Most welders are familiar with the phenomenon called stress corrosion cracking (SCC). It occurs when the wrong filler metal or base material is combined with unsuitable environmental conditions. Getty Images

In this installment we will take a break from welding processes for fabricating aluminum and instead focus on how the filler metal and base metal are affected by welding and how cracking problems can sometimes occur.

Most people are familiar with the phenomenon called stress corrosion cracking (SCC). It occurs when the wrong filler metal or base material is combined with unsuitable environmental conditions.

It is well-known that any Al alloy containing more than 3% Mg is subject to SCC. This includes most of the 5XXX alloys, such as 5083 and 5086, which are sheet and plate alloys. However, you must remember that almost all of the common Al 5XXX filler alloys, such as 5356, 5183, and 5556, also contain more than 3% Mg. Any of them can be susceptible to SCC under certain conditions, regardless of which Al alloy is used to weld.

However, the sensitivity to SCC depends largely on four factors:

Keep in mind that all four of these conditions do not need to happen simultaneously. Once a weld is exposed to high service temperatures, a sensitive microstructure will have formed. Even if the service temperature is lowered, the presence of factors 2 and 3 will be enough to cause SCC.

What does an SCC look like? It is usually a jagged crack down the center of the weld, and it can be short or long depending on the stress level. In fact, I have seen SCCs that were 24 in. long.

Here are three tips to avoid an SCC:

As a closing thought, never use high-Mg base materials such as 5083 at service temperatures above 150 degrees F regardless of whether or not they are welded.

See More by Frank Armao

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