Pitting corrosion continued

Secondly, under certain conditions copper may suffer intense localised pitting corrosion, leading sometimes to perforation of the tube, in quite a short time. This form of attack is not common and depends on a combination of unusual circumstances, one of which is the possession by the tube of a fairly, but not entirely, continuous film or scale that is cathodic to the copper pipe in the supply water this can set up corrosion at the small anodes of bare copper exposed at faults or cracks in the film. Carbon films give rise to such corrosion, but since 1950, when the importance of carbon films was... 

A chloride-based DA will not reduce the TDS of the water, and therefore the tangible benefit of continuous cost savings through lower boiler BD rates is never realized. Also, the introduction of additional chlorides into a boiler may provide the potential for pitting corrosion to take place. 

As corrosion products develop, so the rate of 02 diffusion reduces and the rate of general etch corrosion slows down. But in practice, the presence of surface deposits tends to promote various forms of localized corrosion such as tuberculation, pitting corrosion, and stress corrosion, and consequently the rate of corrosion continues unabated. 

The initiation of pitting corrosion starts, then, with some kind of local irregularity containing metallic inclusions and continues with the penetration of Cl- (or other aggressive ion) into the protecting layer at this point. Until the work of Brown... 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

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 scope of application of CP is enormous and continuously increasing. It is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete structures exposed to the atmosphere, as well as underground parts, tunnels, and other metal equipments using cathodic protection. Apart from reduction of general corrosion, cathodic protection reduces SCC, pitting corrosion, corrosion fatigue, and erosion-corrosion of metallic materials. 

In the interim period before the new deionization equipment for the L and K basins was received, portable equipment was installed in July 1995 and used to lower the L basin water conductivity from 110 to below 8 pS/cm in 2.5 months. The equipment was then moved to the K basin, and within three months the conductivity was lowered to below 10 pS/cm. Continued deionization in both basins for two more months lowered the conductivity further, to less than 3 pS/cm, and the chlorides, nitrates and sulphates were lowered to about 0.5 ppm. The corrosion surveillance programme continued in the three reactor basins and in the RBOF while the basin and water quality improvements were being carried out, i.e. until mid-1996. Results of the component immersion tests through September 1997 (the last withdrawal) showed no pitting corrosion on any of the corrosion coupons. These coupons were exposed to a variety of conditions for 37-49 months as conditions improved in the basins. Table 1.1 presents a summary of component immersion tests for the period 1992-2000, when corrosion coupons accumulated exposure time in extremely high quahty water and withdrawal intervals were extended. 

Kurchatov Institute is continuing to expose the coupons of racks 1 and 3 to the storage basin water in order to (a) obtain more data about the low corrosion rates observed so far (b) investigate further the pitting corrosion on 6061 and 6063 test coupons and (c) clarify the reasons for the differences in corrosion behaviour between the coupons of racks 1 and 3. 

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