Corrosion
References 1. Engineering Materials: Properties and Selection by Kenneth G.
Budinski and Michael K. Budinski, Pearson Prentice Hall, 8th edition, 2004 2. Corrosion Engineering by Mars G. Fontana, McGraw-Hill Book Company, 3rd edition, 1987 3. Corrosion Basics: An Introduction, National Association of Corrosion Engineers (NACE), Houston Texas, 1984
Why study corrosion ?
Corrosion is a worldwide problem Constructions, chemical companies, marine industries, petroleum companies, food companies, automobiles, aircraft/aerospace, etc. What is corrosion? Deterioration of a material (or its properties) because of reaction with its environment The result? Rust ! Study in the US on cost of corrosion per year 1975: $70 billion 1982: $126 billion 1998: $275 billion USD (in US alone) = $370 billion NZD
Effects of Corrosion which can result in ..
Loss of load-bearing cross-section or penetration Crack Initiation (due to pits) & Growth (due to H) Blockage
H H
Effects of Corrosion which can result in .. Maintenance and operating Cost Plant Shutdowns Lost valuable products Effects on Safety and Reliability Product Liability .$100 billion per year (in the US)to cover legal cost
Loss of Electrical Contact Decrease in Heat Transfer Debonding of Coatings, Spalling of Concrete Contamination of products Degradation of appearance
H2O Pb External Rust Spots
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Nature of corrosion
Why corrosion occurs?
Most metals want to return to their natural and stable form, i.e. ore. Iron ores contain oxide of iron. A few metals, e.g. gold, is in metallic form The process allowing the metals to return to ore (oxide or sulfide) is called corrosion.
Corrosion Reactions
Fe + H2O + 1/2O2
Iron Water Oxygen
Fe(OH)2
Ferrous Hydroxide
Fe + 2HCl Practically, all environments are corrosive to some degree Iron Acids, e.g. hydrochloric, sulfuric, nitric Air and moisture Bases, e.g. fresh water, distilled water Salt, e.g. seawater Industrial atmosphere Steam and other gases, e.g. chlorine, hydrogen sulfide Acid
FeCl2 + H2 Ferrous Gas Chloride
Forms of Corrosion
Coating
Metal or nonmetal
General or Uniform corrosion
The most common form of corrosion Normally occurs on the entire exposed surface or over a large area Represents the greatest destruction of metal on a tonnage basis
Uniform
Pitting
Filiform
Crevice
More noble metal
Least damaging from technical point of view because the life of component can be predicted from comparatively simple tests
Intergranular
Flowing Corrodent
Exfoliation
Selective
Load
Galvanic
Erosion-Corrosion & Impingement
Fretting Corrosion
High Temperature Dry Corrosion
Control of General Corrosion
General Corrosion
Positions of Anodes & Cathodes change with time
Use protective coatings Use cathodic or anodic protection Treat environment, e.g. remove O2 , add inhibitors
Use both for underground steel structures
occurs due to
use of wrong material general breakdown of coatings unanticipated change of environment
Select more resistant materials, (Consider non-metals) Ensure overheating of liquids cannot occur
testing should simulate all relevant details e.g. welds, anode/cathode areas, degree of aeration, etc.
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Pitting Corrosion
Localized attack that results in holes, mainly small holes Serious form of corrosion (one of the most destructive) Often difficult to predict because their small size and are often covered with corrosion products Stainless steels are prone to pitting corrosion especially in salt water. Brasses (Cu-Zn), bronzes (Cu-Sn) have better pitting resistance than stainless steels
Size, Shape & Density of Pits can vary enormously
Pitting often leads to initiation of fatigue, SCC, etc.
Crevice corrosion
Usually associated with stagnant solution caused by gasket surfaces, lap joints. Occurs especially under bolt & rivet heads, gaskets, seals and within small holes and cracks Different oxygen content in the stagnant area vs. open surface Stainless steels and aluminium are susceptible to crevice corrosion
Galvanic Corrosion
Occurs when two dissimilar metals are in contact (or electrically connected) Metal with less corrosion resistant become anodic, and the more resistant metal become cathodic (see galvanic series) The farther apart two metals are in the galvanic series, the greater potential galvanic corrosion to occur. Magnesium to steel is a very bad combination; Monel (Ni-Cu) to stainless steel will show negligible activity.
Galvanic Series
Platinum Most cathodic or resistant to corrosion (noble) Gold Titanium Silver 18-8 austenitic stainless steels (passive cond.) Iron-chromium alloys (passive cond.) Inconel (passive) Nickel Monel Bronzes Copper Brasses Inconel (active) Nickel (active) Tin Lead 18-8 Austenitic stainless steels (active) Cast iron Mild steel and iron Aluminum alloys Zinc Most anodic or easy to corrode (active) Magnesium and magnesium alloys
Filiform Corrosion
Occurs under protective films, e.g. lacquered (transparent surface films) Commonly observed on food or beverage cans Observed on steel, magnesium and aluminium surfaces covered by tin, silver, gold, phosphate Does not weaken or destroy metallic components but affects surface appearance Appears as network with an active head and red-brown corrosion product tail
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Exfoliation Corrosion
Specifically attack on flattened and elongated grains Appears as layers below metal surface (flakes, layer and peel off) Typically found in aluminium alloys on aircraft components (skins, spar, etc.)
Stress Corrosion Cracking (SCC)
Material deterioration (cracking) due to combination of corrosive environment and tensile stress Found in aluminium alloys, stainless steels, copper alloys, titanium alloys, etc Stainless steels and aluminium are prone to chloride-containing solutions; brasses crack in ammonia-containing solutions Strongly influenced by materials treatment (heat treatment and stresses) Normally intergranular
Intergranular Corrosion
Occurs preferentially along grain boundaries Usually caused by segregation along grain boundaries Segregation leads to dissimilar composition between grain boundaries (anodic) and the grains (cathodic) The most common case: sensitization and welding of stainless steels When stainless steels are heated in the temperature range of 400 to 850oC, chromium carbides (CrC) tend to form along grain boundaries Also observed in high-strength aluminium alloys and copper alloys
Intergranular Corrosion of Stainless Steel (Weld Decay)
Holding or slow cooling in the range 500-800C is responsible
Dealloying (selective leaching)
Occurs on metal where one constituent of metal alloy is removed from the alloy There are two forms: Dezincification: removal of zinc from brass (Cu-Zn) Graphitization: dissolution of iron from gray cast irons leaving only the graphite Dezincification occurs when brass exposed/operates at a relatively high temperatures, e.g. 80oC for several months Graphitization occurs on gray cast irons pipes over a period of time
Erosion-Corrosion
Turbulent Flow
(relative motion of fluid and metal surface) Mechanical Force of Liquid removes Corrosion Products /Protective Films Pits / Grooves with directional pattern produced
Occurs in Pipes at changes of section, bends Propellers, etc. Valves at seals that leak Exacerbated by solid particles in liquid
dezincification graphitization
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Erosion - Corrosion of Steel Pipe
Section of Large diameter 0.2%C Steel Pipe Weld
Erosion -Corrosion of Sensor Tube
- exposed to flowing sea water at 4m/s Specified material : 70:30 Cu-Ni
water flow
12mm
Erosion Corrosion due to weld metal protruding into flow causing turbulence
Erosion- corrosion due to use of 90:10 Cu:Ni material (which is rated to 3m/s maximum flow rate)
Cavitation Corrosion
Formation & Collapse of Vapour Bubbles due to Hydrodynamic Pressure Changes
Examples of Cavitation
Pressure of Shock Wave produces spalling of Oxide & localised plasticity
316 Stainless Steel Pump Impeller (Vacuum Evaporation Unit) 70C Skimmed Milk 1 yr. in service C Steel Pipe Condensate+Steam leaking from trap Several yrs. service
Oxide reformsBubble reforms in same place
Cavitation Corrosion of Pipe
Sectioned Mild Steel pipe
Corrosion Control
Design & Fabrication Practice Material Selection / Surface Modification Environment Modification e.g. Inhibition, Oxygen Removal
Deep, Steep-sided Pits caused by Cavitation due to vibration of pipe filled with water
Cathodic Protection (Impressed Current, Sacrificial Anodes) Coatings
Remedy: Secure pipe to wall with more brackets to reduce vibration
dont just repair (by welding) without addressing cause
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Cathodic Protection
How to minimise or avoid corrosion
Examples of Design Features to Avoid
Avoid Dissimilar Metals see Galvanic Series Avoid stagnant regions / deposits / crevices
Material Selection
Remember to: Anticipate range of conditions (e.g. Commisioning, Shut Down, Cleaning) Consider Local Factors e.g. Cl- concn of cooling water even if designs/materials used successfully elsewhere Consider effects of Fabrication / Heat-treatment Remember that: Corrosion resistance of minor components e.g. screws, pins, etc. may be just as important as major ones
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