How damage occurs to a metallic material during Pitting corrosion

Corrosion of metallic materials by pitting comprises of a major failure. Damage initiates through perforation and engender stress corrosion cracks. Pitting is a common damaging mechanism to several metals. It is normally associated with specific anions in solutions, particularly the chloride ions. Early identification of pits needs techniques that measure nominal events.

Stainless steels are used in numerous applications for their corrosion resistance. Chromium in steel interacts with oxygen in the air to develop a thin and invisible layer of chrome based oxide called the passive layer. The size of chromium atoms and their oxides are similar that are neatly packed on the metal surface, developing a stable layer. If a metal is cut or scratched and the passive film is disrupted, more oxides are formed to recover the open surface, securing it from oxidative corrosion.

Oxygen is required to repair the passive layer, so the stainless steels have nominal corrosion resistance in low-oxygen and poor circulation conditions. In the seawater, chlorides in the salt will damage the passive layer rapidly.

Few metals show preferential sites of pit nucleation with metallurgical microstructural and microcompositional characteristics stating the sensitivity. Although, it is not the phenomenal origin of pitting, as the site specification is featured for a few metals only. Nucleated pits that do not propagate should repassivate. Although there are numerous states of propagation with a finite survival probability, various factors add to this survival probability.

The evolution of corrosion pits on stainless steels occurred in chloride solution occurs in three different stages- nucleation, metastable growth and stable growth. A microcrack developed by pitting corrosion, develops the initial origin for fatigue fracture. However fatigue failure is not specifically resulted at the deepest interior pitting deep. Considering the different shape of the interior pitting deeps, some are hard to begin or to continue the crack propagation and has a major role as crack locker. If the fracture is created by the pitting deep, continuous plastic deformation can be discovered around the pitting deep. The dimples are normally equalized by the overloaded tension and elongated by shear or tearing.

Cleavage failure occurs by the separation along crystallographic planes. The transgranular fracture is classified to be the brittle fracture in the fracture area. There are many characteristic that can be identified to be the cleavage..

At the grain boundaries the fracture surface cleavage surface changes due to differences between crystallographic orientations. Cleavages are not just related with transgranular fracture, infact also with brittle particles.

Few metals can fail in brittle way however do not cleave. These fractures are recognized as quasi-cleavage. They are identical to cleavage however their characteristic are often flat and smaller.

You should choose a metallic material that is resistant to pitting corrosion in the diverse conditions. The recommended pittingresistant alloys are Hastelloy bars and Inconel bars.