What is Stainless Steel?
Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by making it easier for more iron oxide to form. Since iron oxide has lower density than steel, the film expands and tends to flake and fall away. In comparison, stainless steels contain sufficient chromium to undergo passivation, forming an inert film of chromium oxide on the surface. This layer prevents further corrosion by blocking oxygen diffusion to the steel surface and stops corrosion from spreading into the bulk of the metal. Passivation occurs only if the proportion of chromium is high enough and oxygen is present in it.
What is The Difference between Type 304 and Type 316 Stainless Steel?
316 stainless steel has molybdenum added to its alloy content during production. Molybdenum provides an even higher degree of corrosion resistance than 304 stainless steel. This is an advantage for objects that require resistance to extreme environmental conditions such as salt water, de-icing salts, brine solutions, or other chemical forms of chemical exposure. In laboratory and pharmaceutical settings, 316 stainless steel excels in resistance to acids, bromides, and iodides at high temperature.
It is difficult to tell the difference between 304 and 316 stainless steel by simply looking at the two. They both offer the same polished, chromed, grained, colored, or blasted appearance but 316 stainless steel costs more due to its enhanced chemical and production properties.
Because it is Stainless Steel, Doesn’t That Mean it is also Non-Magnetic?
Not necessarily so. In fact, Type 304ss often has a detectable magnetic draw. And it is possible for Type 316ss to have a very slight magnetic draw.
The reason for this is fairly technical, but it has to do with the amount of chromium in the alloy and the process in which the part was made. Type 304ss has less chromium than Type 316ss, and it is the chromium that affects the microstructure of the surface of the steel. Because Type 316ss has more nickel in it and the presence of this additional nickel serves to enhance the affect of the chromium in the steel, Type 316ss will have little to no detectable magnetic draw.
What typically can create a magnetic draw is cold working or welding of the metal. Cold working in the form of bending, forming or machining can cause a change in the microstructure on the surface of the metal and reduce the effect of the chromium layer that exists on the surface. And again, because Type 304ss has less nickel than Type 316ss, it is more susceptible to developing a degree of magnetism.
How Can “Stainless Steel” Rust?
Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is “stainless” not “stain-impossible.”
Type 304ss is more susceptible to forming rust than Type 316ss. This is due to the presence of higher levels of both nickel and molybdenum. Stamping, forming and welding can disrupt the surface microstructure of stainless steel and this is more so the case for Type 304ss.
Salt water, salt in the aire and chemicals can attack Type 304ss causing pitting to occur. The additional nickel and molybdenum in Type 316ss make it less likely for corrosion to occur when exposed to these same elements.
Galvanic corrosion can occur when stainless comes into contact with dissimilar metals when an electrolyte, water or other solution is present.