Unit 2 Corrosion Testing and Prevention methods
Corrosion Testing –Introduction of Corrosion Testing by Physical (only
weight loss & salt spray method) and Electrochemical Methods such as
ASTM standard methods only G-5&A262-A. Corrosion Prevention methods
–Metallurgical and Environmental aspects of corrosion, Inhibitors, Internal
& External coating, Cathodic & Anodic protection, use of special alloys,
Improvement in design/ changes in design to control corrosion.
Unit 2 Corrosion Testing and Prevention methods
Corrosion Rate, CR
W is weight loss, ρ is metal density, A is exposed rebar specimen
surface area, and T is exposure time
From the density of iron (steel) and typical experimental
parameters:
W = 1.6 grams
ρ = 7.87·106 grams/m3,
A = 76·10-4 m2, and
T = 0.0767 years (28 days),
ASTM standard methods only G-5&A262-A
The ASTM standard G5 is
a method to test the corrosion of Type 430 stainless steel,
with a potentio dynamic anodic polarization measurement.
Experimental setup
For the experiment,
- 430 stainless steel disk of 1 cm2 was immersed in an 1 N aqueous sulfuric acid solution (0.5 mol/L).
ASTM A262 is a common intergranular corrosion testing method that can quickly screen batches of
material to determine corrosion susceptibility.
The ASTM A262 testing specification contains five unique intergranular corrosion tests
Oxalic Acid Test (ASTM A262 Practice A)
This simple etching technique is used as a quick screening method to ensure that a material is free of intergranular
corrosion susceptibility. The Oxalic Acid test is only used to ensure that no corrosion exists; samples are labeled either
"Acceptable" or "Suspect". A different ASTM A262 test method must be used to quantify the level of corrosion or to
disqualify a material for use. The Oxalic Acid test is also only applicable to corrosion associated with chromium carbide
precipitates and is only useful for specific material grades.
Metallurgical Aspects
Polycrystalline would corrode much faster than single crystal. And is due to the
presence of grain. Boundaries in polycrystalline grain boundaries are the
regions of high energy, for their chemical reactivity is higher than the rest of
materials
Effect of Grain Boundary
Metal Purity
Effect of various phases in the alloys presence of phases alloys have the same
effect as Galvanic – coupling
Different Metals
Residual Stresses
The Strain hardening deformation.
Environmental Aspects
of Corrosion
Iron Corrosion
Environmental and Metallurgical
Aspects of Corrosion
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Internal Pipe Coating
3 LAYER POLYETHYLENE COATING AND FBE LINING ON
STEEL PIPES
PRODUCT DESCRIPTION:
3 Layer polyethylene externally coated and fusion bonded
epoxy internally lined steel pipes in diameter ranges from
300 mm to 3000 mm. This coating provides excellent
corrosion resistance, chemical resistance, electrical
resistance, and has very low moisture permeability.
Internal lining of potable water contact approved FBE
provides low frictional resistance to flow. Ideally suited for
potable water and oil and gas applications. The three
coating layers in 3-LPE coated pipe are as follows:
Layer 1: This is the corrosion protective layer. This layer is
of fusion bonded epoxy which offers very good corrosion
protection. The fusion bonded epoxy has a very good
bonding with the blasted steel surface.
Layer 2: This layer is the copolymer adhesive. The
copolymer adhesive is a maleic anhydride grafted
polyethylene compound. This material has good chemical
bonding to the fusion bonded epoxy and the top layer
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Layer 3: This layer is for physical protection and consists of
polyethylene. Since the copolymer adhesive and
polyethylene are similar, they bond well with each other.
Design Rules
Designing structures and parts to prevent or
control corrosion is more cost-effective than
waiting for the equipment to fail in service
Adjust Wall Thickness
Because corrosion involves the degradation
of materials, the process continuously eats
up the material and decreases its thickness.
Therefore, part of corrosion-resistant design
involves making allowances for this
reduction (i.e., wall loss) in the thickness in
pipes, storage tanks and other parts. A
general method is to make the wall thickness
twice of that which is required for the
Adequate Drainage desired life of the structure. However, the
Tanks and other storage containers should be designed in such wall thickness must meet mechanical
a way that they can be easily drained and cleaned. Therefore, requirements for stress, pressure and
all transitions should be smooth, and taps should be located so weight.
that the tank can be completely drained.
Minimize Bi-Metallic Corrosion Cells
Avoid galvanic corrosion by using similar metal
throughout the structure, if possible, or by
avoiding electrical contact by insulating different
materials.
Avoid Differential Aeration Cells
Differential aeration should be avoided. For the
components immersed in water, sufficient
aeration should be ensured to cause passivation,
which slows the corrosion. Otherwise, aeration
should be prevented as much as possible.
Minimize Temperature Gradients
The equipment for heat transport should be designed so that surface temperature varies as little as possible. Cold and
hot spots should be avoided. Superheated spots are prone to thermos galvanic corrosion and cold spots can enhance
local condensation, which leads to corrosion. Therefore, the thermal gradient should be kept to a minimum.
Minimize Stress Gradients
Stress concentrations in the components
exposed to corrosive mediums should be
avoided, especially when using materials
susceptible to stress-corrosion cracking.
Therefore, designers should aim for simple
geometry, as abrupt changes in dimensions can
provide sites for stress concentration.
Separate Environments
The surroundings should be considered to
minimize the consequences of corrosion. This
involves making sure that separate systems do not
impair the environments of others. For example, if
a copper alloy corrodes and the moisture
containing copper ions come in contact with an
aluminum component, it will result in galvanic
corrosion.
Sharp bends should be avoided in piping systems
with high velocity fluids or solids in suspension to
prevent erosion corrosion
Avoid Heterogeneity
The most general rule for proper design is to
avoid heterogeneity. Heterogeneity consists
of different metals, uneven stress and
temperature distribution. Sharp corners
should be avoided because they are difficult
to paint with uniform thickness. Complex
geometries and narrow gaps impede surface
treatments like painting, thermal spraying
and blast cleaning; increase the cost; and
make it difficult for cleaning and drying. All
the relevant codes and standards should be
met. Rules for minimum gaps between
profiles are given in ISO 12944-3:2017.
