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.

Coating and Lining Failure

DESCRIPTION

Coating failures are usually the result of problems associated with either coating application (i.e. surface preparation, primer, or topcoat), chemical or environmental durability or wear and abrasion. Such failures are typically related to loss of adhesion, cracking or wear.

PREVENTION OR REMEDIAL ACTION

1. surface preparation
   • surface must be properly cleaned to remove all dirt, corrosion and contamination.
   • surface must be roughened to provide an adequate anchor pattern for coating.
2. selection of primer
   • provide corrosion inhibition
   • provide additional barrier between metal environment.n\
3. selection of topcoat
   • need adequate coating thickness to cover all of the surface (free of defects or holidays). This may require multiple coats.
   • must have chemical or moisture resistance and temperature resistance.
   • in some applications, it must also have abrasion resistance.n\
4. applications involving cathodic protection
   • must have resistance to cathodic disbonding.

STANDARD TEST METHODS

• NACE TM0175 - visual standard for surfaces of new steel centrifugally blast cleaned with steel grit and shot.
• NACE TM0174 - laboratory methods for the evaluation of protective coatings used as lining materials in immersion service.
• NACE TM0183 - evaluation of internal plastic coatings for corrosion control of tubular goods in an aqueous flowing envrionment.
• NACE TM0375 - abrasion resistance testing of thin film basked coatings and linings using the falling sand method.
• ASTM G-8, G-80, G-95 - tests for cathodic disbonding of pipeline coatings.
• ASTM G-42 - test for pipeline coatings subjected to high or cyclic temperatures.
• ASTM B-117 - salt fog testing.
• ASTM G-85 - modified salt spray (fog) testing.
• conversion coatings: ASTM B-449 (Al-alloys); ASTM B-201 (Zn and Cd alloys); ASTM B-281 (Cu-alloys).
• ASTM G-7 - practice for atmospheric environmental exposure of nonmetallic materials.
• water resistance of coatings in 100% humidity.

Evaluation of Coatings and Lining

In general, coatings and linings work to prevent corrosion by imposing a dielectric barrier between the corrosive environment and the material surface. Therefore, an evaluation of coating performance is one of determining the chemical compatibility and thermal stability of the coating in the service environment and the adhesion of the coating on the substrate. Many coatings tests use post exposure visual examination and/or mechanical performance to rate its resistance to the corrosive environment. Newer evaluation techniques such as electrochemical impedance spectroscopy (EIS) can be used in-situ to assess changes in coating parameters such as film capacitance and pore resistance well in advance of the point where damage can be determined visually. Another important aspect of coating evaluation is the use of service life testing methodologies. In their simplest form, these take the form of evaluations of coating performance after multiple exosure period instead of the commonly used single point testing. This allows for the tends in coating performance versus time to be obtained. Additionally, the use of theoretical and empirical models can be used to accelerate the test by increasing temperature or by changing other parameters (e.g. applied potential). Short term failures obtained at multiple conditions can be used to predict longer terms limits of serviceability in various service environments.

The grand dimensions of this subject immediately limit attempts to make simplistic application of only a single method of testing for all cases. Factors such as,

1. material type,
2. process history,
3. product form,
4. active cracking mechanism(s),
5. loading configuration and geometry, and
6. service environment conditions,

to name a few, can have major consequences in determining the type of specimen and test condition to be utilized. The prudent approach to selection of testing methods is usually to assess these considerations along with a survey of previous experiences provided from prior investigations for similar applications or from those found in the published literature.

It can be said that there is no single perfect testing technique for the evaluation of EAC. However, the evaluation of materials for EAC typically involve the use of the specimen and technique that takes into account as many necessary factors as possible for the particular material and environment under consideration. In some cases, this may mean the use of

1. More than one type of test specimen
2. Various alternative configurations of the same specimen
3. Alternative test techniques with the same specimen (e.g. crevices applied potential, constant load, and slow strain rate)

Most of all, it is important to provide a link between the results of laboratory evaluations and real-world service applications. This is often developed through studies involving:

1. Integrated laboratory and field or in-plant tests
2. Correlation of laboratory data with service experience
3. Reviews of published literature on the service performance of similar materials

In any case, the evaluation of EAC susceptibility using laboratory testing methods can provide data resulting in an increased confidence level. This often allows for an optimization of the materials of construction. By this it is meant that the allowance for unpredictable service performance can be reduced resulting in a lower material cost, reduced downtime, and a reduction in the number of costly failures.