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Corrosion in metallic components occurs when pure metals and their alloys form stable compounds with the process fluid by chemical reaction or electrochemical processes resulting in surface wastage. Appreciable corrosion can be permitted for tanks and piping if anticipated and allowed for in design thickness, but essentially no corrosion can be permitted in fine mesh wire screens, orifice plates and other items in which small changes in dimensions are critical. Rates of corrosion can be heavily affected by temperature changes and whilst a material of construction may be suitable at one temperature it may not be appropriate for use at a higher temperature with the same process fluid. (reference)
The corrosion of non-metallic materials is essentially a physiochemical process that manifests itself as swelling, cracking or softening of the material of construction. In many instances non-metallic materials will prove to be attractive from an economic and performance view.
The use of various substances as additives to process streams to inhibit corrosion has found widespread use and is generally most economically attractive in recirculation systems, however it has also been found to be attractive in some once through systems such as those encountered in the petroleum industry. Typical inhibitors used to prevent corrosion of iron or steel in aqueous solutions are chromates, phosphates, and silicates. In acid solutions organic sulfides and amides are effective.
Process equipment handling hazardous materials should be inspected at regular frequencies, both internally and externally. Localized corrosion can be unpredictable and fabrication defects such as poor welds can be present. Linings can deform or be damaged. Typically the glass lining on a jacketed reactor can suffer thermal shock or a static discharge may occur through the lining. The frequency of inspection can be amended once an inspection history has been built up and the condition of a piece of equipment can be reasonably predicted. The operator should demonstrate that it has inspection and maintenance programs in place for hazardous process equipment including lagged systems. Where equipment is lined electrical continuity tests for lining defects should be carried out where appropriate. Cathodic and anodic protection systems should be regularly checked to ensure continued protection.
Where control of corrosion is dependent on the concentration of contaminants or moisture the operator should demonstrate that procedures and the necessary controls are in place to maintain a safe operating condition. Similarly where inhibitors are added or systems such as cathodic protection are used the operator should demonstrate that these systems are inspected and adequately maintained to ensure continued protection of the process.
