Stainless steel is an alloy steel with 12 wt. % of chromium or above and corrosion resistance. Since carbon steel has weak adhesion of oxides formed on the surface and many pores on the inside, oxygen can easily react with carbon steel. On the other hand, stainless steel has an oxide film with high adhesion and small defect known as passive film, which protects the base metal from oxygen. When surface of stainless steel is damaged by scratch, it reacts with surrounding oxygen to recreate the passive film by self-healing effect.
Such passive film made of iron and chromium oxides plays the most important role in determining anti-corrosiveness of stainless steel and it has extremely low thickness of about 2nm. Composition and structure of the film is still unknown. Songdo PAC conducts various anti-corrosion analyses on the passive film of stainless steel to examine corrosion mechanism of stainless steel under different corrosive environments and presents guidelines for the applications of each stainless steels type.
For example, stainless steel is a semi-permanent material but it can be occasionally attacked as pitting corrosion, intergranular corrosion and stress corrosion cracking if wrong type of stainless steel is applied under specific corrosive environment. Therefore, it is extremely important to select appropriate steel type with consideration on corrosive environment and economic feasibility. Typical corrosion of stainless steel is pitting corrosion that occurs from local destruction of passive film caused by chlorine ion. When metal cation increases inside pitting corrosion, it electrically attracts anion to increase the content of chlorine ion inside. This causes difficulty in formation of passive film on the inside, and pH is also lowered by hydrolysis to create hydrochloric acid (HCl) environment. As a consequence, pitting corrosion is quickly progressed and leads to penetration in severe cases. Therefore, Songdo PAC quantitatively evaluates pitting corrosion resistance of different stainless steel types using electro-chemical methods such as polarization test and suggests optimal steel type to each client.