Pitting Corrosion

Description

Pitting corrosion is highly localized corrosion occurring on a metal surface. Pitting is commonly observed on surfaces with little or no general corrosion. Pitting typically occurs as a process of local anodic dissolution where metal loss is exacerbated by the presence of a small anode and a large cathode.

Prevention or Remedial Action

• increase velocity of media and/or remove deposits of solids from exposed metal surface.
• selection of alloy with higher alloy content (e.g. in stainless alloys higher Cr, Mo and N content according to the following formula):

  PI = Cr + 3.3(Mo) + X(N) where PI is pitting index and
  x = 0 for ferritic stainless steels
  x = 16 for duplex (austenitic/ferritic) stainless steels
  x = 30 for austenitic stainless steels
  

  For more severe pitting service in some environments Ti - and Zr - alloys may also be appropriate.

• use of effective chemical inhibitor to enhance resistance to localized attack.
• Deaeration of aerated environments to reduce localized corrosion through elimination of oxygen concentration cell mechanism.

Standard Test Methods

• ASTM G-46 - practice for examination and evaluation of pitting corrosion.
• ASTM G-48 - test methods for pitting and crevice corrosion resistance of stainless steels and related alloys by the use of ferric chloride solution.
• ASTM G - standard reference test method for making poteniostatic and potentiodynamic anodic polarization measurements.
• ASTM G-61 - test method for conducting cyclic potentiodynamic polarization measurements for localized corrosion susceptibility of iron, nickel or cobalt based alloys.
• NACE TM0274 - dynamic corrosion testing of metals in high temperature water.
• ASTM G-85 - modified salt spray (fog) testing.

Evaluation of Pitting Corrosion

The extent of pitting corrosion can vary greatly depending on the exposure conditions and surface condition of the material. Commonly used methods to determine the pitting corrosion resistance are

• Simple exposure of corrosion coupons to standardized environments of know severity (ASTM G48).
• Evaluaiton of coupons and metal surfaces with standardized techniques to categorize the nature of the pitting attack (ASTM G46).
• Use of electrochemical techniques (ASTM G61) to characterize the current-potential polarization behavior of the material in specific service environments to identify materials susceptible to pitting attack.

Most important in studies of pitting corrosion are the use of visual examination and/or metallographic techniques to characterize the physical nature of the localized corrosive attack. Electrochemical measurements should always be supplemented by such techniques to obtain the most accurate indications. Typically, the most relevant information is the maximum attack depth and/or rate since these parameters will most directly indicate the serviceability of actual components in service.