Chloride-induced corrosion

Polymers, unlike metals, are subject to swelling rather than corrosion. Chloride-induced pitting corrosion, to which austenitic Cr-Ni steels are particularly susceptible, is unknown in polymeric materials. Stress corrosion cracking, however, is possible under conditions that cause the medium to attack the polymer chemically. Stress corrosion cracking can only occur at the same time that stresses resulting from manufacture, coating or lining, or the conditions of use are present. 

Duplex stainless steels are mostly composed of alternate austenite and ferrite grains. Their structure improves resistance to chloride-induced stress corrosion cracking. In certain reducing acids, such as acetic and formic, preferential attack of the ferrite is a serious problem. 

Nickel-chromium alloys can be used in place of austenitic stainless steels where additional corrosion resistance is required. These alloys are still austenitic but are highly resistant to chloride-induced stress corrosion cracking when their nickel content exceeds 40 per cent. 

Titanium is immune to chloride induced stress-corrosion cracking but more expensive than type 300 series stainless steels. 

This type of stress-related corrosion process may result in boiler failure through a sudden and violent rupturing of the boiler tube metal. Austenitic stainless steels also are corroded by SCC mechanisms in the presence of concentrated chlorides (chloride-induced SCC). 

Certain Inconel alloys have been used with varying degrees of success. Inconel 600 (70% Ni, 15% Cr, 15% Fe) has been employed because of its improved resistance to chloride-induced, SCC, but it has failed on occasions because of intergranular attack and general etch corrosion. 

Concrete exposed to deicer salts, or to a marine environment is subjected to chloride and sodium loading. The ability of concrete to resist the penetration of chlorides and sodium is a primary design consideration in marine or cold environments. The ingress of chlorides into concrete is a major problem due to chloride-induced corrosion of the reinforcing steel and deicer salt scaling [a process by which a thin layer (< 1 mm) of concrete deteriorates from the surface of the concrete]. The penetration of sodium from sea water or deicer salts is generally... 

Latex-modified mortars and concretes have become promising materials for preventing chloride-induced corrosion and for repairing damaged reinforced concrete structures. In Japan and the USA, latex-modified mortar is widely used as a construction material in bridge deck overlays and patching compounds, and for finishing and repairs [99]. Polymer-cement hydrate-... 

The relatively thick zinc hydroxide film can effectively prevent underdeposit corrosion and other problems. As a further example, in medium hardness waters, with careful control of the water chemistry, this thick Zn(OH)2 film can help offset risks of chloride-induced pitting corrosion with estuarine makeup water. 

The best non-chloride admixture to date appears to be calcium nitrite marketed as a 20% solids solution, it can be readily added to the mix using the usual dispensers. The admixture has therefore been more readily accepted by the ready-mixed concrete industry than the formate-based admixture. Strength development effects produced in concrete are reported to be comparable to those obtained with calcium chloride [40], Table 7.12 shows a comparison of the results for concrete containing calcium chloride, calcium nitrite, and no admixture. In addition to the improvements in both compressive and tensile strengths, calcium nitrite is an effective inhibitor of chloride-induced corrosion. The material is now finding wide acceptance in bridge and parking deck repair. Other non-chloride accelerators used in Russia and special applications include the alkali-metal carbonates (Na CO, K CO, LiCO ). 

Nickel is resistant to chloride-induced SCC, but subject to caustic cracking in aerated solutions under high stress. Nickel is highly resistant to corrosion in natural fresh water and flowing seawater. Pitting occurs under stagnant or crevice conditions. 

W. H. R. Holden, The influence of different cements on chloride-induced corrosion of reinforcing steel , Cement and Concrete Research, 1986, 16, 79-86. 

In summary, the DuraCrete service-life design is based on equations modelling the deterioration processes of the structure and its resistance against the environmental actions. Mathematical models of the deterioration processes are formulated in physical/chemical terms, including the effect of time. These models are the basis for the probabilistic method of service-hfe design. In the following sections, the DuraCrete models for carbonation and chloride-induced corrosion will be illustrated. 

Propagation Time for Carbonation- (and Chloride)-induced Corrosion... 

As mentioned above, the service life for chloride-induced corrosion is traditionally assumed equal to the initiation time and the period of propagation is not... 

Design Equation for Chloride-induced Corrosion Initiation... 

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