Did you know that stainless steel obtains its corrosion resistance from a tiny layer of chromium oxide molecules of only a few nanometers thick? This tiny layer is crucial, but it can be easily damaged by mechanical or chemical attacks.
The interesting part? This "passive" layer can repair itself by using oxygen from the air! Read on for a new article in our chemical ‘Did you know’ series, written by expert Ton van Oeffel.
Seawater, with its high chloride content (1.9% or 19,000 ppm), is the biggest enemy of stainless steel. Chlorides can damage the passive layer, and without enough air or oxygen, they can cause localized corrosion, known as "pitting."
This is especially problematic in high-temperature environments. To protect stainless steel, tanks and lines should be flushed with fresh water after contact with seawater, and seawater should never be stored in cargo tanks.
Pitting corrosion creates small holes or "pits" that grow inward. These pinholes must be repaired by grinding or welding, followed by passivation using nitric acid.
In this case, the passive layer can’t repair itself because of chemical damage from chlorides and iron.
Certain chemicals, like sulfuric acid and potassium hydroxide, can cause discoloration in stainless steel, especially at high temperatures when they mix with moisture in the ullage space. These temperatures lead to oxidation, resulting in colors ranging from blue to black.
This discoloration can only be removed using a pickling liquid (which contains hydrofluoric acid) and subsequent passivation, as pickling liquid destroys the protective chromium oxide layer.
Passivation meters, available at main ports, measure the "passivity" of stainless steel on a scale from 0 to 100. These meters check the electrical potential of free iron on the steel’s surface.
A reading of 100 means the steel is fully passive (no free iron), and a reading of 0 means there’s no chromium, just free iron. If the result is below 65, passivation is recommended.
