- Weld corrosion can be defined as the process by which the metal deteriorates because of atmospheric gases .The chemical reaction between elements in the weld metal and atmospheric gas / gases occur creating compounds like oxides , nitrates etc. which flake away from the base metal. The corrosion of weld deposits on base metal after welding is an undesirable characteristic.
- While this corroded condition may not be classified as either a structural or appearance defect, it is undesirable and may lead to one or the other of these defects by its persistence. Welds that corrode may eventually fail because of cracks, loss of tensile strength and other defects resulting from corrosion.
- The light metals are especially susceptible to corrosion as are the un stabilized stainless steels. Corrosion failure in weld metals occurs due to various reasons. Some of the weld metals are corrosive resistant where some are not.
- The various types of corrosion that occurs in weld metals are,
- Intergranular corrosion
- Stress corrosion
- Galvanic corrosion
- Pitting corrosion
- Microbiologically influenced corrosion
- Heat-Tint Oxide Formation
Intergranular corrosion:
- Intergranular corrosion is also called as weld decay. Intergranular corrosion occurs mostly in the Heat Affected Zone (HAZ). This occurs at a particular temperature range around 550°C - 850°C, precipitation of chromium carbides is induced when the HAZ reaches the particular temperature range, resulting in depletion of chromium in the region adjacent to the grain boundary. These chromium carbides forms a narrow band along the grain boundaries and this make the zone anodic to the unaffected grains. This region becomes a major region for corrosion. Intergranular corrosion causes a loss of metal in a region that parallels the weld deposit.
Stress
Corrosion:
- This can be caused mainly due to the following reasons proper combination of corrosive media, susceptible microstructure and tensile stress. They have an anodic crack tip and often formation of apparent corrosion products occurs along the fracture region. Cracking is often characterized by crack branching and usually has a delay time prior to crack initiation, with initiation occurring at corrosion pits. Increasing the ferrite content in stainless steel weld metal reduces stress corrosion susceptibility. Approximately 50% of ferrite are resistance to stress corrosion.
Galvanic
corrosion:
- Some of the metals can be welded autogenously where in others filler materials are most commonly used. The usage of filler material with different composition from the base metals produces an electrochemical potential difference which may lead to galvanic corrosion. These electrochemical potential difference make the weldment more active. And generally the corrosive behaviors varies between the base metal and the filler material. There are different combination of filler material and base metal which forms galvanic corrosion. Mostly austenitic stainless steel is used as filler material for many applications in which the weld deposit is to be cathodic stainless steel. In the presence of corrosion environment hydrogen is generated which can prevent the metal from cracking. But these cathodic weld deposits may cause stress corrosion cracks in HAZ. When combination of austenitic steel and ferritic base metal, residual stress occurs which may cause cracks and corrosion.
Pitting
corrosion:
- The breakdown in the passive oxide layer which is generally used to prevent the material from corrosion leads to pitting corrosion. Pits are commonly the result of a concentration cell established by a variation in solution composition that is in contact with the alloy material. Formation of pit on the material acts as an anode region. This anode region is supported be relatively large cathode region. The origin of pits are from specific micro structural features in the weld deposit. Pitting occurs when it exceeds the pitting potential value. Pitting potential value can be referred to as the material/solution combination achieves a potential critical value known to as pitting potential. Pits develop more readily in metallurgically heterogeneous materials. Pits may also initiate at the austenite-ferrite interfaces in stainless steel weld metal.
Micro
biologically influenced corrosion:
- The microbiologically influenced corrosion is caused due to the influence of microorganism which are present in the metal. The best example which can be given is water. When the metal is being exposed to water some of the microorganisms present in it will react with the metal resulting in formation of corrosion. Some other organic media which undergoes metabolic process and produces biofilm when exposed to metal surface which results in initiation of corrosion. Along with the metabolic reactions the microorganisms can also produce a localized acid which may also lead to corrosion. In austenitic steels the micro biological effect are in the form of pit which occurs adjacent to the weld region.
Heat-Tint
Oxide Formation in SS:
- This is caused mainly due to high heat input because of excess current or slow speed during welding or improper shielding of weld region during the time of welding. This produces a variation in oxide formation. This variation in oxide formation will result in a gradient in the degree of chromium depletion adjacent to a stainless steel weld. This may result in formation of corrosion in the weld metal. This is indicated as Heat-Tint Oxide Formation.
General causes and cure:
Some
of the general causes and solutions for welding corrosion is shown below,
Corrosion can be caused because of
Poor
|
Solutions for prevention of cracking by
Good
|
|
|
Disclaimer
No comments:
Post a Comment