• Uniform • Galvanic • Crevice • Pitting • Intergranular • SSC • LME • MIC • SCC • HB-HE-HIC-HMx • Fatigue • Erosion • Stray Current • Index

Different Types of Corrosion
- Recognition, Mechanisms & Prevention

Pitting Corrosion

Recognition of Pitting Corrosion

What is pitting corrosion? Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting corrosion is one of the most damaging forms of corrosion. Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. The following photo shows pitting corrosion of a SAF2304 duplex stainless steel after exposure to 3.5% NaCl solution.

What materials are susceptible to pitting corrosion? Pitting corrosion is usually found  on passive metals and alloys such aluminium alloys, stainless steels and stainless alloys when the ultra-thin passive film (oxide film) is chemically or mechanically damaged and does not immediately re-passivate. The resulting pits can become wide and shallow or narrow and deep which can rapidly perforate the wall thickness of a metal.

ASTM-G46 has a standard visual chart for rating of pitting corrosion.

The shape of pitting corrosion can only be identified through metallography where a pitted sample is cross-sectioned and the pit shape, the pit size, and the pit depth of penetration can be determined.

What causes pitting corrosion? For a defect-free "perfect" material, pitting corrosion is caused by the ENVIRONMENT (chemistry) that may contain aggressive chemical species such as chloride. Chloride is particularly damaging to the passive film (oxide) so pitting can initiate at oxide breaks.

The environment may also set up a differential aeration cell (a water droplet on the surface of a steel, for example) and pitting can initiate at the anodic site (centre of the water droplet).

For a homogeneous environment, pitting IS caused by the MATERIAL that may contain inclusions (MnS is the major culprit for the initiation of pitting in steels) or defects. In most cases, both the environment and the material contribute to pit initiation.

What are the factors influencing pitting corrosion? The ENVIRONMENT (chemistry) and the MATERIAL (metallurgy) factors determine whether an existing pit can be repassivated or not. Sufficient aeration (supply of oxygen to the reaction site) may enhance the formation of oxide at the pitting site and thus repassivate or heal the damaged passive film (oxide) - the pit is repassivated and no pitting occurs. An existing pit can also be repassivated if the material contains sufficient amount of alloying elements such as Cr, Mo, Ti, W, N, etc.. These elements, particularly Mo, can significantly enhance the enrichment of Cr in the oxide and thus heals or repassivates the pit. More details on the alloying effects can be found in the technical paper on "Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail".

How to evaluate the resistance of an alloy to pitting corrosion? A material's resistance to pitting corrosion is usually evaluated and ranked using the critical pitting temperature (CPT) in accordance  with the ASTM Standard
G48-03: Standard Test Methods for Pitting and Crevice Corrosion of Stainless Steels and Alloys by Use of FeCl₃. The critical pitting temperature is the minimum temperature (°C) to produce pitting corrosion and CPT is usually higher than the critical crevice temperature (CPT).

Prevention of Pitting Corrosion

How to prevent pitting corrosion? Pitting corrosion can be prevented through:

• Proper selection of materials with known resistance to the service environment

• Control pH, chloride concentration and temperature

• Cathodic protection and/or Anodic Protection

• Use higher alloys (ASTM G48) for increased resistance to pitting corrosion

EVS-Compass®: Extreme Value Statistics for Corrosion Modeling and Corrosion Life Prediction

Extreme value statistics (EVS) has been used since the 1950s for extrapolating corrosion damages (maximum pit depth, crevice depth, crack depth etc.) from small lab samples, field coupons, or partial coverage inspection blocks to larger area of structures and assets at present or future times. WebCorr's EVS-Compass is the only device and OS independent EVS software on the market for corrosion modeling and life prediction of corrodible structures. Designers, OEM engineers, consultants, operation personnel, maintenance and inspection engineers, and government regulators can quickly and accurately determine:

1. the time to first leak or perforation;

2. the number of leaks or perforation at any given time;

3. the time to Nth leak or perforation for any given number of N;

4. the area of perforation holes;

5. the depth of the largest pit at any given time;

6. the depth of the Nth largest pit at any given time;

7. the number of pits exceeding a given depth D at any given time;

8. the time required for N pits to exceed the depth of D;

9. the probability of failure (POF) at a given time and a given wall thickness;

10. the service life for a given wall thickness at a given POF threshold;

11. the maximum surface area for EVS extrapolation in partial coverage inspection

12. the recommended area for lab coupons or inspection blocks

13. the recommended number of lab coupons or inspection blocks;

14. the charts showing (a) pit depth vs service life; (b) pit depth vs area; (c) probability of failure vs service life; (d) probability of failure vs area; (e) probability of failure vs wall thickness.

CRA-Compass®: Your Guide to Corrosion Resistant Alloys
- Corrosion Prediction, Selection and Application Limits for Resistance to Pitting, Crevice Corrosion and SSC/SCC

Overview of CRA-Compass for Waters and Brines

This module deals with the application limits of 55 common corrosion resistant alloys used in water systems including natural seawater, chlorinated seawater, brines, produced water, formation water, brackish water, groundwater, fresh water, and potable water. Users can define their own alloys for CRA-Compass to evaluate the application limits for their resistance to pitting, crevice corrosion, and stress corrosion cracking (SCC) under the specified operating conditions. The performance of the CRAs in coastal/marine environment is also included in this module. More detailed information on CRA-Compass is available here.

CIPAL-Compass: Copper-Induced Pitting in Aluminium Alloys

This software predicts pitting depth, pitting rate and time to perforation of aluminum alloys in contact waters and process fluids that contain trace amount of copper ions.

Corrosion and Its Prevention (5-day module)
API 571 Damage Mechanisms Affecting Fixed Equipment in the Refining and Petrochemical Industries (5 days)

Corrosion Inspection, Testing and Monitoring: Techniques and Applications (5 days)
Corrosion, Metallurgy, Failure Analysis and Prevention (5 days)
Marine Corrosion, Causes and Prevention (2 days)
Materials Selection and Corrosion (5 days)
Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)

Corrosion in Fire Protection Systems (FPS): Detection, Mitigation and Prevention (1 day)

Microbiologically Influenced Corrosion (MIC): Recognition, Mitigation and Prevention (1 day)

Corrosion Control and Prevention in Seawater Desalination Plants (1 day)

If you require corrosion expert witness or corrosion consulting service on pitting corrosion, our NACE certified Corrosion Specialist is able to help. Contact us for a quote.

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