Corrosion in welded joints is a phenomenon that can compromise the structural integrity of metal components, affecting their durability and performance. This problem arises due to a combination of chemical, physical, and environmental factors that interact in a complex manner. Below are the main causes of corrosion in welded joints:
Metallurgical Heterogeneity
Welded joints exhibit metallurgical heterogeneity resulting from the heating and cooling processes during welding. This heterogeneity creates different microstructures in the heat-affected zone (HAZ) and the weld metal compared to the base metal. Differences in composition and structure can generate areas with different electrochemical potentials, favoring the formation of galvanic cells that accelerate corrosion.
Residual Stresses
During the welding process, high temperatures and rapid cooling can induce residual stresses in the welded structure. These stresses can cause microcracks and defects in the joint, facilitating the penetration of corrosive agents and the initiation of stress corrosion cracking, a type of corrosion that occurs in the presence of tensile stresses and a corrosive environment.
Welding defects
Defects in welds, such as porosity, inclusions, cracks, and lack of fusion, act as stress concentrators and preferential sites for corrosion. These defects can trap contaminants and moisture, creating aggressive local environments that accelerate the corrosion process.
Filler Metal Composition
The selection of filler metal is crucial for the corrosion resistance of the welded joint. If the filler metal is not compatible with the base metal or has an inadequate composition, there can be a significant difference in electrochemical potential between the weld metal and the base metal, promoting galvanic corrosion.
Surface Contamination
During preparation and welding, surface contamination by agents such as oils, greases, oxides, and salts can introduce corrosive elements into the joint. These contaminants can remain on the welded surface or within the structure, acting as initiators of corrosion.
Service Environment
The environment to which the welded joint is exposed also plays a crucial role in corrosion. Factors such as humidity, temperature, the presence of corrosive gases (like chlorides and sulfides), and exposure to salt water or acids can significantly increase the corrosion rate. Extreme temperature and pressure conditions can also accelerate corrosion processes.
Inadequate Protection
The lack of protective coatings or the use of inadequate coatings can leave welded joints vulnerable to corrosion. Coatings such as paints, galvanized coatings, or anodized coatings can provide a physical barrier against corrosive agents. However, if these coatings are applied incorrectly or damaged, the corrosion protection can be insufficient.
Electrolytes in the Environment
The presence of electrolytes, such as saline solutions, in the environment can facilitate electrochemical corrosion. Electrolytes promote ion conduction, allowing corrosion reactions to occur more easily in welded joints.
