Corrosion localized attack

Mortar Uniform corrosion localized attack Copper release Perforation of pipe and leakage... 

In addition to ductile iron and PVC, copper and lead are used in pipes, and brass in fixtures and connections. Lead is released because of uniform corrosion. Copper is also released because of uniform corrosion, localized-attack cold water pitting, hot water pitting, MIC, corrosion fatigue, and erosion-corrosion. Lead pipes and lead-tin solder exhibit uniform corrosion. Brass corrosion includes erosion-corrosion, impingement corrosion, dezincification, and SCC. The direct health impacts are because of increased copper, lead, and zinc concentrations in the drinking water. Mechanical problems because of corrosion include leaks from perforated pipes, rupture of pipes, and the loss of water pressure because of blockage of pipes by corrosion products. 

Corrosive chemicals and external exposure can cause tank failure. Materials of construction should be chosen so that they are compatible with the chemicals and exposure involved. Welding reduces the corrosion resistance of many alloys, leading to localized attack at the heat-affected zones. This may be prevented by the use of the proper alloys and weld materials, in some cases combined with annealing heat treatment. 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

Pitting can occur when normally protective corrosion-product or deposit layers are locally breached. Localized attack occurs during upsets or when protracted idle periods change water conditions abruptly. Regions adjacent to localized corrosion sites often remain... 

Corrosion morphologies. Sulfate-reducing bacteria frequently cause intense localized attack (Figs. 6.2 through 6.7). Discrete hemispherical depressions form on most alloys, including stainless steels, aluminum. Carpenter 20, and carbon steels. Few cases occur on titanium. Copper alloy attack is not well defined. 

Alloys whose corrosion resistance depends on forming a protective oxide layer, such as stainless steel, are susceptible to severe localized attack when pH falls as a result of nonoxidizing acid excursions. How-... 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

Generally, pitting corrosion only occurs on passivated metals when the passive film is destroyed locally. In most cases chloride ions cause this local attack at potentials U > U q. Bromide ions also act in the same way [51], The critical potential for pitting corrosion UpQ is called the pitting potential. It has the same significance as in Eqs. (2-39) and (2-48). 

Localized Attack—corrosion in which one area of the metal surface is primarily anodic and another predominantly cathodic. 

Pitting is a form of extreme, localized attack. The rate of corrosion is greater at some areas than at others, resulting in holes in the metal. Heterogenous metal... 

Items of plant fabricated from stainless steels should be inspected before first use and after any maintenance work or unplanned shutdown. All materials that rely for their corrosion resistance on the presence of an oxide or similar passive layer are susceptible to localized attack where that layer is absent or damaged. Damage is most commonly caused by scratching, metallic contamination (nearby grinding or touching with ferrous tools), embedding of grit and weld spatter. 

The importance of occluded cells cannot be overemphasised, and Brown considers that pitting, crevice corrosion, intergranular attack, filiform corrosion and hydrogen cracking are characterised by local acidification due to hydrolysis of metal ions, and that this phenomenon is of major significance in the overall mechanism. 

The addition of a minimum of 2% molybdenum content in type 316 stainless steel has been shown to reduce the tendency for pitting-type corrosion in chloride environments. Hoar and Mears postulated that chloride ions accelerate the corrosion of stainless steel by penetrating the oxide him. The chloride-contaminated him then loses its passivating quality and a local attack on the metal follows, creating a pit. The exact mechanism by which molybdenum strengthens the oxide him is not clearly understood. ... 

Evidently, therefore, correct design can play an important part in reducing the corrosion of steel structures through lessening the danger of local attack due to mass metal effects and condensation. 

It should be made clear that all the rates of rusting in the atmosphere just quoted, relate to average general penetration and take no account of pitting. Serious pitting of steel exposed to atmospheric corrosion is uncomm.on on simple test plates, but it may be necessary to allow for this in some practical cases, where local attack may be occasioned by faulty design and other factors. 

Evaluation of Resiuts After the specimens have been reweighed, they should be examined carefully. Localized attack such as pits, crevice corrosion, stress-accelerated corrosion, cracking, or intergranular corrosion should be measured for depth and area affected. 

This is also the case with methods that yield information on localized corrosion. The overall corrosion rate may be small when localized attack occurs, but failure due to perforation or loss of function may be the consequence of localized attack. 

This term describes the more or less uniform wastage of material by corrosion, with no pitting or other forms of local attack. If the corrosion of a material can be considered to be uniform the life of the material in service can be predicted from experimentally determined corrosion rates. 

Fig. 5.10 State of lead coins after different conservation conditions (a) coin protected by a passive surface (b) coin with corrosion localized on protruding features (c) coin nearly disintegrated due to corrosive attack (courtesy Societe Archeologique de Namur, from [267])...

In the previous analysis, homogeneous current distribution has been assumed but, on many occasions, corrosion occurs with localized attack, pitting, crevice, stress corrosion cracking, etc., due to heterogeneities at the electrode surface and failure of the passivating films to protect the metal. In these types of corrosion processes with very high local current densities in small areas of attack, anodic and cathodic reactions may occur in different areas of disparate dimensions. 

One effect is that bio-fouling of the metal surface often promotes localized attack. When a barnacle or mussel attaches itself to a plate of stainless steel in sea water, a differential aeration corrosion cell is formed. Intense local pitting results which may lead to complete perforation in a relatively short time. In one case a 0.75-inch thick stainless steel plate was perforated beneath a barnacle base after 9 months of immersion in sea water. 

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