The general perception towards stainless steel is that it is a super alloy that never rusts. Well, this assumption is anything but true. The reality is that stainless steel does rust. The word “stain-less” does not imply free from stain or “stain-impossible”. It simply means that the alloy stains less. This is exactly the case with stainless steel. When compared to other metals and alloys, stainless steel the most resistant to corrosion. Ordinary carbon or alloy steels are not in the same league as stainless steel when it comes to corrosion resistance. So Why does stainless steel rust at all?
Aggressive Conditions Cause Rusting
Although stainless steel does corrode, it is important to note that the alloy will not rust under ordinary atmospheric or water based environments. In other words, the corrosion of stainless steel takes place in specific aggressive conditions which are conducive for corrosion. Therefore, the use of basic forms of stainless steel is not recommended under such hostile conditions. To make sure that the stainless steel application or structure remains unharmed, unscathed and intact in these disadvantageous conditions, the use of highly alloyed stainless steel is required.
Corrosion Resistance is Quantitative
In other words, stainless steels are more or less corrosion resistant and they work very well in almost every environment. The characteristic of corrosion resistance is both qualitative as well as quantitative. Each type of stainless steel has a certain degree of corrosion resistance. Once the limit of resistance is crossed, the formation of rust is bound to occur on the stainless steel. The limit of the corrosion resistance of a stainless steel is determined by what goes into making the stainless steel in the first place. This means that the limit of resistance is dependent on the constituent elements. As a result of this, different grades of stainless steel respond differently when left in an environment that aids corrosion. The grade of the stainless steel is not the only way to judge its corrosion resistance. The detailing and workmanship of each and every stainless steel goes a long way in deciding how vulnerable the steel will be to staining and corrosion.
Understanding the corrosion mechanisms of stainless steels helps us understand why stainless steels rust. Corrosion mechanisms in stainless steel can be broadly divided into 6 categories. They are:
a) Pitting Corrosion
This is a localized version of corrosion that takes place in stainless steel when it is exposed to environments that contain chlorides.
b) Crevice Corrosion
This is another form of localized corrosion which is triggered when the oxygen levels are extremely low in a crevice. This particular type of corrosion is not much of a problem unless the stainless steel is in a stagnant solution where chlorides can accumulate.
c) Bimetallic corrosion
When dissimilar metals in a common electrolyte come into contact with another, then bimetallic corrosion, also known as galvanic corrosion can take place. The most common scenario is stainless steels corroding in rain.
d) Stress Corrosion Cracking
Tensile stresses coupled with specific environmental conditions lead to stress corrosion cracking.
e) General Corrosion
When the stainless steel has a pH value of less than 1, then general corrosion takes place.
f) Intergranular Attack
When you heat an austenitic stainless steel at around 450 to 850 degrees Celsius, the carbon in the steel converts to grain boundaries which lead to corrosion.
Cleaning up stainless steel rust
Cougartron weld cleaners are very effective at removing rust and corrosion stains from stainless steel and fully repassivating the surface to ensure that corrosion doesn’t recur.
See more here about stainless steel rust removing at Cougartron ProPlus page.