Corrosion failures

Because of the inherently greater susceptibility of expansion bellows to failure from unexpected corrosion, failure of guides to control joint movements, etc., it is advisable to examine critically their design choice in comparison with a stiff system. 

Liquid-Metal Corrosion Liquid metals can also cause corrosion failures. The most damaging are liqmd metals which penetrate the metal along grain boundaries to cause catastrophic failure. Examples include mercury attack on aluminum alloys and attack of stainless steels by molten zinc or aluminum. A fairly common problem occurs when galvanized-structural-steel attachments are welded to stainless piping or eqmpment. In such cases it is mandatoty to remove the galvanizing completely from the area which will be heated above 260°C (500°F). 

The selection of materials to be used in design dictates a basic understanding of the behavior of materials and the principles that govern such behavior. If proper design of suitable materials of construction is incorporated, the eqiiipment should deteriorate at a uniform and anticipated gradual rate, which will allow scheduled maintenance or replacement at regular inteivals. If localized forms of corrosion are characteristic of the combination of materials and environment, the materials engineer should still be able to predict the probable life of equipment, or devise an appropriate inspection schedule to preclude unexpected failures. The concepts of predictive, or at least preventive, maintenance are minimum requirements to proper materials selection. This approach to maintenance is certainly intended to minimize the possibility of unscheduled production shutdowns because of corrosion failures, with their attendant possible financial losses, hazard to personnel and equipment, and resultant environmental pollution. 

To summarise, unexpected corrosion failures are much more likely to occur by localised attack than by uniform attack (which can easily be detected) and although corrosion handbooks are useful for making initial choices of materials for applications where corrosion is important, critical components must be checked for life-to-fracture in closely controlled experiments resembling the actual environment as nearly as possible. 

Initially, knowledge of the process is required. It is assumed that the component is free m defects, e.g. porosity, as this will affect surface integrity, and free from residual stresses caused by any previous manufacturing process. There is also a risk in the reduction of component fatigue life associated with some surface coating processes. The compatibility between mating surfaces in service must also be addressed because of possible galvanic corrosion failure... 

Consideration must be given to possible equipment corrosion from such external sources as a corrosive atmosphere, spills, insulation, or gland leakage. For example, insulation containing trace quantities of chlorides can cause stress corrosion failure of 18-8 stainless steel vessels and piping. ... 

Pitting is regarded as one of the most insidious forms of corrosion, since it frequently leads to perforation and to a consequent corrosion failure. In other cases pitting may result in loss of appearance, which is of major importance when the metal concerned is used for decorative architectural purposes. However, aluminium saucepans that have been in service for some time are invariably pitted, although the pits seldom penetrate the metal, i.e. the saucepan remains functional and the pitted appearance is of no significance in that particular application. 

The implications of the terms predictable and unpredictable used in the context of corrosion require further consideration, since they are clearly dependent on the knowledge and expertise of the engineer, designer or corrosion designer who takes the decision on the metal or alloy to be used, or the procedure to be adopted, to control corrosion in a specific environmental situation. On this basis a corrosion failure (i.e. failure of the function of the metal due to corrosion within a period that is significantly less than the anticipated life of the structure) may be the result of one or more of the following possibilities ... 

Professor M. Fontana has made the statement that Virtually all premature corrosion failures these days occur for reasons which were already well known and these failures can be prevented . It is apparent from this statement, and from the conclusions reached by the Committee on Corrosion and Protection, that category 1 is responsible for the majority of incidents of corrosion failure that could have been avoided if those responsible were better informed on the hazards of corrosion and on the methods that should have been used to control it. 

Westbrook, J. H., The Role of Precipitation and Segregation at Grain Boundaries in Corrosion Failure , J. Electrochem. Soc., 119, 216C (1972)... 

The two principal forms of stress-corrosion failure are (a) hot salt cracking and (Z)) room-temperature cracking, the latter occurring in both aqueous and methanolic chloride environments, and in N2O4. In addition, environmental failures can occur in alloys in direct contact with some liquid and solid metals, and certain gases. 

The representation of the results from slow strain-rate tests may be through the usual ductility parameters such as reduction in area, the maximum load achieved, the crack velocity or even the time to failure, although as with all tests, metallographic or fractographic examination, whilst not readily quantifiable, should also be involved. Since stress-corrosion failures are usually associated with relatively little plastic deformation, the ductility... 

Fig. 8.94 Nominal stress-extension curves for mild steel in oil giving ductile failure, and in 4n NaN03 producing stress-corrosion failure, at the same test temperature (104°C)...

Regular shutdowns cannot be tolerated in large integrated factories and replacement sections of plant have to be maintained in readiness to operate when corrosion failure occurs. This method of dealing with corrosion can lead to a considerable increase in capital investment. 

A recent survey by du Pont on all failures in their metallic piping and equipment taking place during a 4-year period showed corrosion accounting for 55% of total failures. Table 9.3 lists the major causes of corrosion failure in this wide ranging survey. 

CORROSION IN CHEMICAL AND PETROCHEMICAL PLANT 9 17 Table 9.3 Corrosion failures analysed by type (after du Pont (1%8 to 1971))... 

S. W. Borenstein and P. B. Lindsay. MIC (microbiologically influenced corrosion) failure analysis. Mater Performance, 33(4) 43-45, April 1994. 

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