Mechanism of Pitting corrosion and how it affects a service material?

Alloying elements added during the production of the steel increase corrosion resistance, hardness or strength. The metals widely used as alloying elements in stainless steel include chromium, nickel and molybdenum. Stainless steels are separated as martensitic stainless steels,  ferritic stainless steels, austenitic stainless steels, duplex stainless steels and precipitation hardening steels.

Stainless steel prevents corrosion, maintains its strength at high temperatures and is easily retained, it is widely used in fields automotive, propulsion shaft for high speed craft and food processing products, medical and health equipment. The replacement of a high speed craft is lighter than the displacement of a traditional ship. The displacement factor is essential to receive quick and suitable sea transportations. High speed craft permits for the use of non-traditional ship manufacturing materials provided a safety standard similar to a conventional ship is received.

Stainless steels are used in numerous applications for offering corrosion resistance. However steels have excellent general resistance, they are not sensitive to pitting corrosion. This localized dissolution of an oxide-covered metal in specific severe conditions is one of the most common and catastrophic causes of failure of metallic structure.

Pitting corrosion is a localized accelerated dissolution of metal that occurs due to a breakdown of the security passive film on the metal surface. The mechanism of pitting corrosion is similar to crevice corrosion, dissolution of the passivating layer and regular acidification of the electrolyte due to inadequate aeration.

In the pits, a highly severe corrosive environment occurs that may tolerate nominal similarity to the bulk corrosive condition. For instance, in the pitting of stainless steels in water containing chloride, a micro- environment necessarily showing hydrochloric acid may occur in the pits. The pH in the pits reduced considerably, in combination with an increase in chloride ion concentration due to electrochemical pitting mechanisms in these systems. Pitting is usually discovered in conditions where general corrosion resistance is conferred by passive surface layers.

Localized pitting corrosion is discovered where these passive layers have damaged. Pitting corrosion caused by microbial activity like sulfate reducing bacteria required special consideration. Generally pitting corrosion in stainless steels occurs in neutral to acid solutions with chloride or ions containing chlorine.

The identification and valuable evaluation of pitting corrosion often shows an extreme challenge. Evaluating pitting corrosion can be further complicated by a distinction between the beginning and distribution phases of pitting processes. Extremely sensitive electrochemical noise method may create early alert of extreme damage by featured pit. Pitting can occur without any anticipitation and with nominal overall metal loss.

The pits may be hidden under surface deposits and other corrosion products. A small narrow pit with nominal overall metal loss can cause to the failure of an whole engineering system. Pitting corrosion is almost a common denominator of all types of a localized corrosion, may be with different shapes. Corrosion of metals and alloys by pitting comprises one of the extreme failure mechanisms. Failure caused by pits by perforation and engender stress corrosion cracking that affects the life cycle of application material.

We produce Pitting corrosion resistant alloys Inconel 718wire and Hastelloy grades for use in the severe application environments.