Corrosion in stainless steel is one of the most troublesome technical concerns for fabricators and users. Whether localized or more widespread, corrosion takes on many forms including general corrosion, crevice corrosion, intergranular corrosion, pitting corrosion and stress corrosion.

Corrosion is defined as an undesirable deterioration of the metal as a result of a chemical or electrochemical reaction caused by the environment. Welds often do not exhibit the same level of corrosion resistance found in the base metal, even when everything is done correctly. But if not done correctly, the corrosion in the welds can be higher. Here are some tips to prevent this potentially catastrophic problem.

First, choose the right filler metal for the job. Your welding supplier can advise you on this if the designer has not specified a filler metal. The filler metal should meet or exceed the level of molybdenum in the base metal. This could require the use of a nickel-based alloy, especially when the molybdenum content exceeds 4%.

Second, the welding procedure must control the heat input. This becomes increasingly more critical as the nickel content rises. Excessive heat can cause the formation of undesirable structures and create undue stresses that will exacerbate corrosion or cause cracking in service. With duplex stainless steels, too much heat or even too little heat can cause undesirable phases to be formed.

Third, pay close attention to the selection of tools for grinding, brushing and buffing the final weldments. Be sure to use stainless steel cleaning tools that have not previously been contaminated from use on carbon or low alloy steels. Also, ensure that all traces of slag and welding fume are removed from the base metal. Even if the welded area looks clean, an invisible film may be present that should be removed. Eliminating any cracks or crevices, such as undercut, is imperative.

A seldom-used practice is to use chemical inhibitors as a final passivation of the welded areas. The choice of such a chemical is not without risks but if properly used it can serve its intended purpose.

Of the many methods to control corrosion, none is better than the knowledge and familiarity of “best practices.”  Controlling corrosion is best accomplished by selecting the proper filler metal and following sound fabrication practices. This will ultimately provide maximum corrosion resistance.