Pitting corrosion temperature

AHoy/Laboratoiy Method C—CPT Critical Pitting Corrosion Temperature (C) Method D—CCT Critical Crevice Corrosion Temperature (C) ... 

The materials Monit and Sea-Cure are characterised by good resistance to pitting, crevice and stress corrosion cracking in seawater. The critical pitting corrosion temperature in the FeCls test is 328 K (55 °C) and the critical crevice corrosion temperature is 318 K (45 °C). In Table 36, the pitting potentials of the two superferrites and the austenitic steels 1.4539 (SAE 904 L, XlNiCrMoCu25-20-5) and X3CrNiMol7-13-3 (SAE 316,1.4436) measured in 5% NaCl solution are presented. 

In a superferritic steel (ELI steel) with the composition 0.022% C, 24.9% Cr, 3.63% Ni, 3.52% Mo and 0.50% Ti a critical pitting corrosion temperature of over 333 K (60 °C) was determined in a seawater-like chloride sulphate solution. A chlorine addition to the solution shifts the corrosion potential in the positive direction, reducing the pitting resistance equivalent in the critical temperature range [116]. 

Steel Pitting corrosion temperature. Crevice corrosion temperature,... 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

Enamel coatings are used for the internal protection of storage tanks that in most cases have built-in components (e.g., fittings with exits, probes, temperature detectors) that usually exhibit cathodic effectivity. These constitute a considerable danger of pitting corrosion at small pores in the enamel. Corrosion protection is achieved by additional cathodic protection which neutralizes the effectiveness of the cathodic objects. 

In applying electrolytic protection, galvanized tubes can be installed downstream from copper components in water boilers without danger of Cu " -induced pitting corrosion. The protection process extends the application range for galvanized tubes with respect to water parameters, temperature and material quality beyond that in the technical regulations [16, 17]. 

In systems with considerable temperature variations over the metal surface, the warmer areas will be anodic to the cooler areas. The creation of a cell usually leads to pitting corrosion of the anodic area. 

Tatsuya, K. and Shuichi, F., Pitting Corrosion of Titanium in High-temperature Halide Solutions , Proc. 2nd Ini. Conf. Titanium Sci. Technol., 4, 2383 (1973)... 

The arbitrary division of behaviour has been made because of the extreme behaviour of some chemicals that initiate small areas of attack on a well-passivated metal surface. The form of attack may manifest itself as stress-corrosion cracking, crevice attack or pitting. At certain temperatures and pressures, minute quantities of certain chemicals can result in this form of attack. Chloride ions, in particular, are responsible for many of the failures observed, and it can be present as an impurity in a large number of raw materials. This has led to the development of metals and alloys that can withstand pitting and crevice corrosion, but on the whole these are comparatively expensive. It has become important, therefore, to be able to predict the conditions where more conventional materials may be used. The effect of an increase in concentration on pitting corrosion follows a similar relationship to the Freundlich equation where... 

Aluminum Foil. Studies of various foods wrapped in aluminum foil show that food products to which aluminum offers only fair resistance cause little or no corrosion when the foil is in contact with a nonmetallic object (glass, plastic, ceramic, etc.) The reactions, when found, are essentially chemical, and the effect on the foil is insignificant. However, when the same foods are wrapped or covered with foil that is in contact with another metallic object (steel, tinplate, silver, etc.), an electrochemical or galvanic reaction occurs with aluminum acting as the sacrificial anode. In such cases, there is pitting corrosion of the foil, and the severity of the attack depends primarily on the food composition and the exposure time and temperature. Results obtained with various foods cov-... 

Water treatment monitoring and control is often a knife-edge operation and must be tailored to the overall operation of the boiler because waterside and gas-side problems usually are interlinked. Consequently (and as with other types of WT boiler), not only should the utility boiler FW be essentially free of dissolved oxygen to prevent waterside pitting corrosion of the economizer and other boiler components, but also the temperature must be high enough to prevent dewpoint condensation and subsequent acid attack on the gas side of the economizer tubes. 

Economizers are always subject to a risk of very severe pitting corrosion as a result of the release of DO with rapid increase of water temperature. Therefore, they should be inspected at every available opportunity. 

These detailed microscopic studies show that it is possible to predict how and where pitting corrosion will occur on the surface. Like the titanium surface, an aluminum surface is passivated at normal temperatures by formation of an oxide layer in the ambient atmosphere. Despite formation of an oxide layer, aluminum surfaces can also be studied by STM. Pitting corrosion can be observed after 10 h of immersion of an aluminum surface at -1.2 V/normal hydrogen electrode in a IO-2 A/ NaCl electrolyte. The pitting on aluminum is observed as a general roughening... 

In the early 1970s, Brigham and Tozer were the first to make a systematic study, using temperature as the variable, of the connection between potential, temperature, and pitting corrosion (Fig. 12). They argued that in principle a critical pitting temperature should exist, but the data obtained showed a transition over a range of temperatures. The sharp transition was demonstrated experimentally by Quarfort in 1989. 

With small modifications, ASTM standard G48 can be used to determine a CPT. The test is used as a ranking parameter for the resistance to pitting of high-alloyed austenitic stainless steels. In this method, material coupons are typically exposed for 24 or 72 h to a 6% FeCl3 (=1.11 mole/liter) solution at fixed temperatures (typically with 2.5°C intervals). The CPT is defined as the lowest temperature at which the specimen is attacked by pitting corrosion. 

Table 4.43 Critical pitting and crevice corrosion temperature per ASTM G-48...

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