Corrosion mechanism chloride

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

The rate of all these corrosion mechanisms increases when the water contains a high alkaline, chloride, or sulfate content. 

Sulfate ions have reactions similar to those of chloride. They are corrosion-causative agents (similar to oxygen and hydrogen) of the various types of concentration cell corrosion. In addition, they also are depassivation agents and may greatly accelerate the risk of stress corrosion mechanisms. Saline corrosion pits resulting from high concentrations of chloride and sulfate salts also may be associated with low pH corrosion because hydrochloric acid and sulfuric acid can form within the pit, under deposits. 

In the past ten years the number of chemistry-related research problems in the nuclear industry has increased dramatically. Many of these are related to surface or interfacial chemistry. Some applications are reviewed in the areas of waste management, activity transport in coolants, fuel fabrication, component development, reactor safety studies, and fuel reprocessing. Three recent studies in surface analysis are discussed in further detail in this paper. The first concerns the initial corrosion mechanisms of borosilicate glass used in high level waste encapsulation. The second deals with the effects of residual chloride contamination on nuclear reactor contaminants. Finally, some surface studies of the high temperature oxidation of Alloys 600 and 800 are outlined such characterizations are part of the effort to develop more protective surface films for nuclear reactor applications. ... 

Cruz, R.P., Nishikata A., Tsum T., Pitting corrosion mechanism of stainless steel under wet- dry exposure in chloride containing environments, corrosion science, 40, pp 125-139, 1998. 

R. B. Polder, Cathodic protection of concrete ground floor elements with mixed in chloride , in Corrosion of Reinforcement in Concrete, Corrosion Mechanisms and Corrosion Protection, Papers from Eurocorr 99, J. Mietz,... 

Regarding molten chlorides, mainly KCl-ZnCl2 mixtures are present in the ashes and fast corrosion occurs at relatively low temperatures. Figure 21 presents results of a thermogravimetric measurement on 2.25Cr-lMo-steel beneath a molten 50 wt. % KCl-50 wt. % ZnCl2 mixture at different temperatures in He-5 vol.% O2 gas mixture. At 350 °C, significant corrosion occurs by the molten salt. The main corrosion mechanism is the dissolution of... 

R.P. Vera Cruz, A. Nishikata, T. Tsuru, Pitting corrosion mechanisms of stainless steels under wet-dry exposure in chloride-containing environments, Corros. Sci. 40 (1998) 125—139. 

Atmospheric corrosion of metals is differentiated from the other forms of corrosion due to exposure of metals to different atmospheres rather than immersion in electrolytes. The spontaneous atmospheric corrosion of materials is controlled by the temperature, the relative humidity, the time of wetness, the pH of the electrolyte, and the presence of contaminants such as chlorides, NH3, SO2, NO2, and acidic fogs. In most cases, the rate equations have hmited validity due to different local atmospheric conditions. Metals spontaneously form a solid metal oxide film when exposed to dry atmospheres. The barrier oxide film reaches a maximum thickness of 2-5 nm [1-6]. The corrosion rate of metals exposed to a wet atmosphere is similar to that observed during immenion in aerated water in the presence of dissolved oxygen. Atmospheric corrosion rates decrease in dry atmospheres with corrosion mechanisms that are different from those in wet atmospheres. 

The three following mechanisms of concrete destruction as a result of chloride corrosion are reported ... 

It is obvious that the scaling resistance of the heat-resisting steels will be detrimentally influenced by any other corrosion mechanism which may be destroying the oxide layer, e.g., by chemical reactions with other metal oxides, chlorine, or chlorides. Thus in general, the heat resistance cannot be characterized by a single test method or measuring parameter but will depend on the speciflc environmental conditions. 

The concrete environment is often simulated by a concentrated calcium hydroxide solution [7,8]. Addition of additional sodium or potassium hydroxide, or both, to increase the pH has no effect on changing the corrosion mechanisms, but increases the chloride threshold level [9]. Typically the effects of chlorides and corrosion inhibitors on the susceptibihty of steel to pitting can be determined in these environments [7-9]. 

The most widely used cabinet test is the neutral salt spray (Fog) test (ASTM B 117), which consists of a fog of 5 % sodium chloride within the chamber at 35 C [46. Controversy exists over the validity of B 117 as a performance test because corrosion mechanisms are not always the same as those observed in automobile service. Also, not all materials can be successfully evaluated in the test. However, the value of the salt spray test as a quality assurance test is well documented [46]. Several modifications to the salt spray test have been developed including acetic acid salt spray (ASTM G 85, Annex 1), copper accelerated acetic acid salt spray (ASTM B 368), acidified synthetic seawater fog (ASTM G 43, Method of Acidified Synthetic Seawater (Fog) Testing), and modified salt spray (ASTM G 85). ASTM G 85 also includes cyclic tests. 

Marine environments are very aggressive conditions for structural steels. Chlorides promote the corrosion and reduce the critical relative humidity. The corrosion mechanism can be expressed as ... 

The martensitic chromium steels can be used in natural water at ambient temperature up to a maximum chloride content of 200 to 300 mg L (Fot and Heitz, 1967 Effertz and Forchhammer, 1977). No general limits for the maximum chloride level can be given if additional corrosive mechanisms, e.g. microbial corrosion, are active. 

Lyon, Thompson, and Johnson [56] point out that the high sodium chloride content of the salt spray test can resnlt in corrosion morphologies and behaviors that are not representative of natnral conditions. Harrison has pointed out that the test is inappropriate for use on zinc—galvanized snbstrates or primers with zinc phosphate pigments, for example — becanse, in the constant wetness of the salt spray test, zinc undergoes a corrosion mechanism that it wonld not nndergo in real service [57]. This is a well-known and well-docnmented phenomenon and is discnssed in depth in chapter 7. 

Figure 4 Schematic diagram of the effect of the three primary corrosion mechanisms that occur in integrated circuits due to the presence of moisture and an applied electrical bias. The photograph in the upper right is a typical result of either electrolytic dissolution or cathodic corrosion (depends on location of defects in the passivation layer). The lower right photograph shows a short-producing electrolytic migration of solder that occurred in a ceramic capacitor in the presence of high humidity and a chloride-containing flux residue.

Describe very briefly the corrosion mechanism of zinc in an atmosphere containing SO2 and chlorides. 

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