Wide pitting corrosion

Laser-based profilometry is now being applied to a wide variety of both NDT and Quality Control gauging applications. In the world of NDT, the primary interest is in the details associated with surface topography or deformation of a particular component. Laser-based profilometry systems are commonly used to inspect surfaces for defects such as pitting, corrosion, deformation and cracking. Quality control gauges are used for absolute measurement of dimensions, such as the diameter and thickness of a given part. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

The onset of pitting corrosion occurs suddenly If one performs electrochemical experiments with stainless steel, e. g. by applying a constant electrical potential to a sample immersed in dilute NaCl solution, the electrical current - which is an indicator for chemical activity (corrosion) on the metal surface - is low over a wide parameter range. But if critical parameters like temperature, potential, or electrolyte concentration exceed a certain critical value, the current rises abruptly and the metal surface is severely affected by pitting corrosion. The transition to high corrosion rates is preceded by the appearance of metastable corrosion pits. 

Specimen 7 displays irregular corrosion spread over the entire surface of the specimen. It takes the form of wide depressions with pitting corrosion. The test solution is rust-brown and cloudy with a rust-coloured sediment, the main component of which is ferric hydroxide. This picture is typical of contact corrosion between unalloyed steel and copper in sodium chloride solutions (Figure 11.85(g)). 

The corrosion resistance offered by GRP to such a wide range of chemicals means that it is the material of choice, particularly where a mixture of gases is being transported. In any gas stream, condensation resulting in accumulation and concentration will occur at some point in the hne. This wiU lead to pitting corrosion in traditional materials and will result in failure of the... 

In the final inspection, severe crevice and pitting corrosion was observed on the aluminium alloys. Pits as deep as 0.5 mm and as wide as 1.1 mm were observed on coupon 1100/57 (Fig. 9.4).The SZAV coupons showed only crevice corrosion. The results of the three inspections showed that ... 

Temperature can also be used as an acceleration factor in a fashion similar to potential. Many materials wiU not pit at a temperature below a critical value that is often extremely sharp and reproducible [56-62]. At low temperatures, extremely high breakdown potentials are observed, corresponding to transpassive dissolution, not localized corrosion. Just above the critical pitting temperature (CPT), pitting corrosion occurs at a potential that is far below the transpassive breakdown potential. This value of CPT is independent of environmental parameters and applied potential over a wide range, and is a measure of the resistance... 

In the presence of chlorides (as an impurity in an ionic liquid) or of fluorides (which are formed as a result of hydrolysis of tetrafluoroborate anion in a wet ionic liquid) the passive layer is destroyed and pitting corrosion develops. This is why the first plateau discussed above for dry and chloride-free BMIBF4 (Fig. 3) almost disappears in the presence of water and chlorides when the corrosion inhibitor is absent (Fig. 4). In contrast, in the presence of MBT the first plateau in wet and chloride-containing BMIBF4 (Fig. 4) is almost as wide as in dry and chloride-free BMIBF4 (Fig. 3). Apparently the MBT layer protects the surface from chloride- and fluoride attack. 

The results of the study (Figure 3 below) showed that the corrosion increases as the O2 content and flow rate increase, with an increased tendency towards local attack (wide pitting). The water adjusted to pH 8.0 by addition of sodium hydroxide solution had a favourable effect, although the local corrosive attacks were not suppressed completely. 

The initially apparently contradictory results of the experiments in [19] and [20] must be seen from the point of view of the types of corrosion. [19] deals primarily with uniform general corrosion (removal of iron) and the formation of msty water, while the conclusions in [20] primarily relate to wide or deep pitting corrosion. 

Inhibitors for pitting corrosion are by far less widely studied (De Berry, 1993). They can act ... 

Pitting cavities may fill with corrosion products and form caps over the pit cavities sometimes creating nodules or tubercles (Fig. 6.10). While the shapes of pits vary widely (Fig.6.11) they are usually roughly saucer-shaped, conical, or hemispherical for steel and many associated alloys. The following are some factors contributing to initiation and propagation of pitting corrosion ... 

Cobalt-chromium alloys, tike titanium, are passive in the human body. They are widely in use in orthopedic applications. They do not exhibit pitting corrosion. 

It is therefore now widely accepted that, even if the so-called depolarization of the cathodic reaction by the enz5me hydrogenase occurs, it plays no more than a secondary role in the pitting corrosion of iron in anaerobic conditions. This theory is therefore reported here more for historical interest than as a potential mechanism for pitting corrosion of iron in presence of SRB. 

It has long been known that pitting corrosion develops in the presence of chloride. Chloride ions Cl are adsorbed on the natural oxide film [5], followed by the rupture of the film at weak points, with formation of microcracks that are a few nanometres wide. Many pits are initiated within a very short time, up to lO cm . Their density depends on the alloy from 10" cm on 1199 to 10 ° cm on an alloy containing 4% copper. 

---