2 PROTECTION AGAINST CORROSION 43.Constructional design to reduce corrosion In order to prevent corrosion we must have knowledge about the selection of mate- rials and the importance of constructional design. There are numerous examples of constructions which have been quite satisfac- tory, both in respect of strength and aesthetics, but where nevertheless the structure has had weaknesses. Unsuitable designs may cause entrapment of water, dirt and pollution. This results in extensive maintenance costs. Consequently, we must try to reduce corrosion at an early stage, preferably already on the drawing board. Crevices Construction designs often create areas with narrow crevices. Crevices may be created in many ways, for example by fastening parts by spot welding or bolts. In areas with difficult access, this will give rise to problems. In such a case rust formation constitutes a high risk of expansion and rupture. This is due to the fact that rust takes up a larger volume than the steel from which it is formed. The result may be deformation of thin sheets etc. There are several ways to avoid the formation of crevices. In many cases crevices can be avoided if materials are welded together instead of flanged and bolted. Another possibility is to deliberately make the crevices bigger to enable easier and better maintenance work. Also steel structures at the ground level may be subject to splashes and collec- tion of moisture. In many cases the steel can be protected by embedding it in a concrete foundation or by applying extra thick coats of paint in such areas. Bolts © Outdoor exposure om Narrow crevice Deformation due to corrosion Products =“ Figure 2.6 Corrosion products have a higher volume than metal. This may result in deformation Geometrical effects The geometrical designs should be as simple as possible. The simpler the surfaces, the easier it is to carry out surface preparation and paint application. This makes cleaning and later maintenance easier and reduces the risk of corrosion. Quite many problems are caused by variations in profile shapes. This is why large areas are easier to prepare and paint than for instance sections which are welded, riveted or bolted together. It is difficult to obtain sufficient film thickness on sharp edges. If edges are 44 Corrosion PRoTECTIONrounded in connection with painting, the paint coating will be more even and better than on sharp edges. Many specifications now include the requirement that edges must be rounded and have a radius of minimum 2 mm. Metal Paint Bad \ Better \ Sharp edge Rounded edge: fow thickness on edges even thickness Figure 2.7 Sharp edges should be rounded in order to obtain a sufficiently thick paint coating SL CJ} I. Unsuitable Usable Best Ea] L Residual moisture Draining hole : Unsuitable Usable VAN Figure 2.8 Designs and shapes which are difficult to maintain and/or collect dirt and moisture should be avoided Residual moisture The international standard EN ISO 12944-3 “Design considerations” explains, both in text and figures, that in connection with the planning of constructions important aspects are whether they function properly and have adequate stability, strength and durability. The entire design must be planned and must include surface prepa- 2 PROTECTION AGAINST CORROSION = 45 :ration, painting, inspection and maintenance. Especially the aspects of design and access must be considered of the utmost importance in order to obtain the best possible base and possibility for maintenance later on. In connection with projects it is still possible to meet inappropriate designs like those shown in the Figures 2.8, 2.10, and 2.11. Narrow gaps, crevices and lap joints are places which are always subject to corrosion attacks because moisture, dirt, etc. are collected in such places. In some cases sealants should be used in order to reduce the risk of corrosion, or possibly welding should be carried out once more. Any openings in for example stiffeners, frames or similar structural members should have a gap of minimum 50 mm to provide access for sufficient surface preparation and application of an anticorrosive paint. | — Stiffener Bottom flange Figure 2.9 Stiffener design recommended for corrosion protection With due consideration of the risk of corrosion the design should also be in accord- ance with the recommended corrosion-protective treatments. It must be possible to carry out proper maintenance with blast cleaning and airless spray application. It should also be ensured that the design preferably eliminates any entrapment of moisture and dirt. [6] Moisture condenses. and causes corrosion Insulated support Heat loss through support Figure 2.10 In tanks incomplete insulation of supports may result in internal condensation and risk of corrosion In buildings where plating is used attention must be paid to the risk of thermal 46 Corrosion protectionbridges at low outdoor temperatures. Thermal bridges may cause a risk of conden- sation on the inside of the plating. If suitable damp-proofing and insulation materi- als are used, this may be prevented. In order to avoid similar problems, for example in tanks, the entire tank and supports must be insulated. Tanks should also be designed in such a way that draining is possible. Otherwise, any liquid retained in the tank may cause a risk of corrosion. Unsuitable Better Best Cannot be Completely emptied empty completely Figure 2.11 Tanks should be designed in such a way that no liquids are collected Metallic joints In many cases damage is due to more or less noble metals being in direct contact with each other. This causes a risk of galvanic corrosion. The larger the potential difference between the metals, the faster the corrosion rate on the ignoble metal. If at all possible, care should be taken to keep the metals completely insulated from each other. Insulating gaskets and washers Insulating gaskets Insulating gaskets Figure 2.12 Direct electric contact between different metals can be prevented by means of insulating gaskets 2 PROTECTION AGAINST CORROSION = 47 PpettnenWhen welding, it is of great importance to select the correct welding electrode. It must be ensured that the weld does not become anodic in relation to the parent material. If the welds are less noble than the parent material, the welds may be severely attacked. In the offshore industry accidents have occurred due to use of wrong welding electrodes. As a result of this the welds corroded rapidly and leakages occurred. ‘The cause of this damage was galvanic corrosion caused by the large and unfa- vourable difference in size between the area of the pipes (the cathode) and the area of the welds (the anode). Before painting welds it is important that the welds are prepared in such a way that they are suitable for being coated. The surface preparation should include removal of irregularities and weld spatter by grinding, and removal of weld smoke residues by fresh water hosing. Depending on the agreed specification, the extent of work applied to the grinding may vary. Specifications may include relatively sim- ple information such as “weld seams to be treated in a suitable manner...” without any specific requirements to the way they look. This may of course result in discus- sions and disputes in a project. The NACE standard RP 0178-2003 “Standard Recommended Practice: Fabri- cation Details, Surface Finish Requirements, and Proper Design Considerations for Tanks and Vessels to be Lined for Immersion Service” is an excellent tool. (7] The standard contains detailed text and illustrations describing welds prepared by grinding. The standard also incudes a molded plastic replica with examples of butt, fillet and lap welds. Completely untreated welds have been coded F, and after this the welds have received better and better grinding. On welds ground to the degree of Aall weld spatter and irregularities have been removed and the surface has been ground completely even. Electrolyte Stee! wf The welding material is less noble than stee! Figure 2.13 It is important that no welds are less noble than the surrounding material Weld beads and weld smoke Ground weld Figure 2.14 Residues of weld smoke and weld beads provide an unsuitable substrate ‘for painting, 48 — Corrosion protection e iDesign to prevent unnecessary turbulence In flowing media, for example in liquid-bearing pipes, erosion corrosion may arise. Strongly susceptible to this are pipes, bends, T-pieces, heat exchanger pipes, etc. carrying mixtures of water and solid matter. It is very important that the pipes have a suitable design in order to avoid unnecessary turbulence. Figure 2.15 shows that insertion of pipes by welding should be carried out in such a way that any unneccessary change of the flow within the pipe is avoided. Pipes should be designed in such a way that any dimensional changes are gradual. Use of right-angled bends where a change of flow may cause erosion corrosion should also be avoided. Figure 2.15 Sharp bends are areas at risk in pipes Using paints and coatings Paints The most widely used method for protecting steel is by use of paints. When paint, or a similar coating, is applied, a film is built up to prevent corrosive gases or liquids from getting into direct contact with the metal. Generally, a corrosion-protective paint can be said to protect the stee! in three different ways: 2 PROTECTION AGAINST CORROSION = 49.1 Paints can protect steel against corrosion by excluding water and air from the surface. The paints are said to protect by means of barrier protection. Although some water vapour and oxygen will diffuse through the coating, barrier coat- ings generally have low permeability. Typical barrier coatings are two-compo- nent epoxy and polyurethane paints, viny! and chlorinated rubber paints, and tar-based paints, just to mention a few. 2 Paints can also protect steel against corrosion if special pigments are added to the paint which provide an inhibirive effect to the steel, for example by inhibit- ing the corrosion process by passivation. Paints with such pigments must not be too dense. Water must be able to permeate and react with the pigments in order for them to contribute to the corrosion control. You may have heard mention of pigments providing protection of steel in this way. Previously, the most widely used pigments included red lead and zinc chromate. Due to health hazards these pigments are very little used today. Instead the pigment zinc phosphate is used to an increasing extent. 3. Paints may protect steel against corrosion by “sacrificing” themselves. This is called the cathodic effect. Paints used for this method of protecting steel are the zine-rich paints, The zinc in the paint is sacrificed and provides the necessary protection to the steel. The barrier formed when we apply paint is relatively thin. The film thickness re- quired for protecting the steel depends on the environment the metal is exposed to. ‘When exposed in air, paint systems in a total film thickness from approximately 100 um up to 300 pm are usually applied. The paint must often be selected according to several requirements to the paint. Residual rust may be present on areas of a structure with difficult access. On such areas a paint type is chosen which is tolerant of a rusty surface. On areas where regular maintenance is required, it is advantageous to chose a maintenance-friendly paint. i Consequently, there are many circumstances to take into consideration when selecting paints. However, the most important property of an anticorrosion paint is its resistance towards moisture and water. Water has an impact on almost all or- ganic materials and is close to being a universal solvent. Paint system selection and use of EN ISO 12944 The selection of paint systems for major projects within the onshore or offshore industries can be based on the standard EN ISO 12944 “Paints and Varnishes - Corrosion Protection of Steel Structures by Protective Paint Systems”. (8] EN ISO 12944 consists of 8 parts in total and was published in 1998. Accord- ing to the European Committee for Standardization, CEN (Comité Européen de Normalisation), a large part of the European countries have committed themselves to implement this standard, resulting in EN ISO 12944 also becoming national standards. 50 — Corrosion PROTECTIONEN ISO 12944 “Paints and Varnishes - Corrosion Protection of Steel Structures by Protective Paint Systems” Part 1 General introduction Part 2 Classification of environments Part 3 Design considerations Part 4 Types of surface and surface preparation Part 5 Protective paint systems Part 6 — Laboratory performance test methods Part 7 Execution and supervision of paint work Part 8 Development of specifications for new work and maintenance There are different ways of protecting steel structures from corrosion. EN ISO 12944 deals with protection by paint systems and covers, in the various parts, all features that are important in achieving adequate corrosion protection. ‘The standard deals with a number of surfaces consisting of unalloyed or low alloy steel and their treatment, for example by thermal spraying, hot dip galvaniz- ing, surfaces painted with prefabrication primer, zinc-electroplated surfaces, etc. ISO 9226:1992 “Corrosion of metals and alloys ~ Corrosivity of atmospheres — Determination of corrosion rate of standard specimens for the evaluation of corrosivity” classifies the environment according to 6 atmospheric corrosivity cut- egories. The same classification is used in EN ISO 12944 where the following terms are used: Atmospheric corrosivity category ci very low C2. low C3. medium C4 high C5-1 very higti (industrial) C5-M__ very high (marine) Additionally, three corrosivity categories have been defined for structures immersed in water or buried in soil. In the case of structures immersed in water or buried in soil the corrosion is of a general nature. It is therefore difficult to indicate any specific corrosivity categories. Corrosivity category Environment Im! Immersed in fresh water Im2 Immersed in sea water or brackish water Im3 Buried in soil The standard defines the corrosivity categories based on mass or thickness loss afier first year of exposure of standard test specimens made of unalloyed carbon steel or zinc. Both typical interior and exterior environments have been described. Examples of the various corrosivity categories as well as examples of paint systems within each category are given below. Many other modifications or combi- nations are possible. 2 PROTECTION AGAINST CORROSION = 51.Corrosivity category C1 Interior: Heated buildings with clean atmospheres (offices, shops, schools or hotels) Exterior: - Aggressivity: Insignificant Paint systems: None, systems in C2 can be used for aesthetical reasons Corrosivity category C2 Interior: Unheated buildings where condensation may occur (depots, sports halls) Exterior: Atmosphere with low levels of pollution (rural atmosphere) Aggressivity: Low Paint systems: Alkyd systems: 2-5 coats (80-200 pm) Acrylic, chlorinated rubber, PVC: 2-4 coats (100-160 pm) Epoxy, polyurethane: 2-4 coats (120-160 ym) Zinc epoxy: 1-2 coats (80 pm) Zinc ethy| silicate: 1 coat (80 pm) Corrosivity category C3 Interior: Production rooms with high humidity and some air pollution (food processing plants, production halls) Exterior: Urban and industrial, moderate SO, pollution, coastal areas with low salinity Aggressivity: Medium Paint systems: Alkyd systems? 3-5 coats (200 ym) Acrylic, chlorinated rubber, PVC: 35 coats (200-240 um) Epoxy, polyurethane: 3-5 coats (160-240 ym) Zinc epoxy + acrylic, chlorinated rubber: 1-2 coats (160-200 um) Zine ethyl silicate + epoxy + polyurethane: 3-4 coats (160-200 pm) Corrosivity category C4 Interior Chemical plants, indoor swimming pools, coastal shipyards Exterior: Urban and industrial, moderate SO, pollution, coastal areas with moderate salinity Aggressivity: High Paint systems: Alkyd systems: 3-5 coats (200 ym) | Acrylic, chlorinated rubber, PVC: 3-5 coats (200-240 um) Epoxy, polyurethane: 3-6 coats (200-320 um) Zinc epoxy + epoxy + polyurethane: 4-5 coats (160-320 ym) ‘ate + epoxy + polyurethane: 3-4 coats (240 jm) icate + acrylic, chlorinated rubber, PVC: 3-4 coats (240 pm) 52 — CorROSION PROTECTIONCorrosivity category C5 Interior: Buildings or areas with almost permanent condensation and highly polluted atmosphere Exterior: I: Industrial areas with high relative humidity and aggressive atmosphere M: Coastal and offshore areas with high salinity Aggressivity: Very high Paint systems: Chlorinated rubber systems: 3-4 coats (200 pm) Epoxy, polyurethane: 3-6 coats (300-500 pm) Zinc epoxy + coal tar epoxy: 4 coats (400 ym) Zinc ethy! silicate + epoxy + polyurethane: 3-5 coats (320 um) Epoxy solvent-free: 1-2 coats (800 um) The corrosivity categories listed above are all intended for corrosion protection of steel under atmospheric conditions. However, in case of selection of paint systems for structures or objects to be immersed in fresh water or sea water or to be buried in soil, the following categories apply: Paint systems for the categories water and soil, Im1, Im2, and Im3" Exterior: Immersed in fresh water or sea water / brackish water or buried in soil Paint systems: Epoxy, polyurethane: 2-4 coats (360-380 pm) Zinc epoxy + coal tar epoxy: 5 coats (540 ym) Epoxy solvent-free: 1-2 coats (800 ym) Coal tar epoxy: 1-5 coats (360-1000 ym) Coal tar polyurethane: 2 coats (400 pm) The durability of the paint systems depends on factors such as surface preparation, conditions during application, quality of application, paint system, exposure condi- tions, etc. Within each corrosion class the paints systems are divided into 3 durability classes: class L (low), class M (medium) and class H (high). The durability is described by the following intervals: Low (L) 2-5 years Medium (M) 5-15 years High (H) more than 15 years The durability time is not linked to any “guarantee time”, but is a consideration which can help the owner set up a maintenance program. The following conditions largely apply to all systems: surface preparation requirements primer types number of coats and total film thickness 2 PROTECTION AGAINST CORROSION = 53What are the influences by factors such as surface preparation, selected paint sys- tem, and dry film thickness on the durability of a paint system? Studies often show that the higher the requirements to surface preparation, sys- tem build-up as well as number of coats and total film thickness, the longer the durability of the paint system. Linings (organic) Not all surfaces are suitable for the use of paint as protection. There may be several reasons for this, including extensive mechanical or chemical impact, difficult ar- eas, or economic considerations. Linings largely protect the metal in the same way as paints by forming a barrier "between the ambient environment and the metal. The term lining is often associated with higher film thicknesses than when using paint. Some of these linings can be applied in thicknesses of several millimetres, some may be several centimetres thick. ‘Among the organic lining types currently in use are tar coatings, asphalt, vari- ous plastic coatings, bitumen, grease-like coatings, tapes, etc. The methods of application of linings are numerou: halt and tar linings are spray-applied, while various types of plastic and bitumen linings are wrapped around pipes in automatic machines. To a certain extent this is also the case of grease-like linings and other tapes. Metal coatings A widely used methad for corrosion protection is to apply a thin layer of another metal to the metal to be protected. Very many structures around us have external surfaces consisting of a metal coating. Familiar examples are hot dip galvanized lamp posts, crash barriers, gar- bage stands, and chromium-plated and nickel-plated parts on cars. Metal coatings are applied by various methods. They can be applied electrolyti- cally (aqueous metal solutions), by dipping in molten metals (hot dip galvanizing) or by thermal spraying (metallization). Electroplating Metal coating of small steel components is done by means of electrolysis. Elec- trolysis utilizes an electrolyte with metal ions, anode and cathode, and a power source. By this method, dissolved metal ions from the electrolyte are deposited on the steel surface. The coatings applied to metals by means of electrolysis can be both more noble and less noble than the metal to be protected. For example, if zinc is applied on steel. the zinc will function as a sacrificial anode layer. If damage occurs down to the bulk metal, the zinc layer will corrode and not the steel. The thickness of electrolytic zinc layers is usually low, approxi- mately 5-20 ym. 54 Corrosion PROTECTIONChromium and nickel layers are usually applied for aesthetical reasons. The thickness of the layer is usually 5-40 ym for nickel and 0.3-2 um for chromium. Such layers are more noble than steel, and in order to protect against corrosion they must be absolutely dense and holiday-free. If coating damage down to the steel occurs, the nickel/chromium layer will become the cathode and the steel anode. In other words, there is a high area ratio between the cathode and the anode. This results in increased corrosion of the coated metal in the damaged area. Zine —E = Sy steei s Nickel -f z steel + Figure 2.16 Damage of the nickel/chromium layer down to steel may result in severe pitting corrosion of the steel Dipping in molten metals Another method for coating steel is dipping it ina bath with molten metal. The most widely used method is hor dip galvanizing. By dipping steel in a bath with molten zinc, a zinc layer of considerably higher thickness can be obtained than is the case of electroplated steel. The zinc thickness depends on the stéel type. On unkilled steel zinc coatings above approximately 70 um are rarely seen, On silicon-killed steel a zinc thickness of over 200 ym can be obtained. Even though dipping in molten zinc is the most common protection process, steel can also be applied with coatings of aluminium, aluminium-zine alloy and lead by dipping it in molten metals of these types. The corrosion rates of metal coatings depend on the ambient environment. A zinc coating of 75 ym can protect steel in an inland area for 20 years, perhaps longer, but when exposed in aggressive industrial atmosphere the protection may perhaps only last for approximately 10 years. AYP BES. SOE SESE SS Zs, Steel Figure 2.17 Cross cut of the coating on hot dip galvanized steel 2 PROTECTION AGAINST CORROSION = 55Thermal spraying (metallization) Another method for coating steel is by spray applying either a metal wire or metal powder melted by a gas flame or an electric arc in a spray gun. The most common metals sprayed on for corrosion protection of steel are zinc and aluminium and their alloys. Thermal spraying of zinc in connection with corrosion protection of bridges has generally been combined with paint systems. A common thickness of the zinc coat- ing is 100-150 ym plus approximately 200 um paint. The lifetime of the zinc coat- ing is extended quite considerably when overcoated. In Norway some structures have been in use for more than twenty years, more or Jess without any damage. Such systems employing thermal spraying and paint systems are usually called duplex systems. Within the offshore oil industry thermal spraying has been in use for many years. In case of thermal spraying of pure aluminium and aluminium alloys a coat- ing of 150-400 jim is usually applied as well as a paint system. If the temperature of the structure is below approximately 120°C, an epoxy or polyurethane system in a thickness of approximately 200 pm is usually applied. When the temperature is higher than this, heat-resistant paints are used, for example ny silicone paints in a thickness of approximately 50 um. Cathodic proctection Cathodic protection can, in short, be described as a process where the metal to be protected becomes the cathode in a galvanic cell. As mentioned above, the cathode is not attacked by corrosion. The attack takes place at the anode. In practice, this is utilized by putting the metal to be protected into galvanic contact with a less noble metal (the sacrificial anode) or by using impressed current. Cathodic protection, both in theory and in practice, has been known since 1824 when Sir Humphrey Davy described this form of corrosion protection. The method is used for the protection of ships, offshore platforms, and oil and gas pipes in sea water, etc. The method is also used to prevent localized corrosion on aluminium. and stainless steel alloys. In recent years cathodic protection has also been used for protecting concrete constructions. ; How does this type of cathodic protection work? | The rate of corrosion of a metal in an electrolyte to a very high extent depends on the potential of the metal. The attack on the metal can be reduced by making the ; metal more negative, i.e. lowering the potential. i Asa further illustration, a diagram showing the potential (voltage) relative to j the pH value for iron in water will be discussed. The diagram is called the Pourbaix- | diagram after Marcel Pourbaix who invented it. Similar diagrams are also avail- | able for other metals. | ‘The solid lines in the diagram indicate a transition to areas where the steel either 56 — CorrosiON PROTECTION SESE Eet SSR EEES=SIEEEEECESIESTEIEE SITET ESTESITLISIEESSSITEEEST=STISEEISESIISCSESCEILIISEISEEILSTECEEIIESTcorrodes, is passive (slight corrosion) or is immune (no corrosion). The dashed lines show the lower and upper stability limits for water. Below the lower line water is transformed into hydrogen gas (H,), above the upper line water is transformed into oxygen gas (O,). Whether the metal corrodes or not (see the diagram) depends on the potential of the metal and the pH value of the solution. Corrosion GLI ob ee Nar Passivation te en aio Immune. [Corrosion| Figure 2.18 Pourbaix-diagram for steel in water In the Pourbaix-diagram reference is made to the Standard Hydrogen Electrode (SHE). Whenever potentials are to be measured under practical conditions, it is always necessary to use other sorts of reference electrode, for example copper/ copper sulphate, silver/silver chloride or zinc. The reference electrodes will have this potential in relation to the standard hy- drogen electrode, see also Table 1.2: Silver/silver chloride in sea water (SSC) +0.25 volt Copper/copper sulphate in sea water (SCE) — +0.32 volt Zinc in sea water -0.78 volt Example If steel is immersed in sea water with a pH value of 7, a potential of the steel of approximately -0.40 volt versus SHE will be measured. This potential corresponds to the approximately ~0.65 volt versus the silver/silver chloride electrode and the saturated calomel electrode shown in the galvanic series in Table 1.3. The point is plotted on the Pourbaix-diagram, and we will now take a look at what happens if the conditions around the steel are slightly changed. 2 PROTECTION AGAINST CORROSION 57a) If we lower the pH value, making the water more acidic, this will result in continued corrosion since the steel remains within the corrosive area. The corro- sion rate increases with the reduction of the pH value. b) If we increase the pH value, the water becomes more alkaline, and the steel is moved into the area where it is passivated. The corrosion rate is reduced, but not entirely stopped. If we apply a more positive potential, the metal will again be transferred to the passive area. This method does not make the metal immune against corrosion, but a layer is formed on the metal surface which reduces the rate of corrosion. This is called anodic protection. If we apply a more negative potential, the steel will be seen to move down into the immune area. In the border area where corrosion stops at -0.65 volt meas- ured versus SHE slight corrosion may still arise, but at potentials lower than -0.65 volt measured versus SHE the metal dissolution will be close to zero. This type of protection is called cathodic protection. © = It has now been demonstrated that it is possible to protect steel by changing the potential in a more negative direction. The above example is for stee!/water but in principle it also applies to steel in sea water. Practical measurements For practical corrosion protection of steel in sea water we may for example use sacrificial anodes in order to lower the potential of the steel down into the immune area. The pH value of sea water varies only slightly and is usually around 8. As illustrated in Table’ 1.3 the potential of steel in sea water is approximately -0.65 volt versus SCE (saturated calomel electrode) and SSC (silver/silver chloride elec- trode). Under these’conditions the stee! will corrode. The established potential criterion for obtaining protection of steel at normal temperatures in sea water is -0.80 volt versus SSC and SCE, which corresponds to -0.87 volt versus CSE (copper/copper sulphate), see Table 1.2. The potential of a piece of zinc in sea water measured in the same way is ap- proximately -1.0 volt, see Table 1.3. Under these conditions the zinc will corrode. If zinc and steel are electrically connected, the potential will adjust toa potential between the potentials of each of the metals. A potential of -0.85 volt versus SSC is easily obtained and the corrosion of the steel is stopped. However, after the cou- pling the zinc will corrode — sacrifice itself - at a higher rate. From the diagram it can also be seen why reinforcing steel in concrete does not corrode. At a pH value of 12-13 the steel is passive. It is not until a pH value of around 9-9.5 that corrosion of the reinforcing steel occurs. The more negative the potential of the steel, the lower the rate of corrosion. It is not necessary to achieve a potential of the steel lower than -0.80 to -0.85 V versus SSC in order to obtain satisfactory protection. At even lower potentials hydrogen gas is formed which may result in hydrogen brittleness of the steel. Paint systems on steel will form blisters under such conditions and the coating will detach com- pletely due to extensive formation of hydroxy! ions (OH). 58 — Corrosion proTEctionCathodic protection is used to protect structures both in water and soil. Both environments provide the necessary distribution of protective current in such a way that the structure is sufficiently protected. Reference electrode _DC rectifier -0,85v sso e £ | Permanent — if anode (grphite) _ (EEEEES & Steal LU Figure 2.19 Cathodic protection of steel in sea water The Norwegian Oil Directorate has provided some guidelines for the dimensioning of cathodic protection of both uncoated and coated steel in sea water. In the North Sea the following protective current requirements have been established: Table 2.2 Guidelines for selection of current densities in order to dimension cathodic protection of uncoated steel at sea water temperature [9] Current density, mA/m? Area (°N) oun iniialcapaciy | Average eurent | Final capaci peeky | densi for ween ae Sateuiaion in seopeetion Uncoated stee! | Southern part of North Sea | | (up to 57) Nae 150 85 100 | | | l | Northern part of North Sea | | j (57-62) ; 180 | 85 120 Bottom sediments | | sand, clay {20 5 5 silt 40 10 10 | 2 PROTECTION AGAINST CORROSION 59An offshore structure should reach the desired protective value within three months after having been permanently erected. In order to achieve this a high initial capac- ity is necessary. A high initial capacity results in the formation of calcareous depos- its on the steel surface. For this reason, a lower average current density is required to maintain the protective potential. Guidelines for the selection of current density at the dimensioning of cathodic protection of thin film coated or painted steel surfaces are given in the table below: Table 2.3 Guidelines for selection of current densities in order to dimension cathodic protection of thin film coated steel surfaces at sea water temperature [9] | Lifetime _____ Current density, mA/m2 i (years) initial___ In mid-operation a 5 25 10 oa 6 12 | 20 | 4 15 30 i | 30 i 4 22 } 60 | 4 35 | 90 | 40 1 The metal to be protected is coupled to sacrificial anodes of a less noble metal. like for example zinc, aluminium or magnesium. 2 The metal to be protected is coupled to an external power source, usually a DC rectifier. Often inert, permanent anodes of lead, graphite, magnetite, platinum or platinized titanium are used. Inert anodes are anode types which are con- sumed at a minimum rate over time. Using sacrificial anodes As mentioned above, cathodic protection works because the metal in the galvanic coupling becomes the cathode. Which material can then become the anode in such a circuit or such a system? From the galvanic series in sea water all alloys which are more ignoble than steel can theoretically be used. However, usually anodes made of aluminium, zinc or magnesium are used. The question arises, is it really as simple as this? The answer is both yes and no. An aluminium anode weighing for example 100 kg mounted on an offshore oil platform will not reduce the corrosion on the platform to any great extent. There- fore, it is important during the dimensioning of the cathodic protection to consider the relative area sizes of the cathode and the anode. The protection depends on the distance between the anode and the area to be protected. Consequently, the posi- tioning of the anodes is often based on previous experience. In recent years, simu- lation/calculation models for cathodic protection have been worked out by means of advanced computer programs. In case of cathodic protection by means of sacrificial anodes the anodes supply 60 Corrosion Protectionthe current necessary to protect the steel. If the anodes work satisfactorily, they will eventually be totally consumed. After some years it will be necessary to replace the anodes. However, this depends on how much current the structure requires over time. ‘Sacrificial anodes Figure 2.20 Cathodic protection using sacrificial anodes Aluminium anodes are commonly used for under water protection in the offshore industry. This is mainly due to the weight savings obtained compared, with zinc. The current output from aluminium anodes is higher than for zinc anodes. This is important with a view to the lifetimes of the anodes and how often they need to be replaced Magnesium anodes cannot be used for cathodic protection in sea water. The self-corrosion of magnesium in sea water is very high. On the other hand, magne- sium wire finds widespread use for stripping of tanks on board ships. Stripping is the removal of rust and mill scale. Large quantities of magnesium wire are welded to the steel surface in for example ballast tanks. After the wire has been fixed the tanks are filled with sea water, which acts as the electrolyte. The steel becomes highly overprotected. The overprotection causes the formation of large volumes of hydrogen gas at the surface, causing mill scale and rust to detach from the surface. Proper cleaning with high pressure hosing before subsequent surface preparation and painting of the tanks is of course a must. Consequently, the use of this type of anodes is limited to water tanks or pipelines in soil. In case of protection in soil, the anodes (zinc or magnesium anodes) are usually placed in a coke filling mixed with gypsum and common salt to ensure suitable conductivity. On vessels both the method of attachment and the shape of the sacrificial anodes are of major importance due to the drag of the ship in the water. The best method is to weld on the anodes in preference to attaching them with screwed-on fasteners which may be shaken loose. In case of cathodic protection in sea water calcium and magnesium salts are deposited which have a protective effect and with time reduce the current require- ments. The West Sole B-platform in the British sector of the North Sea was put into operation in 1966. Here a combination of paint and cathodic protection was used. Under water the platform was painted with coal tar and additionally protected by 236 zinc anodes with a total weight of a little more than 19 tons. After almost 20 years in service this was still reported to provide protection of the platform [10]. 2 PROTECTION AGAINST CORROSION = 61Figure 2.21 Riser's on the Oseberg-platform protected by use of coal-tar epoxy, epoxy and sacrificial anodes of aluminium Cathodic protection with impressed current The principle of an impressed current installation is quite similar to that described for the sacrificial anodes, however, some differences should be noted. A system based on impressed current is more complicated and more expensive to install than the systems with sacrificial anodes. ‘The necessary protection is provided by the permanent anodes through a recti- fier wansmitting direct current. One great advantage of the impressed current in- stallation for cathodic protection is the use of anodes which are scarcely consumed. The anode material is more noble than the metal to be protected. It is subject to very little attack. Anode materials often used are graphite, magnetite, platinum and plati- nized titanium. Protection with impressed current is used for a number of purposes, including the protection of pipelines, steel pilings in soil, floating docks, etc. Impressed cur- rent installations are today very rarely used on offshore structures. But CONOCO’s 62 CorRosioN PROTECTIONMurchison-platform in the North Sea having a surface area of 80 000 m? is pro- tected by one hundred anodes, and fifty reference electrodes are in place [11]. Pre- vious projects with impressed current installations have experienced problems with breakdown of current-carrying cables to anodes and broken down anodes. As a result of these circumstances protection with sacrificial anodes is often chosen. 4. - Permanent anodes 2- Reference electrodes 3 - Control unit, 4- Registration unit 5 - Rectifier unit, 6 - Galvanic connection between rudder and hull 7 - Slipting on propeller shaft Figure 2.22 Inpressed current cathodic protection installation on a ship In order to obtain even protection of a painted ship’s hull the anodes are fitted below the waterline along the ship’s side. On suitable sites reference electrodes are placed. They are used for monitoring that the ship has the right protection. Curren’ voltage are automatically adjusted on the basis of the readings taken. The anodes are insulated from the underlying paint system by use of an anode shield created by a 0.8-2 mm thick layer of epoxy filler applied in a diameter of 2- 3 metres around the anodes. This is done in order to prevent the paint system from breaking down due to the alkaline environment developed around the anodes. Elec- tronic control makes the system self-regulating. And if defects arise, for example damage to the paint coating system, the current supply is automatically increased. It is not unproblematic to use impressed current installations for the protection of buried pipelines. It is important that the buried anodes can supply current over large distances. Usually a coke or coal filling with gypsum and salt is used to increase the conductivity. It was mentioned at the beginning of this chapter that cathodic protection had been used for protecting reinforcing iron in concrete constructions. Impressed cur- rent installation is used as a protective system on bridges and road systems world- wide. When impressed current is used, it is possible to change the potential if need arises. This cannot be done in the case of sacrificial anodes. Combination of cathodic protection and paint coating systems Before the Oseberg B-platform was built, very few submerged steel parts of the 2 PROTECTION AGAINST CORROSION = 63.fixed platforms in the North Sea were painted below the water line. They are solely protected by cathodic protection with sacrificial anodes. In spite of the deposit of calcium on the unpainted steel, a large number of anodes are required for cathodic protection. For the most recent offshore projects, systems combining paint coatings and cathodic protection have been used under water on platforms in the North Sea. In order to save weight and to obtain an expected lifetime of forty years, the jacket of the Oseberg B-platform was painted with coal tar epoxy in a total of 400 pm [12] At the same time such a combined protective system reduces the quantity of sacri- ficial anodes required. Cathodic protection is in many cases used as a supplement to ensure satisfactory protection of the steel in case of damage to the paint coating film. In order for a paint coating system to be suitable together with cathodic protec- tion, the following conditions must be fulfilled: The paint coating selected should be able to withstand alkalis as alkalis will be formed at the cathode. Epoxy, coal tar epoxy and vinyl coatings are suitable in combination with cathodic protection. Additionally, also asphalt coatings are used. However, the binders of drying oils, alkyd, phenolic resins, polyester and epoxy ester and modified chlorinated rubber paints are broken down in contact with alka- lis. Paint coating film i Rectifier 77.3 V SSC + Reference electrode Steel Gas and hydroxide formation Permanent Painted Hydroxide formation, the anode steel paint coating blisters Figure 2.23 Overprotection Overprotection In summary, cathodic protection is a technique used for reducing corrosion. Sacri- ficial anodes or noble permanent anodes supply sufficient amounts of cathodic current for the dissolution of metal at the anode to be insignificant. In case of correct cathodic protection of steel in sea water, a voltmeter, inserted between the steel and a reference electrode (SSC), should show a potential between 0.80 and 0.85 volt. If the potential is further reduced, down towards —1.00 volt, overprotection may arise. Overprotection causes the formation of hydroxy] ions (OH) and hydrogen gas (H,). 64 — Corrosion pRoTEcTIONThis phenomenon is usually called cathodic disbonding. Cathodic disbonding often appears as blistering together with the formation of white or beige calcareous deposits on and around the damaged areas. How extensive the calcareous deposit- ing is, will depend on the hardness of the water. Normally the paint system will not detach, but the blistering is relatively dense. Cathodic disbonding may arise both in the case of sacrificial anodes and im- pressed current with potentials exceeding approximately ~1.15 to -1.20 volt meas- ured versus CuSO, [13] The formation of alkali (OH’) in the blisters protects the steel. If the blisters are broken, for example in connection with the docking of a ship, the steel under the paint system can be observed to have a grey appearance. Rupture of the blisters increases the area to be protected and the power supply requirements increase. Severe corrosion 0,60 Corrosion ‘Some protection 0.85 100% cathodic protection Slight overprotection aa Overprotection and possible paint coating damage 18 Figure 2.24 Potentials of steel measured versus a copper/copper sulphate refer- ence electrode Corrosion inhibitors A corrosion inhibitor is the common term for a chemical or substance added in small quantities to the corrosive medium to reduce or obstruct a corrosion process. Some people may perhaps link the use of inhibitors to the addition of chemicals to pickling (acid) baths in order to limit the corrosion attack on steel. But the inhibi- tors find a wider area of use than that, including the use in connection with oil production and in the cooling water of circulating piping systems. Depending on whether they affect the anode or cathode reaction, they are called anodic or cathodic inhibitors. The purpose of an anodic inhibitor is to retard the reaction taking place at the anode: the metal dissolution. The cathodic inhibitor 2 PROTECTION AGAINST CORROSION 65,must retard the reaction at the cathode: the oxygen reduction. A few substances can inhibit both reactions. What do the inhibitors consist of? And how do they work? The inhibitors are often chemicals like chromate, nitrite, phosphate or polyphosphate. They may have many different ways of working. Some form a thin film on the surface to inhibit the reactions. Others are absorbed at anodic areas of the surface. Cathodic inhibitors are often called “safe”, for even if too small amounts of inhibitor are added so that not all cathodic areas are eliminated, at any rate some of the active cathode surface will be reduced. Too little addition of anodic inhibitor may be “dangerous”, because in this case not all anodic areas are removed. As a result, the corrosion will intensify at the remaining areas. How much the rate of corrosion will be retarded when adding the inhibitor de- pends on how effective it is. It is important to continuously control how much inhibitor is present in a system. In this way an eye can be kept on whether there is sufficient inhibitor available to protect the metal. Corrosion inhibitors are used in a number of areas: in acids for cleaning of steel surfaces for mill scale and rust before hot dip galvanizing (pickling) in anti-freeze (ethylene glycol) for cars in the coolant used for the cutting, turning, milling and drilling of metals The paint industry also uses inhibiting pigments which, in the same way as de- scribed above, are to provide the metal with an effective protection. As examples of such pigments iron oxide and zinc phosphate can be mentioned. Corrosion monitoring Corrosion attacks may occur in spite of all the best efforts done to prevent it. It is therefore important to have control routines which include inspection and monitor- ing in order to prevent a structure from breaking down. Visual inspections carried out on a regular basis are important parts of the monitoring process. But in order to be able to carry out visual controls we depend upon the access possibilities for taking a closer look at for example damaged areas. This is not always possible. Examining corrosion attacks inside process systems presents a problem. In order to discover corrosion attacks on inaccessible areas, for example inside piping systems, other methods must be employed. There are many different ways of determining the condition of the material. One way is ultrasonic measurements. This method is used for searching for cracks or pittings on metals. The instruments used are small and the readings are taken on the outside of the pipes. In liquid-carrying systems weight loss coupons can be utilized. This method involves inserting small coupons or specimens of the same material quality as the pipe into the flow of liquid. The coupons are placed parallel to the direction of flow in such a way that the corrosion attack will be equivalent to the one on the pipe 66 Corrosion PROTECTIONwall. After a period of time the samples are examined. The weighing enables us to determine the weight loss of the metal. This can then be converted into a corrosion rate of for example millimetres per year. The method is relatively time-consuming if reliable results are to be obtained. Linear polarization probe Fasteners for weight loss coupons oe Ml t f.@ Closing valve Closing valve Figure 2.25 Measuring methods for control of corrosion rate Two other measuring methods widely used are measurement of electrical resistance and polarization resistance. When measuring electrical resistance the resistance of a small sample, often a thin wire, of the metal is measured. A probe of the metal in question is placed in the corrosive liquid. As the wire corrodes, the resistance of the wire increases due to the cross-section of the wire becoming smaller. The increase in resistance becomes a measure of the corrosion. This method tells us how much of the metal has corroded. But the method provides no information on which type of corrosion is occurring on the wire or in the system. This method is often used in connection with dosage of water treatment. - The method does not require so long exposure times to provide results as when the weight loss is calculated. The measurement of polarization resistance is an electrochemical method where a probe is provided with two electrodes of the material to be examined. The probe is mounted inside the system and a DC voltage applied between the electrodes. The method provides an instantaneous value of how fast the metal corrodes. The method depends upon the liquid having a sufficiently high conductivity. Both of the latter methods have gained great practical importance, in particular for the monitoring of corrosion on offshore installations. Literature 1 Gullmann, Jan; Knotkova, Dagmar et al; Weathering steels in building — cases of corrosion damage and their prevention; Research cooperation between the Swedish Corrosion Institute and G.V. Akimov’s State Research Institute for the Protection of Materials, Czechoslovakia; Bulletin No. 94, Stockholm 1985. 2 John W. Peart: Unpainted weathering steel bridges. Corrosion mechanisms and maintenance alternatives. Journal of Protective Coatings and Linings. January 1991. Pages 36-42. 2 PROTECTION AGAINST CORROSION = 67.3 E. Bardal: Korrosjon og korrosjonsvern. Tapir, Trondheim 1985. 4 E.V. Schmidt: Exterior Durability of Organic Coatings, FM) International Pub- lications Ltd., Surrey 1988. 5 E. Mattsson: Elektrokemi och Korrosionsléra, Bulletin nr. 100. Stockholm 1987. 6 K.A. Chandler and D.A. Bayliss: Corrosion Protection of Steel Structures. Elsevier Applied Science Publishers, London 1985. 7 NACE Standard RP 0178-2003. Standard Recommended Practice: Fabrica- tion Details, Surface Finish Requirements, and Proper Design Considerations ‘for Tanks and Vessels to Be Lined for Immersion Service. 8 EN ISO 12944:1998 “Paints and Varnishes - Corrosion Protection of Steel Structures by Protective Paint Systems”. 9 Oljedirektoratet: Veiledning for korrosjonsbeskyttelse av innreminger. 10 D. Fairhurst: Konferansen — UK Corrosion 84 — Lecture 1/1, ICorrSt. Bir- mingham, England. 11 K.R. Trethewey and J. Chamberlain: Corrosion — For students of science and engineering. Longman Group UK Limited 1988. 12 RE. Lye: A Corrosion Protection System for a North Sea Jacket. Materials Performance, May 1990, pages 13-18. 13 JPCL, Journal of Protective Coatings and Linings, June 1990, pages 9-11. Control questions 1 Aluminium railings are to be mounted on a steel structure by means of suitable Jong-life fastening materials. Describe which selection of materials you would recommend, and how you will join and protect the construction, Describe which types of corrosion may arise over time, and which measures you will take to reduce the risk. 2 You must present a proposal for the selection of materials for a piping system for the transportation of sea water at a constant temperature 20°C. Describe the corrosion attacks you expect might arise on pipes of steel, stainless steel (ASTM 316) and copper-nickel. 3. ‘The facades of a large number of buildings in the centre of a city with medium corrosive atmosphere (corrosivity category C3) are to be renovated. New bal- conies are to be fitted, and the load-bearing structure for the balconies is to be made of steel. But we are in doubt about the further treatment. We want a system with a minimum lifetime of 50-60 years and a maintenance interval of approximately 15-20 years. Draw up a proposal for the pretreatment and coat- ing systems which in your opinion will fulfil these requirements. 68 — Corrosion protection