Pitting Corrosion

Pitting is a localized form of corrosive attack.  Pitting corrosion is typified by the formation of holes or pits on the metal surface.  Pitting can cause failure due to perforation while the total corrosion, as measured by weight loss, might be rather minimal.  The rate of penetration may be 10 to 100 times that by general corrosion.

Pits may be rather small and difficult to detect.  In some cases pits may be masked due to general corrosion.  Pitting may take some time to initiate and develop to an easily viewable size.

Pitting occurs more readily in a stagnant environment.  The aggressiveness of the corrodent will affect the rate of pitting.  Some methods for reducing the effects of pitting corrosion are listed below:

  Reduce the aggressiveness of the environment   Use more pitting resistant materials   Improve the design of the system

Pitting corrosionFrom Wikipedia, the free encyclopedia

Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the

creation of small holes in the metal. The driving power for pitting corrosion is the

depassivation of a small area, which becomes anodic while an unknown but potentially vast

area becomes cathodic, leading to very localized galvanic corrosion. The corrosion

penetrates the mass of the metal, with limited diffusion of ions. The mechanism of pitting

corrosion is probably the same as crevice corrosion.

Contents

 [hide]

1 Mechanism

2 Susceptible alloys

3 Environment

4 Examples

5 See also

6 References

7 External links

[edit]Mechanism

Diagram showing a mechanism of localized corrosion developing on metal in a solution containing oxygen

It is supposed by some that gravitation causes downward-oriented concentration gradient of

the dissolved ions in the hole caused by the corrosion, as the concentrated solution is

denser. This however is unlikely. The more conventional explanation is that the acidity

inside the pit is maintained by the spatial separation of the cathodic and anodic half-

reactions, which creates a potential gradient and electromigration of aggressive anions into

the pit[1].

This kind of corrosion is extremely insidious, as it causes little loss of material with small

effect on its surface, while it damages the deep structures of the metal. The pits on the

surface are often obscured by corrosion products.

Pitting can be initiated by a small surface defect, being a scratch or a local change in

composition, or a damage to protective coating. Polished surfaces display higher resistance

to pitting.

[edit]Susceptible alloys

Alloys most susceptible to pitting corrosion are usually the ones where corrosion resistance

is caused by a passivation layer: stainless steels, nickel alloys, aluminum alloys. Metals that

are susceptible to uniform corrosion in turn do not tend to suffer from pitting. Thus, a regular

carbon steel will corrode uniformly in sea water, while stainless steel will pit. Additions of

about 2% of molybdenum increases pitting resistance of stainless steels.

[edit]Environment

The presence of chlorides, e.g. in sea water, significantly aggravates the conditions for

formation and growth of the pits through an autocatalytic process. The pits becomes loaded

with positive metal ions through anodic dissociation. The Cl− ions become concentrated in

the pits for charge neutrality and encourage the reaction of positive metal ions with water to

form a hydroxide corrosion product and H+ ions. Now, the pits are weakly acidic, which

accelerates the process.

Besides chlorides, other anions implicated in pitting

include thiosulfates (S2O32−), fluorides and iodides. Stagnant water conditions favour pitting.

Thiosulfates are particularly aggressive species and are formed by partial oxidation of

pyrite, or partial reduction of sulfate. Thiosulfates are a concern for corrosion in many

industries: sulfide ores processing, oil wells and pipelines transporting soured oils, Kraft

paper production plants, photographic industry, methionine and lysine factories.

Corrosion inhibitors, when present in sufficient amount, will provide protection against

pitting. However, too low level of them can aggravate pitting by forming local anodes.

[edit]Examples

A corrosion pit on the outside wall of apipeline at a coating defect before and afterabrasive blasting.

The collapsed Silver Bridge, as seen from the Ohio side

A single pit in a critical point can cause a great deal of damage. One example is the

explosion in Guadalajara, Mexico on April 22, 1992, whengasoline fumes accumulated

in sewers destroyed kilometers of streets. The vapors originated from a leak of gasoline

through a single hole formed by corrosion between a steel gasoline pipe and a zinc-plated

water pipe.[2] Firearms can also suffer from pitting, most notably in the bore of the barrel

when corrosive ammunition is used and the barrel is not cleaned soon afterward.

Deformities in the bore caused by pitting can greatly reduce the firearms accuracy. To

prevent pitting in firearm bores, most modern firearms have a bore lined with chromium.

Pitting corrosion can also help initiate stress corrosion cracking, as happened when a

single eyebar on the Silver Bridge, West Virginia and killed 46 people on the bridge in

December, 1967.[clarification needed]

[edit]

Methods of protection from corrosion

Surface treatments

Galvanized surface

Applied coatings

Main article: Galvanization

Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by

providing a barrier of corrosion-resistant material between the damaging environment and the (often cheaper,

tougher, and/or easier-to-process) structural material. Aside from cosmetic and manufacturing issues, there are

tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature. Platings usually fail only

in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a

galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. For

this reason, it is often wise to plate with a more active metal such as zinc or cadmium. Painting either by roller

or brush is more desirable for tight spaces; spray would be better for larger coating areas such as steel decks

and waterfront applications. Flexible polyurethane coatings, like Durabak-M26 for example, can provide an anti-

corrosive seal with a highly durable slip resistant membrane. Painted coatings are relatively easy to apply and

have fast drying times although temperature and humidity may cause dry times to vary.

Reactive coatings

If the environment is controlled (especially in recirculating systems), corrosion inhibitors can often be added to

it. These form an electrically insulating and/or chemically impermeable coating on exposed metal surfaces, to

suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or

defects in the coating, since extra inhibitors can be made available wherever metal becomes exposed.

Chemicals that inhibit corrosion include some of the salts in hard water (Roman water systems are famous for

theirmineral deposits), chromates, phosphates, polyaniline, other conducting polymers and a wide range of

specially-designed chemicals that resemble surfactants (i.e. long-chain organic molecules with ionic end

groups).

This figure-8 descender is annodized with a yellow finish. Climbing equipment is available in a wide range of anodized

colors.

Anodization

Main article: Anodising

Aluminium alloys often undergo a surface treatment. Electrochemical conditions in the bath are carefully

adjusted so that uniform pores severalnanometers wide appear in the metal's oxide film. These pores allow the

oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are

allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation

processes take over to protect the damaged area. Anodizing is very resilient to weathering and corrosion, so it

is commonly used for building facades and other areas that the surface will come into regular contact with the

elements. Whilst being resilient, it must be cleaned frequently. If left without cleaning Panel Edge Staining will

naturally occur.

Controlled permeability formwork

Main article: Controlled permeability formwork

Controlled permeability formwork (CPF) is a method of preventing the corrosion of reinforcement by naturally

enhancing the durability of thecover during concrete placement, . CPF has been used in environments to

combat the effects of carbonation, chlorides, frost and abrasion.

Cathodic protection

Main article: Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the

cathode of an electrochemical cell. Cathodic protection systems are most commonly used to protect steel,

water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms.

Sacrificial anode protection

Sacrificial anode in the hull of a ship.

For effective CP, the potential of the steel surface is polarized (pushed) more negative until the metal surface

has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For

galvanic CP systems, the anode material corrodes under the influence of the steel, and eventually it must be

replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference

in electrochemical potential between the anode and the cathode.

Impressed current cathodic protection

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete

protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power

source (such as a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes of

various specialized materials. These include high siliconcast iron, graphite, mixed metal oxide or platinum

coated titanium or niobium coated rod and wires.

Anodic protection

Main article: Anodic protection

Anodic protection impresses anodic current on the structure to be protected (opposite to the cathodic

protection). It is appropriate for metals that exhibit passivity (e.g., stainless steel) and suitably small passive

current over a wide range of potentials. It is used in aggressive environments, e.g., solutions of sulfuric acid.

Economic impact

The collapsed Silver Bridge, as seen from the Ohio side

The US Federal Highway Administration released a study, entitled Corrosion Costs and Preventive Strategies

in the United States, in 2002 on the direct costs associated with metallic corrosion in nearly every U.S. industry

sector. The study showed that for 1998 the total annual estimated direct cost of corrosion in the U.S. was

approximately $276 billion (approximately 3.2% of the US gross domestic product).[2]

Rust is one of the most common causes of bridge accidents. As rust has a much higher volume than the

originating mass of iron, its build-up can also cause failure by forcing apart adjacent parts. It was the cause of

the collapse of the Mianus river bridge in 1983, when the bearings rusted internally and pushed one corner of

the road slab off its support. Three drivers on the roadway at the time died as the slab fell into the river below.

The following NTSB investigation showed that a drain in the road had been blocked for road re-surfacing, and

had not been unblocked so that runoff water penetrated the support hangers. It was also difficult for

maintenance engineers to see the bearings from the inspection walkway. Rust was also an important factor in

the Silver Bridge disaster of 1967 in West Virginia, when a steel suspension bridgecollapsed in less than a

minute, killing 46 drivers and passengers on the bridge at the time.

Similarly corrosion of concrete-covered steel and iron can cause the concrete to spall, creating severe

structural problems. It is one of the most common failure modes of reinforced concrete bridges. Measuring

instruments based on the half-cell potential are able to detect the potential corrosion spots before total failure of

the concrete structure is reached.

Root Cause Analysis

Proper root cause analysis identifies the basic source or origin of your problem. Root cause analysis is a step by step approach that leads to the identification of a fault's first or root cause. Every system, equipment, or component failure happens for a reason.  There are specific succession of events that lead to a failure. A root cause analysis investigation follows the cause and effect path from the final failure back to the root cause.

Root Cause Analysis Procedure

The procedure investigates the failure using facts left behind from the initial flaw. By evaluating the remaining evidence after the fault, and information from people associated with the incident, the analyst can identify both the contributing and non-contributing causes that caused the event. 

Root cause analysis provides a methodology for investigating, categorizing, and eliminating, root causes of incidents with safety, quality, reliability, and manufacturing process consequences.

AMC collects the data, analyses the data,

Root Cause Failure Analysis

Failure of a component indicates it has become completely or partially unusable or has deteriorated to the point that it is undependable or unsafe for normal sustained service.

Failure analysis   is an engineering approach to determining how and why equipment or a component has failed.  Some general causes for failure are structural loading, wear, corrosion, and latent defects.  The goal of a failure

develops appropriate corrective action, presents the data clearly and generates practical recommendations.  Root cause analysis is a tool to better explain what happened, to determine how it happened, and to understand why it happened. 

The root cause analysis methodology provides clients specific, concrete recommendations for preventing incident recurrences.  AMC identifies the processes and procedures that need changing to improve clients businesses.

Understanding the existing data of the incident, the root cause analysis method allows safety, quality, and risk and reliability managers an opportunity to implement more reliable and more cost effective policies that result in significant, enduring opportunities for improvement.  These procedural improvements increase a business' capability to recover from and prevent disasters with both financial and safety consequences.

analysis is to understand the root cause of the failure so as to prevent similar failures in the future. 

In addition to verifying the failure mode it is important to determine the factors that explain the how and why of the failure event.  Identifying the root cause of the failure event allows us to explain the how and why of failure.

AMC specializes in industrial product failure, corrosion, expert witness testimony, industrial accident investigation, materials and metallurgical failure analysis, welding,manufacturing,  forensic engineering, product liability, and explosion investigation services.  We have extensive experience in applying root cause failure analysis to solving engineering problems.

Preventing Reoccurrence of the Failure

It is not always necessary to prevent the first, or root cause, from happening. It is merely necessary to break the chain of events at any point and the final failure will not occur.  Frequently the root cause analysis identifies an initial design problem. Then a redesign is commonly enacted. Where the root cause analysis leads back to a failure of procedures it is necessary to either address the procedural weakness or to develop an approach to prevent the damage caused by the procedural failure.

Our clients understand why root causes are important, have identified and defined inherent problems, and enacted practical recommendations.  AMC has extensive engineering and quality assurance experience to provide clients with proven successful techniques to identify the root cause of their problems and appropriate solutions to these problems.

Heat Exchangers

Heat exchangers are commonly used to transfer heat from steam, water, or gases, to gases, or liquids.  Some of the criteria for selecting materials used for heat exchangers are corrosion resistance, strength, heat conduction, and cost.  Corrosion resistance is frequently a difficult criterion to meet.  Damage to heat exchangers is frequently difficult to avoid.

The tubes in a heat exchanger transfer heat from the fluid on the inside of the tube to fluid on the shell side (or vice versa).  Some heat exchanger designs use fins to provide greater thermal conductivity.  To meet corrosion requirements, tubing must be resistant to general corrosion, pitting, stress-corrosion cracking (SCC), selective leaching or dealloying, and oxygen cell attack in service. 

Failures of Heat Exchangers

Some common causes of failures in heat exchangers are listed below:

  Pipe and tubing imperfections   Welding   Fabrication   Improper design   Improper materials   Improper operating conditions   Pitting   Stress-corrosion cracking (SCC)   Corrosion fatigue   General corrosion   Crevice corrosion   Design errors   Selective leaching, or dealloying   Erosion corrosion

 

           

Failure analysis can identify the root cause or causes that have contributed to your heat exchanger failure.

Stainless Steels

Stainless Steels are iron-base alloys containing Chromium.  Stainless steels usually contain less than 30% Cr and more than 50% Fe. They attain their stainless characteristics because of the formation of an invisible and adherent chromium-rich oxide surface film. This oxide establishes on the surface and heals itself in the presence of oxygen.  Some other alloying elements added to enhance specific characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, and nitrogen.  Carbon is usually present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades.  Corrosion resistance and mechanical properties are commonly the principal factors in selecting a grade of stainless steel for a given application.

 Stainless steels are commonly divided into five groups:

  Martensitic stainless steels   Ferritic stainless steels   Austenitic stainless steels   Duplex (ferritic-austenitic) stainless steels   Precipitation-hardening stainless steels.

Martensitic stainless steels are essentially alloys of chromium and carbon that possess a martensitic crystal structure in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are usually less resistant to corrosion than some other grades of stainless steel.  Chromium content usually does not exceed 18%, while carbon content may exceed 1.0 %.  The chromium and carbon contents are adjusted to ensure a martensitic structure after hardening. Excess carbides may be present to enhance wear resistance or as in the case of knife blades, to maintain cutting edges.

Ferritic stainless steels are chromium containing alloys with Ferritic, body centered cubic (bcc) crystal structures. Chromium content is typically less than 30%.  The ferritic stainless steels are ferromagnetic.  They may have good ductility and formability, but high-temperature mechanical properties are relatively inferior to the austenitic stainless steels.  Toughness is limited at low temperatures and in heavy sections. 

Austenitic stainless steels have a austenitic, face centered cubic (fcc) crystal structure. Austenite is formed through the generous use of austenitizing elements such as nickel, manganese, and nitrogen.  Austenitic stainless steels are effectively nonmagnetic in the annealed condition and can be hardened only by cold working.  Some ferromagnetism may be noticed due to cold working or welding.  They typically have reasonable cryogenic and high temperature strength properties. Chromium content typically is in the range of 16 to 26%; nickel content is commonly less than 35%.

Duplex stainless steels are a mixture of bcc ferrite and fcc austenite crystal structures. The percentage each phase is a dependent on the composition and heat treatment. Most Duplex stainless steels are intended to contain around equal amounts of ferrite and austenite phases in the annealed condition. The primary alloying elements are chromium and nickel.  Duplex stainless steels generally have similar corrosion resistance to austenitic alloys except they typically have better stress corrosion cracking resistance.  Duplex stainless steels also generally have greater tensile and yield strengths, but poorer toughness than austenitic stainless steels.

Precipitation hardening stainless steels are chromium-nickel alloys. Precipitation-hardening stainless steels may be either austenitic or martensitic in the annealed condition.  In most cases, precipitation hardening stainless steels attain high strength by precipitation hardening of the martensitic structure.

Selecting a Stainless Steel

There are a large number of stainless steels produced.  Corrosion resistance, physical properties, and mechanical properties are generally among the properties considered when selecting stainless steel for an application.  A more detailed list of selection criteria is listed below:

  Corrosion resistance   Resistance to oxidation and

sulfidation   Toughness   Cryogenic strength   Resistance to abrasion and

erosion   Resistance to galling and seizing   Surface finish   Magnetic properties

  Ambient strength   Ductility   Elevated temperature strength   Suitability for intended cleaning

procedures   Stability of properties in service   Thermal conductivity   Electrical resistivity   Suitability for intended fabrication 

techniques

  Retention of cutting edge

Corrosion resistance is commonly the most significant characteristic of a stainless steel, but can also be the most difficult to assess for a specific application. General corrosion resistance is comparatively easy to determine, but real environments are usually more complex.  An evaluation of other pertinent variables such as fluid velocity, stagnation, turbulence, galvanic couples, welds, crevices, deposits, impurities, variation in temperature, and variation from planned operating chemistry among others issues need to be factored in to selecting the proper stainless steel for a specific environment.

AMC can provide engineering services to determine how to optimize the selection of stainless steel for your application.  Our engineering analysis can reduce overall costs, minimize service problems, and optimize fabrication of your structure.

Corrosion Failures 

Corrosion is chemically induced damage to a material that results in deterioration of the material and its properties.  This may result in failure of the component.  Several factors should be considered during a failure analysis to determine the affect corrosion played in a failure.  Examples are listed below:

 Type of corrosion   Corrosion rate  The extent of the corrosion  Interaction between corrosion and

other failure mechanisms

Corrosion is is a normal, natural process.  Corrosion can seldom be totally prevented, but it can be minimized or controlled by proper choice of material, design, coatings, and occasionally by changing the environment.  Various types of metallic and nonmetallic coatings are regularly used to protect metal parts from corrosion.

Stress corrosion cracking necessitates a tensile stress, which may be caused by residual stresses, and a specific environment to cause progressive fracture of a metal.  Aluminum and stainless steel are well known for stress corrosion cracking problems. However, all metals are susceptible to stress corrosion cracking in the right environment.  

Laboratory corrosion testing is frequently used in analysis but is difficult to correlate with actual service conditions.  Variations in service conditions are sometimes difficult to duplicate in laboratory testing

Corrosion Failures Analysis

Identification of the metal or metals, environment the metal was subjected to, foreign matter and/or surface layer of the metal is beneficial in failure determination.  Examples of some common types of corrosion are listed below:

 Uniform corrosion    Pitting corrosion  Intergranular corrosion  Crevice corrosion  Galvanic corrosion  Stress corrosion cracking

Not all corrosion failures need a comprehensive failure analysis.  At times a preliminary examination will provide enough information to show a simple analysis is adequate.  

 

Stainless Steels

Stainless Steels are iron-base alloys containing Chromium.  Stainless steels usually contain less than 30% Cr and more than 50% Fe. They attain their stainless characteristics because of the formation of an invisible and adherent chromium-rich oxide surface film. This oxide establishes on the surface and heals itself in the presence of oxygen.  Some other alloying elements added to enhance specific characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, and nitrogen.  Carbon is usually present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades.  Corrosion resistance and mechanical properties are commonly the principal factors in selecting a grade of stainless steel for a given application.

 Stainless steels are commonly divided into five groups:

  Martensitic stainless steels   Ferritic stainless steels   Austenitic stainless steels   Duplex (ferritic-austenitic) stainless steels   Precipitation-hardening stainless steels.

Martensitic stainless steels are essentially alloys of chromium and carbon that possess a martensitic crystal structure in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are usually less resistant to corrosion than some other grades of stainless steel.  Chromium content usually does not exceed 18%, while carbon content may exceed 1.0 %.  The chromium and carbon contents are adjusted to ensure a martensitic structure after hardening. Excess carbides may be present to enhance wear resistance or as in the case of knife blades, to maintain cutting edges.

Ferritic stainless steels are chromium containing alloys with Ferritic, body centered cubic (bcc) crystal structures. Chromium content is typically less than 30%.  The ferritic stainless steels are ferromagnetic.  They may have good ductility and formability, but high-temperature mechanical properties are relatively inferior to the austenitic stainless steels.  Toughness is limited at low temperatures and in heavy sections. 

Austenitic stainless steels have a austenitic, face centered cubic (fcc) crystal structure. Austenite is formed through the generous use of austenitizing elements such as nickel, manganese, and nitrogen.  Austenitic stainless steels are effectively nonmagnetic in the annealed condition and can be hardened only by cold working.  Some ferromagnetism may be noticed due to cold working or welding.  They typically have reasonable cryogenic and high temperature strength properties. Chromium content typically is in the range of 16 to 26%; nickel content is commonly less than 35%.

Duplex stainless steels are a mixture of bcc ferrite and fcc austenite crystal structures. The percentage each phase is a dependent on the composition and heat treatment. Most Duplex stainless steels are intended to contain around equal amounts of ferrite and austenite phases in the annealed condition. The primary alloying elements are chromium and nickel.  Duplex stainless steels generally have similar corrosion resistance to austenitic alloys except they typically have better stress corrosion cracking resistance.  Duplex stainless steels also generally have greater tensile and yield strengths, but poorer toughness than austenitic stainless steels.

Precipitation hardening stainless steels are chromium-nickel alloys. Precipitation-hardening stainless steels may be either austenitic or martensitic in the annealed condition.  In most cases, precipitation hardening stainless steels attain high strength by precipitation hardening of the martensitic structure.

Selecting a Stainless Steel

There are a large number of stainless steels produced.  Corrosion resistance, physical properties, and mechanical properties are generally among the properties considered when selecting stainless steel for an application.  A more detailed list of selection criteria is listed below:

  Corrosion resistance   Resistance to oxidation and

sulfidation

  Ambient strength   Ductility   Elevated temperature strength

  Toughness   Cryogenic strength   Resistance to abrasion and

erosion   Resistance to galling and seizing   Surface finish   Magnetic properties   Retention of cutting edge

  Suitability for intended cleaning procedures

  Stability of properties in service   Thermal conductivity   Electrical resistivity   Suitability for intended fabrication 

techniques

Corrosion resistance is commonly the most significant characteristic of a stainless steel, but can also be the most difficult to assess for a specific application. General corrosion resistance is comparatively easy to determine, but real environments are usually more complex.  An evaluation of other pertinent variables such as fluid velocity, stagnation, turbulence, galvanic couples, welds, crevices, deposits, impurities, variation in temperature, and variation from planned operating chemistry among others issues need to be factored in to selecting the proper stainless steel for a specific environment.

AMC can provide engineering services to determine how to optimize the selection of stainless steel for your application.  Our engineering analysis can reduce overall costs, minimize service problems, and optimize fabrication of your structure.

 

Different Types of Corrosion- Recognition, Mechanisms & Prevention

Pitting Corrosion Recognition   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 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. This following photo show pitting corrosion of SAF2304 duplex stainless steel exposed to 3.5% NaCl solution.

    

Pitting corrosion forms on passive metals and alloys like stainless steel 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 shape the size and the depth of penetration can be determined.

 Mechanisms   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.

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 here .

 Prevention

  

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

Corrosion Factors

The factors listed earlier have been organized in a framework of six categories with a number of subfactors as shown in the following Table. According to Staehle's materials degradation model, all engineering materials are reactive and their strength is quantifiable, provided that all the variables involved in a given situation are properly diagnosed and their interactions understood [7]. The corrosion based design analysis (CBDA) approach was further developed from the initial framework as a series of knowledge elicitation steps to guide maintenance and inspection decisions on the basis on first principles [8].

Factors and contributing elements controlling the incidence of a corrosion situation [7]

The two most important of these steps are described in the following Figures for respectively the environment and the material definitions. Each of the numbers in brackets in these Figures identifies an explicit action that needs to be considered for each definition.

Analysis sequence for determining environment at a location for analysis

Analysis sequence for determining materials at a location for analysis (LA) matrix

A brief explanation of the individual elements in the environment definition follows:

1. "Nominal Chemistry" refers to the bulk chemistry. For components exposed to ordinary air atmospheres, the "Major" elements mean humid air. The "Minor" elements refer to industrial contaminants such as SO2 and NO2

2. "Prior Chemistry History" refers to exposures to environmental species that might still reside on the surfaces or inside crevices

3. "System Sources" refers to those environments that do not come directly from a component but from an outside source

4. "Physical Features" includes occluded geometries, flow, and long range electrochemical cells

5. "Transformations" refers, for example, to microbial actions that can change relatively innocuous chemicals such as sulfates into very corrosive sulfide species that may accelerate hydrogen entry and increase corrosion rates

6. "Concentration" refers to accumulations much greater than that in the bulk environment due to various actions of wetting and drying, evaporation, potential gradients, and crevices actions that prevent dilution

7. "Inhibition" refers to actions taken to minimize corrosive actions. This usually involves additions of oxygen scavengers or other chemicals that interfere directly with the anodic or cathodic corrosion reactions.

The end point of the process is an input to a location for analysis (LA) matrix that is illustrated inthe following Figure for the locations in a steam generator.

Schematic view of steam generator with different locations for analysis

The LA template of the locations that correspond to most likely failure sites along tubes in a steam generator of a pressurized water nuclear power plant is detailed in the following Table for the main failure modes and sub-modes considered in such analysis. Maintenance and inspection actions can be decided upon by following developing trends monitored in each LA matrix thus produced.

Matrix for organizing Mode-Location cells. The abbreviations, ‘LP,’ ‘HP,’ ‘Ac,’ ‘MR,’ ‘Ak,’ and ‘Pb’ for ‘SCC’ and ‘IGC’ refer to ‘low potential,’ ‘high

potential,’ ‘acidic,’ ‘Mid-range pH,’ ‘alkaline,’ and ‘lead’ for ‘stress corrosion cracking’ and ‘intergranular corrosion’

The framework summarized above, which was initially developed to predict the occurrence of stress corrosion cracking (SCC), was extended to other corrosion modes/forms. Additionally, an empirical correlation was established between the factors listed in Table of factors and the forms of corrosion described earlier in the  previous Module . Recognized corrosion experts were invited to complete an opinion poll listing the main sub-factors and the common forms of corrosion as illustrated in the example shown in the following Figure. Background information on the factors and forms of corrosion was attached to the survey. A total of sixteen completed surveys were returned subsequently analyzed.

Opinion poll sheet for the most recognizable forms of corrosion problems

The following Figure presents the Box-and Whisker plots of the results obtained with pitting corrosion.When presented in this fashion, such results can provide a useful spectrum of factor and sub-factor confidence levels.

Box and whisker plots of the survey results obtained for the factors and sub-factors underlying the appearance of pitting corrosion

Example problem 7.1

Propose some arguments to explain the high variance, visible in the previous Figure, between expert opinions on the factors causing pitting corrosion. 

Linking the corrosion factors with possible forms of corrosion in this fashion may provide guidance to inexperienced corrosion failure investigators who have typically limited knowledge of corrosion processes. A listing of the most important factors should therefore help to increase the awareness of the complexity and interaction of the variables behind most corrosion failures and reveal how ‘experts’ have reduced

such complexity to a reduced set of variables, as the compiled results of the survey indicate in the following Figure.

Results of compiled survey of corrosion experts highlighting the most important correlations between corrosion forms and factors

An application of the compiled framework could be to test one’s skills against the ‘experts’ as illustrated in the following Figure.

Comparison of the answers of one expert with the some of the compiled expert survey results

Another application of this practical correlation would be to use the framework of factors vs. forms for archiving data in an orderly manner. Analysis of numerous corrosion failure analysis reports has revealed that information on important variables is often lacking [9]. The omission of important information from corrosion reports is obviously not always an oversight by the professional author. In many cases, the desirable information will simply not be (readily) available and require a special investigation to be completed.