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 corrosion
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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
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 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:
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 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:
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 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
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.