We continue our series of posts about corrosion. Today we will analyze in detail some key aspects about corrosion phenomena of stainless steel welded components.
What type of corrosion can be triggered?
The choice of the basic material is essential to have stainless steel welded components that maintain good sealing characteristics over time. Especially depending on the environment in which these components will have to work. If you do not choose the material correctly, you will often run into the risk of intergranular corrosion. This metallurgical phenomenon leads to the precipitation of chromium carbides at the boundaries of the grains, where the material will lose its “stainlessity” and thus be sensitive to aggressive agents.
Chemical composition of the material. How important is it?
A fundamental role also plays the chemical composition of the material that will affect the start time of precipitation. The phenomenon, in fact, will begin:
- sooner if the material has a high carbon content
- later if the material has a lower carbon content.
Another relevant parameter is the presence of “stabilizing” elements (e.g. titanium and niobium) in the precipitation dynamics. These avoid the precipitation of chromium carbides and, consequently, the loss of stainlessity at the boundaries of the grains. Even in the presence of stabilizing elements, however, care must be taken to carry out an adequate thermal cycle to not incur into a “knife blade” corrosion (which can develop between the fused and thermally altered areas).
What material to choose?
The choice, therefore, could fall on a low-carbon base material (e.g. 304L, 316L) or on a stabilized (e.g. 321, 316Ti) to prevent the onset of corrosive phenomena. In the presence of welded joints, it is possible to raise awareness in the thermally altered area. This happens, for example, when the component being welded has considerable thicknesses, which makes the thermally altered zone remain exposed to high temperatures for a long time, or when the heat of the welding process is not completely disposed of.
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