Stainless steel comes in different grades, not all of which have the same resistance to oxidation. The choice of the stainless steel grade for a fitting or pipe is therefore not trivial, as it can have an impact on the safety of the installation.
Basically, stainless steel is composed of carbon, iron and chromium. Other metals can be added to improve performance, such as nickel, molybdenum, titanium, etc.
Stainless steel's resistance to corrosion is due to the presence of chromium, which creates a layer of chromium oxide on the surface of the part. This phenomenon is called passivation.
Stainless steel 304 (1.4301 according to the European standard) contains 17 to 19% chromium and 8 to 10% nickel. It is the most widely used stainless steel in the world, as it offers good resistance to oxidising acids and can withstand natural humidity and fresh water. It is used in the food industry, and in standard industrial piping parts.
Its weak point is its vulnerability to salts and chlorides. In a marine environment, or during certain chemical processes, 304 stainless steel parts are subject to corrosion
Stainless steel 316 (1.4401 or 1.4436 according to the European standard) contains 16 to 18% Chromium, 10 to 12.5% Nickel, and 2 to 2.5% Molybdenum. The addition of molybdenum makes the alloy more resistant to chlorides and industrial solvents. 316 stainless steel is therefore recommended for industrial installations involving chemical treatments, coastal areas, and all environments subject to high levels of salinity
Another important parameter for choosing a stainless steel grade is the carbon content. Low carbon stainless steels, suffixed with L (such as 304L or 316L) have less than 0.03% carbon
During the welding process, the carbon forms chromium carbides, which results in less protected areas in the workpiece (less protective chromium oxide film). A low carbon content is therefore an important choice for fittings and parts that are to be welded to industrial pipelines, regardless of salt and chloride resistance.