What is Preferential Weld Corrosion (PWC)?

Welding is a fundamental process in various industries, from construction to aerospace, enabling the fabrication of complex structures and components. However, alongside the benefits of welding come challenges, one of the most significant being the risk of corrosion, particularly Preferential Weld Corrosion (PWC). PWC poses a threat to the structural integrity of welded metal assemblies, demanding attention and proactive measures to mitigate its effects.

Preferential Weld Corrosion (PWC) Explanation.

Preferential Weld corrosion (PWC) is the accelerated corrosion of the weld metal and/or  HAZ, as compared parent metal in a corrosive environment. It is a form of galvanic corrosion which the weld metal acts like anode as compared to the adjacent  parent  metal. The main causes of the preferential weld corrosion are 

1. Difference in composition of parent metal, HAZ and weld metal.
2. Difference in microstructure of parent metal, HAZ and weld metal.  

Both of these causes are usually result of poor welding practices and poor quality control. 

Weld metal is seen to be severely affected with preferential corrosion when the  manganese contents are higher. Further the corrosion occurs more rapidly in hardened  steel in acidic environments than in fully tempered steel. 

Therefore welds in non post weld heat treated equipment and piping are more susceptible to preferential weld corrosion than post weld heat treated structures. Experience has  shown that the preferential corrosion is found more in less critical service systems  like  sea water cooling, slop storage and drain water systems etc.

This is because more stringent quality monitoring is done on the critical systems while these systems are often overlooked.  

Like all other corrosion mechanisms the corrosive environment severity plays basic role in determining the rate of preferential corrosion. Fluids having high electrical conductivity,  such as seawater or the acidic streams with lower pH are the main contributors in  preferential corrosion. 

However, low conductivity environments like water streams with dissolved CO₂ can also  play significant role in preferential corrosion. Similarly Alterations to the environment, such as the addition of a biocide, can change the corrosion characteristics of a system.

For example, a joint may be totally resistant to corrosion in a particular environment, but with the addition of a biocide, the joint may become susceptible to preferential corrosion. 

Understanding Preferential Weld Corrosion.

Preferential Weld Corrosion occurs when there is a localized attack on the weld metal within a welded structure, often leading to accelerated deterioration compared to the surrounding base metal. The root cause of PWC lies in the inherent differences between the weld material and the base metal. These disparities can manifest in variations in chemical composition, microstructure, or residual stresses.

Mechanism of PWC.

At the heart of PWC is a phenomenon known as galvanic corrosion. When two dissimilar metals are in contact in the presence of an electrolyte, such as moisture, an electrochemical cell is formed. In the welding, the weld junction acts as the site of galvanic corrosion due to the differences in composition and microstructure between the weld metal and the base metal.

The weld metal, being distinct from the base metal, assumes the role of the anode in the galvanic couple, while the base metal becomes the cathode. As a result, the weld metal undergoes accelerated corrosion compared to the base metal. This preferential attack weakens the weld joint, jeopardizing the structural integrity of the entire assembly.

Factors Influencing PWC.

Several factors influence the severity of Preferential Weld Corrosion:

1. Material Composition: Variations in alloy composition between the weld metal and the base metal exacerbate the likelihood of galvanic corrosion.
2. Welding Process: Different welding techniques and parameters can introduce varying levels of residual stresses and metallurgical changes, influencing the susceptibility to PWC.
3. Environmental Conditions: Corrosive environments, such as marine or industrial settings, accelerate the progression of PWC.
4. Surface Contaminants: Residual contaminants from the welding process or subsequent surface treatments can promote localized corrosion initiation.

Mitigation Strategies.

Preventing Preferential Weld Corrosion requires a multi-faceted approach:

1. Material Selection: Choosing welding materials with compatible compositions and corrosion resistance properties minimizes the potential for galvanic coupling.
2. Weld Design: Optimizing weld geometry, such as minimizing sharp corners or crevices where corrosion can initiate, reduces susceptibility to PWC.
3. Surface Protection: Applying corrosion-resistant coatings or inhibitors to the welded surfaces provides an additional barrier against corrosion.
4. Post-Weld Treatment: Stress relieving and passivation processes can alleviate residual stresses and enhance the corrosion resistance of welded assemblies.
5. Monitoring and Maintenance: Regular inspection and maintenance protocols are essential for detecting early signs of corrosion and implementing timely corrective actions.

Conclusion:

Preferential Weld Corrosion poses a significant challenge to the reliability and longevity of welded structures across various industries. Understanding the underlying mechanisms and implementing proactive mitigation strategies are paramount to safeguarding against the detrimental effects of PWC. By addressing the root causes and adopting preventive measures, engineers and industry professionals can ensure the integrity and performance of welded assemblies in demanding operating environments.