Seacoast atmospheric exposure

Laboratory corrosion tests for exfoliation corrosion susceptibility are a necessary tool for research and quality control engineers however, the validity of such accelerated tests depends on their relationship to realistic service conditions and their sensitivity to various degrees of susceptibility. The tests must be discriminating and yet not so severe as to be unrealistic. For the majority of engineered structures, exposure to outdoor atmospheres provides a baseline that is representative of many service conditions, except for structures that are subjected to unusual chemical environments. Experience has shown that seacoast conditions are more corrosive to aluminum alloys than inland urban and industrial conditions (see Fig. 2), and seacoast atmospheric exposure tests have been particularly useful for the validation of accelerated exfoliation tests [9]. 

Al—Mg—Si types have excehent corrosion resistance in most weathering exposures including industrial and seacoast atmospheres. Ahoys based on additions... 

Lifka, B. W. and Sprowls, D. O., Relationship of Accelerated Test Methods for Exfoliation Resistance in 7XXX Series Aluminum Alloys with Exposure to a Seacoast Atmosphere, Corrosion in Natural Environments, ASTM STP 558, ASTM International, West Conshohocken, PA, 1974, pp. 306-333. 

Publications on filiform test methodology are anticipated in the near future due to the current interest in aluminum for auto body sheet, together with the need to maintain an aesthetically pleasing painted surface. This form of corrosion can occur during exposure to seacoast atmosphere, and it has been developed at inland atmospheric exp>osure sites by spraying the specimens periodically (about three times per week) with a 3-5 % solution of sodium chloride. 

Plots of maximum depth of pitting corrosion on 1000 and 3003 alloy sheet and the percent loss in strength resulting from exposure to seacoast atmosphere at Point Judith, Rhode Island. Both curves show the self-stopping nature of pitting corrosion of aluminum but at different rates of change. The rationale behind the difference is discussed in Section 19.7. (Courtesy of Alcoa Technical Center, Marsha Egbert.)... 

Annex A2 describes a cyclic salt spray test that uses a 5% NaCl solution acidified to pH 3 with acetic acid in a spray chamber at a temperature of 49 °C (120 °F). The modified ASTM acetic acid salt/intermittent spray (MASTMAASIS) test is applicable for exfoliation testing of 2xxx (dry-bottom operation) and Ixxx (wet-bottom operation that is, with approximately 25 mm, or 1 in., of water present in the bottom of the test chamber) aluminum alloys with a test duration of 1 to 2 weeks. Results with 7075 and 7178 alloys in various metallurgical conditions have been shown to correlate well with results obtained in a seacoast atmosphere (4 year exposure at Point Judith, RI) (Ref 8). There is no record of the MASTMAASIS test being overly aggressive that is, causing exfoliation of a material that did not exfoliate at the seacoast (Ref 7). 

Specific tests frequently used are (a) neutral 5 % Sodium Chloride salt spray (ASTM B 117, Test Method of Salt Spray (Fog) Testing), (b) 3.5 % Sodium Chloride by alternate immersion (ASTM G 44, Practice for Evaluating Stress Corrosion Cracking Resistance of Metals in 3.5 % Sodium Chloride Solution), and (c) exposure to various outdoor atmospheres. Guidelines for outdoor exposure are contained in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Generic types of atmospheres used are seacoast, industrial, urban, and rural. Sometimes specific geographical locations or local chemical conditions are important because they can produce unique results [2i],... 

CF8M has excellent corrosion resistance in normal atmospheric conditions, including seacoast exposure. At worst, some slight staining may develop. It resists most water and brines at ambient temperature. Seawater may cause pitting corrosion particularly imder low-flow or stagnant conditions, or at elevated temperatures. CF8M is used for 80-100% sulfuric acid at ambient temperature. 

Exposure to various outdoor atmospheres is described in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Atmospheres include marine (seacoast), industrial, urban, and rural types. The effects of these generic atmosphere types on the corrosion behavior of aluminum alloys are described in Chapter 8. 

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