With simple everyday materials such as bleach and salt as the enemy, the perfect surface of stainless-steel can come under attack. Corrosion in the form of pitting may occur. Mostly undetected by the human eye, the microscopic indentations that pitting is, compromises the metal’s integrity.
It’s all in the chemistry. Stated as ppm – ‘parts per million’ (0,0001%) it is the chloride content that induces pitting along with the pH (acidity) and the prevailing temperature. Stainless-steel manufacturers have combatted brilliantly by grading their steel according to various endangering factors. Referred to as PRE (pitting resistance equivalent) the chemical composition of each grade of steel is calculated. For the tech-savvy reader, the formula is: “% of Cr + 3.3 of Mo + % of N” – for the rest of us that looks like a super secure pin number for online banking.
Pitting Corrosion Testing
The critical pitting temperature (CPT) is the measurement used in the industry to grade steel’s resistance to pitting. Three primary alloys – Chromium (Cr), Molybdenum (Mo) and Nitrogen (N) have proven to have a positive effect on pitting resistance. Even with all the tests and being able to select the right grade for the task, one cannot ask the impossible. Seawater-bearing pipe systems, for example, fight against the fact the natural seawater is more corrosive than its chloride content leads one to believe. Due to the expected biological activity, chlorination’s as a biocide is frequently used to prevent the growth of micro-organisms in pipes bearing sea water. Even then, continuous chlorination can itself cause pitting, which is why only intermittent chlorination is recommended.
In short, it’s about using the right grade stainless steel for the right job in the right place with the right temperature and the right treatment that takes the pits out of pitting. Mind the rules, as they say, and the rules will mind you.