Corrosion. Also pitting

Finally, pitting may be viewed as a special form of concentration cell corrosion. Most alloys that are susceptible to crevice corrosion also pit. However, many metals may pit but not show crevice attack. Further, although sharing many common features with concentration cell corrosion, pitting is sufficiently different to warrant a separate categorization. 

F Cu-30Ni-2Fe 2Mn 0-014 <1 Very slight pitting attack beneath brown tubercles of corrosion products. Also pitting in areas next to sample holder. No measurable attack opposite jet... 

Pi theorem (Buckingham), 3 582 11 744 Pitot tubes, 11 661-662 Pit sealants, 3 334-335 PIT technique, 23 845. See also PIT (powder-in- tube) conductors Pitting corrosion... 

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 can also occur under atmospheric conditions. The corrosion starts at the break and continues to undercut the coating, forming a rather heavy tubercle of hard rust or scale with the pit underneath the original metal. The corrosion products help to isolate the aggressive medium inside the pit. These are common in marine environments as well as various industries where strong corrosive conditions exist.26 Also, pits with their mouth open (uncovered) exist and are responsible for loss of thickness and can act as stress raisers. 

Termination the propagation of the pit may terminate due to an increased internal resistance to the local cell. This can be the result of the pit being filled with corrosion product or the cathodic area being filmed with corrosion product. Pitting will also terminate if the metal becomes dry. However, the pits may reinitiate once the metal... 

The investigations on corrosion and corrosion inhibition were carried out under the conditions of the oxidation experiments and also at elevated temperatures up to 523 K. The metallic samples were not pre-treated. On the surface of carbon steel St 1203 as well as high-grade steel 1.4541 without addition of an inhibitor corrosion and pitting can clearly be seen after the... 

Resistant to high pitting corrosion. Also available as 316L for welded construction. 

Prevention—The methods outlined above to combat crevice corrosion also apply to pitting. Metals that are resistant to pitting should be used as alloying agents their passive films are more protective and more stable to halogen attack. For example, the addition of 2% molybdenum to 18-8 (type 304) stainless steel to produce 316 stainless steel significantly increases pitting resistance. 

Titanium corrosion also was observed during oxidation of cutting oil that contained sulfur and chlorine. For these tests, the SCWO system was operated at 600°C (1,112 F) and 3,400 psig (SWEC, 1996). Severe crevice and pitting corrosion were noted. In this study, acidic and reducing conditions (indicated by the presence of hydrogen sulfide) were present locally within the reactor. The operating period was approximately 50 hours. 

Lead pipes are the major source of lead in drinking water. In the past, lead pipes were eonsidered to be a eonvenient and suitable material for the conveyance of water. Lead is easily formed, eut and jointed, and its flexibility provides resistance to subsidence and frost. The thickness of lead pipe and its resistance to pitting corrosion also made it a desirable and durable material. 

For materials like stainless steels, the mechanisms are quite different. Corrosion resistance in stainless steels is provided by a passive film that acts as a barrier between the alloy and the water. The passive film is a continuous, non-porous and insoluble film, which, if broken under normal conditions, is self-healing. Due to these characteristics, the uniform corrosion of stainless steels is usually very low and the major risk is pitting corrosion. The pitting corrosion risk of stainless steels is influenced not only ly the composition of the alloy and by water quality but also by service conditions, quality of the material and quality of the installation (fitting, soldering conditions, etc.). 

The effect of Cr is beneficial not only for stainless steels but also for Ni base alloys like Alloy 22 and Hastelloy C4 with ca. 20% Cr. These alloys are resistant to general corrosion and pitting, even in highly chloride+ontaining solutions like Q brine (>8 M Cl ). Pitting is obtained at elevated temperatures at sufficiently positive potentials only [40]. This is again a consequence of the presence of Cr, which... 

Corrosion control as a result of design should be noted here. The design of oil and gas facilities should be done in such a way that there is no room for stagnation, especially on separators, treatment units, or pipelines, because stagnant areas contain some residual materials that over time could cause internal corrosion through pitting, while bulges or dents should also be avoided to curb external facility corrosion. 

It is assumed that the bridge is gradually deteriorating. Two study cases of the uniform corrosion and also pitting corrosion are considered based on formulae (8) and (9). It is considered that the initiation of corrosion starts early after the bridge completion (in 5 years). 

---