Exposure testing

One of the most common accelerated exposure tests is the salt spray or salt fog test described in ASTM B117 [84]. In this test, samples are exposed to a spray or fog of 5% NaCl solution at 30 C. Several US military specifications require salt fog testing, and many corrosion engineers have considerable experience with this test. It is therefore of use as a qualification procedure to evaluate materials. However, the results of salt spray testing do not always correlate with other types of exposure, and considerable variability in test results is not uncommon. 

ASTM G85 describes modified versions of salt spray testing in which the spray solution is acidified with acetic acid, SO2 is added, or the test conditions are cycled. These modifications result in a more corrosive environment than the standard salt spray test. 

This encyclopedia and this chapter are focused on electrochemistry. However, a few words about nonelectrochemical techniques are in order for completeness. Exposure testing of samples to service or accelerated environments is widely used, and there are several ASTM standards describing appropriate methods. Several immersion tests for intergranular and exfoliation corrosion were mentioned in the last section. ASTM G31 describes methods for immersion testing of metals [83]. This standard practice contains mostly common sense, but also has information that will help experimentalists avoid common 

The most common nonelectrochemical approach for the study of corrosion is weight loss measurements. Such measurements are limited by the resolution of the gravimetric device, and, for aqueous corrosion, are usually applied in aggressive environments such as the boiling acids used to evaluate sensitization of stainless steels in ASTM A262 [76]. The Quartz Crystal Microbalance (QCM) is a gravimetric instrument capable of submonolayer sensitivity that has been increasingly applied over recent years in the area of corrosion [85, 86]. 

The QCM takes advantage of the fact that quartz crystals have a well-defined 

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Jacques, L.F.E., Accelerated and outdoor/natural exposure testing of coatings. Prog. Polym. Sci., 25, 1337-1362 (2000). 

Rubber) open cure, -lager, n, bonded warehouse, -lagerprtifung, /. Paints, etc.) exposure test(ing). freilassen, t.t. set free, liberate. 

Determination of bimetallic corrosion in outdoor exposure tests , BS 6682 (1986)... 

The first three authors were concerned with the rusting of motorcar bodies. They found that the rusts formed on steel under sheltered and exposed conditions, respectively, differed markedly in chemical composition, structure and protective properties. The second paper gives the results of exposure tests in many different indoor atmospheres, from which the following representative rates of rusting over one year are taken. 

Table 3.21A Atmospheric exposure test, Sheffield, 5 years. Results loss weight (g/m ) ...

Fig. 3.48 Results of a 7.5 year exposure test programme on 150 X 100 mm panels at Kure...

Fig. 4.38 Atmospheric corrosion of nickel and nickel alloys during exposure tests at sites in the USA. 1, Nickel 200 2, Alloy 600 3, Alloy 800 4, Alloy 825 5, Alloy 400 (after van Rooyen...

Anderson, reporting on a series of 20-year exposure tests carried out by the ASTM, quotes the average rates of corrosion for various types of environment which are given in Table4.35. 

Tantalum is attacked, even at room temperature, by concentrated alkaline solutions. However, tantalum is fairly resistant to dilute alkaline solutions. In one long-term exposure test in a paper mill, tantalum suffered no attack in a solution with a pH of 10. Attack by alkaline solutions is summarised in Table 5.24. 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Quality control tests are intended to detect produced materials which deviate from manufacturing specifications, and thus may result in questionable performance. The materials are usually subjected to spectrographic analysis which is the primary quality control check. The exposure tests are necessarily of short duration (hours or days), in which the test conditions attempt to reflect the environment of operation, for example using artificial seawater for a marine application. Since a property that is reproducible and indicative of a consistent quality anode is all that is required, there is no attempt to mirror, except in the crudest fashion, current density profiles. 

Exposure tests of 5-years duration in the atmosphere and during immersion in fresh water or sea-water by Welding Institute and by Stanners and Watkins have indicated the best paint systems for aluminium-coated steel. For immersed conditions two systems giving good performance are ... 

An extensive compilation of atmospheric exposure test data on zinc is now available and complements the slightly earlier critical study by Schikorr . 

More recently, results of exposure tests for 10 years in a severe industrial environment at Stratford, London, have been reported by the Fulmer Research Institute". A range of pure and alloy specimens, anodised to a maximum film thickness of about 25 m, was exposed at an angle of 45°. 

In order to secure information as to changes in corrosion rates with time, as in atmospheric exposure tests, it is necessary to expose sufficient specimens to allow sets to be taken from test after at least three time intervals. 

In view of possible or probable variations in mechanical properties among different specimens of the same metal cut from different sheets or other pieces, or even from different sections of the same sheet or piece, it is necessary to pay careful attention to the initial sampling of stock to be used for control, as well as exposure, specimens. An interesting case in which several of these considerations were involved was provided by the long-time atmospheric exposure tests of non-ferrous metals carried out by Subcommittee VI of ASTM Committee B-3 on Corrosion of Non-Ferrous Metals and Alloys in which changes in tensile properties were used as one of the means of measuring the extent of corrosion. 

In considering exposure tests, whether in the form of laboratory, field or service tests it is important to consider the purpose of the test and the relevance of the data to the anti-corrosion function of the coating. Thus, in the case of paint coatings, factors such as gloss deterioration, chalking and colour retention are of considerable importance in some industries, for example the automotive industry, but perhaps of minor importance in the painting of structural steelwork. These assessment factors can nevertheless be of significance since they may be the precursors of corrosion of the basis metal. 

Test method for porosity in gold platings on metal substrates by gas exposures Test method for half-cell potentials of uncoated reinforcing steel in concrete Method for detection of copper corrosion from petroleum products by the copper strip tarnish test... 

D 1014 1983 (1988) Method for conducting exterior exposure tests of paints on steel... 

Standard reference test method for making potentiostatic and potentiodynamic anodic polarisation measurements Recommended practice for atmospheric environmental exposure testing of nonmetal-lic materials... 

US ERA, Occupational and Residential Exposure Test Guidelines, OPPTS 875.1000, Background for Application Exposure Monitoring Test Guidelines, Environmental Protection Agency, Washington, DC, p.l4 (1996). 

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