Corrosion types pitting

Analysis of electrochemical current noise Detection of pitting corrosion defects, signatures of specific corrosion types (pitting or crevice vs general) W(Mxl et al. (2002), Sulyma and Roy (2010h), Monticelli et al. (1998)... 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

Local corrosion or pitting is more important for practical purposes than the rate of general corrosion, and may proceed 10 times or so more rapidly than this. Inasmuch as certain types of cast iron are liable to suffer graphitic corrosion, whereas steel does not, steel might theoretically be expected to show to some advantage when used for buried pipelines. In practice, however, a cast-iron pipe has to be of stouter wall than a steel pipe for equal strength, and it is doubtful whether any distinction between the rust resistance of the two materials in the soil is justified. 

Figure 19.21 shows the types of crevices used by Wilde " for studying crevice corrosion and pitting of Cr-Ni-Fe alloys in the laboratory and in the field. Types 1 and 5 were used for anodic polarisation studies in nitrogen-saturated 1 mol dm NaCl and in aerated 3-5 mass% NaCl, respectively, and it can be seen that attachment to the conducting lead is by means of a Stern-Makrides pressure gasket Types 3 and 4 were used for field tests... 

The importance of MIC has been underestimated, because most MIC occurs as a localized, pitting-type attack. In general this corrosion type results in relatively low rates of weight loss, changes in electrical resistance, and changes in total area affected. This makes MIC difficult to detect and to quantify using traditional methods of corrosion monitoring [1447]. 

This method is the simplest of all the methods and is capable of detecting surface flaws such as corrosion, contamination, surface finish and surface discontinuities on joints.47 The discontinuities on joints such as welds, seals, solder connections and adhesive bonds can be detected. General corrosion, qualitative pitting corrosion, stress-corrosion cracking, weld-heat-affected zone attack, erosion corrosion and other type of degradation can be observed by visual examination aided by microscopes with sufficient magnification. Degradation of plastics can also be detected by visual examination. Visual examination is also used in conjunction with other techniques, such as powerful microscopes. 

Pitting corrosion (Table 4.8) involves pit initiation (breakdown of passive film) followed by pit growth. The chloride ion induces pitting corrosion. Type 304 steel undergoes pitting more readily than Type 316 steel. The molybdenum in 316 steel is responsible for its reduced susceptibility to pitting corrosion. Type 316L steels contains... 

Single-component corrosion types, important for heat exchanger design and operation, are as follows (1) uniform attack corrosion, (2) galvanic corrosion, (3) pitting corrosion, (4) stress corrosion cracking, (5) erosion corrosion, (6) deposit corrosion, and (7) selective leaching [153],... 

Standard austenitic stainless steels such as type 316 (18 Cr 10 Ni 3 Mo) have useful if limited resistance, to acids and reasonable resistance to pitting corrosion. Type 304 (18 Cr. 10 Ni) stainless steel has a good resistance to nitric acid. Austenitic stainless steels have relatively low strength, poor antierosion and abrasion properties and do not possess the ability to resist stress corrosion cracking. 

The most widespread corrosion types are point corrosion or pitting, filiform, crevice, contact and intercrystalline corrosion. The highest danger for the metal-polymer system is presented by the crevice type of corrosion. 

Fig. 4 Example of the results of crevice corrosion. Type 304 SS exposed to 6 wt.% ferric chloride for 48 h at room temperature with castellated crevice washer applied around center hole. Pitting also occurred at several sites outside the crevice. The draining of the occluded solution from the crevice sites (and pits) led to the river pattern of...

The shape of the pit is often responsible for its continued growth for the same reasons mentioned under crevice corrosion. A pit can be considered a type of self-forming crevice. Pitting can occur... 

Corrosion can affect an entire surface of a metal or just local spots. Uniform corrosion of the complete surface usually only happens in acidic conditions [2]. This generally results in overall thinning and causes no major damage. On the other hand, a very detrimental form of corrosion is pitting. This type is found at a single location on the surface and creates a pit or cavity which is difficult to prevent and often hard to detect. It can result in structural failure (example a cracked pipe). 

It appears that the type of corrosion (uniform, pitting) and potential consecpiences of failure should be taken into account in the recommendations for the crack width limits given in the current prescriptive documents. The crack width limits recommended for the assessment of the residual working life of a bridge should be further analyzed. 

Stainless steels exposed to seawater develop deep pits within a matter of months, with the pits usually initiating at crevices or other areas of stagnant electrolyte (crevice corrosion). Susceptibility to pitting and crevice corrosion is greater in the martensitic and ferritic steels than in the austenitic steels it decreases in the latter alloys as the nickel content increases. The austenitic 18-8 alloys containing molybdenum (types 316,316L, 317) are still more resistant to seawater however, crevice corrosion and pitting of these alloys eventually develop within a period of 1-2.5 years. 

Corrosion scientists work with two basic types of experiments, (1) controlled and (2) uncontrolled. In controlled experiments, levels of independent variables (factors such as temperature and chemical composition) are controlled and the dependent variables (corrosion rate, pitting potential, etc.) are measured. In uncontrolled experiments such as atmospheric exposures, uncontrolled independent variables such as temperature and relative humidity are measured along with the dependent corrosion results. Properly designed controlled experiments are much more amenable to statistical analysis of variance than are properly designed uncontrolled experiments. Many scientists prefer some form of factorial design for controlled experiments. 

If susceptibility to specific types of corrosion is of interest, specialized sample configurations can be used. If the corrosion performance of the material in seawater is completely unknown, at a minimum it should be evaluated for general corrosion and pitting, galvanic corrosion, crevice corrosion, and stress corrosion. 

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