Chloride ion corrosion

Tingley, I. L, Corrosion Resistance of Type 329 Stainless Steel in HNO3 containing Chloride Ion , Corrosion, 19, 408t (1963)... 

Refait, P.H., Abdelmoula, M. and G6nin, J.-M.R. (1998) Mechanisms of formation and structure of green rust one in aqueous corrosion of iron in the presence of chloride ions. Corrosion Science, 40(9), 1547-60. 

Y. Tsutsumi, A. Nishikata, T. Tsuru, Initial stage of pitting corrosion of type 304 stainless steel under thin electrolyte layers containing chloride ions corrosion, passivation, and anodic films, J. Electrochem. Soc. 152 (2005) B358-B363. 

The chemistry of pitting is quite complex and is explained in most chemistry text books. However, the principle is fairly simple, especially where chlorides are present. At some suitable site on the steel surface (often thought to be a void in the cement paste or a sulphide inclusion in the steel) the passive layer is more vulnerable to attack and an electrochemical potential difference exists that attracts chloride ions. Corrosion is initiated and acids are formed, hydrogen sulphide from the MnS inclusion and HCl from... 

Mercury(II) chloride, HgC, corrosive sublimate, m.p. 280 C, b.p. 302"C. Essentially covalent material (Hg plus CL Hg plus aqua regia). Forms complex halide ions, e.g. (HgCU) (HgCL)" in excess HCl and forms complexes. Very poisonous. 

Many stabilizer systems have been tailored to a particular industry need or for particular areas where dilution water quaUty is poor. These grades are heavily stabilized and may contain organic sequestering agents, ie, staimate, phosphates, and nitrate ions, so that the weak solutions produced by dilution from hard water retain acceptable stabihty. The nitrate is not a stabilizer, but it inhibits corrosion of aluminum storage tanks by chloride ion. 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

The concentration dependence of iron corrosion in potassium chloride [7447-40-7] sodium chloride [7647-14-5] and lithium chloride [7447-44-8] solutions is shown in Figure 5 (21). In all three cases there is a maximum in corrosion rate. For NaCl this maximum is at approximately 0.5 Ai (about 3 wt %). Oxygen solubiUty decreases with increasing salt concentration, thus the lower corrosion rate at higher salt concentrations. The initial iacrease in the iron corrosion rate is related to the action of the chloride ion in concert with oxygen. The corrosion rate of iron reaches a maximum at ca 70°C. As for salt concentration, the increased rate of chemical reaction achieved with increased temperature is balanced by a decrease in oxygen solubiUty. 

Impurities in a corrodent can be good or bad from a corrosion standpoint. An impurity in a stream may act as an inhibitor and actually retard corrosion. However, if this impurity is removed by some process change or improvement, a marked rise in corrosion rates can result. Other impurities, of course, can have very deleterious effec ts on materials. The chloride ion is a good example small amounts of chlorides in a process stream can break down the passive oxide film on stainless steels. The effects of impurities are varied and complex. One must be aware of what they are, how much is present, and where they come from before attempting to recommena a particular material of construction. 

In the stainless group, nickel greatly improves corrosion resistance over straight chromium stainless. Even so, the chromium-nickel steels, particularly the 18-8 alloys, perform best under oxidizing conditions, since resistance depends on an oxide film on the surface of the alloy. Reducing conditions and chloride ions destroy this film and bring on rapid attack. Chloride ions tend to cause pitting and crevice... 

These alloys have extensive applications in sulfuric acid systems. Because of their increased nickefand molybdenum contents they are more tolerant of chloride-ion contamination than standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. 

The corrosion resistance of aluminum and its alloys tends to be veiy sensitive to trace contamination. Veiy small amounts of metalhc mer-cuiy, heavy-metal ions, or chloride ions can frequently cause rapid failure under conditions which otherwise would be fuUy acceptable. 

The lifetime of a conventional exhaust system on an average family car is only 2 years or so. This is hardly surprising - mild steel is the usual material and, as we have shown, it is not noted for its corrosion resistance. The interior of the system is not painted and begins to corrode immediately in the damp exhaust gases from the engine. The single coat of cheap cosmetic paint soon falls off the outside and rusting starts there, too, aided by the chloride ions from road salt, which help break down the iron oxide film. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

Generally, pitting corrosion only occurs on passivated metals when the passive film is destroyed locally. In most cases chloride ions cause this local attack at potentials U > U q. Bromide ions also act in the same way [51], The critical potential for pitting corrosion UpQ is called the pitting potential. It has the same significance as in Eqs. (2-39) and (2-48). 

Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10,40-42]. 

Steel in cement mortar is in the passive state represented by field II in Fig. 2-2. In this state reinforcing steel can act as a foreign cathodic object whose intensity depends on aeration (see Section 4.3). The passivity can be lost by introduction of sufficient chloride ions or by reaction of the mortar with COj-forming carbonates, resulting in a considerable lowering of the pH. The coordinates then lie in field I. The concentration of OH ions can be raised by strong cathodic polarization and the potential lowered, resulting in possible corrosion in field IV (see Section 2.4). 

Due to both carbonization and penetration of chloride ions, steel will pass from a passive to an active condition and (consequently) may corrode. If the mortar is completely surrounded by water, oxygen diffusion in wet mortar is extremely low so that the situation is corrosion resistant because the cathodic partial reaction according to Eq. (2-17) scarcely occurs. For this reason the mortar lining of waste pipes remains protective against corrosion even if it is completely carbonated or if it is penetrated by chloride ions. 

The passivating action of an aqueous solution within porous concrete can be changed by various factors (see Section 5.3.2). The passive film can be destroyed by penetration of chloride ions to the reinforcing steel if a critical concentration of ions is reached. In damp concrete, local corrosion can occur even in the presence of the alkaline water absorbed in the porous concrete (see Section 2.3.2). The Cl content is limited to 0.4% of the cement mass in steel-concrete structures [6] and to 0.2% in prestressed concrete structures [7]. 

A diagnosis of possible damage should be made before beginning repairs with other construction measures [48,49]. There should be a checklist [48] of the important corrosion parameters and the types of corrosion effects to be expected. Of special importance are investigations of the quality of the concrete (strength, type of cement, water/cement ratio, cement content), the depth of carbonization, concentration profile of chloride ions, moisture distribution, and the situation regarding cracks and displacements. The extent of corrosion attack is determined visually. Later the likelihood of corrosion can be assessed using the above data. 

The role of moisture in corrosion of metals and other surfaces is twofold surface wetness acts as a solvent for containments and for metals is a medium for electrolysis. The presence of sulfate and chloride ions acceler-... 

Not understanding details of the corrosion service. Stating only the predominant acid without the other details (such as presence of chloride ion) is an example. The author explains how to use the acronym SPORTSFAN ... 

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