Seacoast Atmosphere

If one wants to retain a shiny metallic surface in seacoast atmosphere, even pure aluminum and all alloys will have to be protected, usually by a clear lacquer or an anodized coating. 

The 3XXX alloys and low-magnesium-content 5XXX alloys are too low in strength to be considered as structural alloys. Therefore, they are usually used as thin sheet or tubing. It is recommended that protection be provided to prevent perforation of such thin products. 

The higher strength 5XXX alloys and the 6XXX alloys have sufficient inherent resistance to corrosion that they can be used without any protective 

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

In addition to the potential for SCC from the inside as a result of the process fluids, austenitic SS s can experience SCC from the outside, if the austenitic SS is neither low carbon nor stabilized, intergranular SCC can occur as low as at room temperature as a result of either chlorides or sulfur dioxide in the atmosphere. In the 140 to 220°F (60 to 104°C) range, transgranular SCC of stabilized, low carbon, and regular grades of austenitic SS s has occurred as a result of chlorides from seacoast atmospheres. 

Two-part epoxy adhesives (RT-curing) give poor results in seacoast atmospheres unless a compatible organic sealer is placed over the edge of the... 

Fig. 2.50 Weight change of thermally sprayed coatings in a Japanese seacoast atmosphere. Key same as in Fig. 2.49 (Butler and Morrow, 1990).

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]. 

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. 

Lifka, B. W., Corrosion Resistance of Aluminum Alloy Plate in Rural, Industrial, and Seacoast Atmospheres— Paper 420, Corrosion/87, National Association of Corrosion Engineers, Houston, TX, 1987. 

FIG. 3—Bar graph comparing tensile yield strength and the highest surviving SCC test stress for 7075-T6 and T73 in the L, LT, ST directions of plate and the Land T directions of rod. The SCC results are representative of tests in ASTM G 44 and in seacoast atmosphere at Point Judith, Rl. 

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. 

Lifka, B. W., "Corrosion Resistance of Alirminum Alloy Plate in Rural, Industrial and Seacoast Atmospheres, Aluminum, Vol. 63, No. 12, 1987, pp. 1256-1261. 

The most damaging natural atmosphere for aluminum is that of a seacoast, with the severest corrosion taking place within 0.5 mile (0.8 km) of the shore line. Corrosive effects will be greatly reduced at distances of 2-5 miles (3-8 km) from the shore. Prevailing wind directions and the roughness of the surf determine the extent of the corrosive effect, which controls how far inland actual salt mist will be carried. In order to maintain the shining metallic surface of aluminum in a seacoast atmosphere, pure aluminum as well as some aluminum alloys must be protected with a coat of clear lacquer or an anodized coating. 

Alloys of the 2XXX and 7XXX with tempers that are susceptible to intergranular corrosion, exfoliation, or stress corrosion cracking should not be used in seacoast atmospheres. Thin products should be afforded protection against perforation even if they are only susceptible to pitting corrosion. For alloys whose tempers are resistant to exfoliation and stress corrosion cracking the need for protection is dependent on two factors ... 

Surface Industrial atmosphere (lap shear in MPa) Seacoast atmosphere (lap shear in MPa) ... 

The general level of corrosion resistance of alloy 15-5PH exceeds that of types 410 and 431, and is approximately equal to that of alloy-4PH. Very little rusting is experienced when exposed to 5% salt fog at 95 (35°C) for a period of 500 h. When exposed to seacoast atmospheres, rust gradually develops. This is similar to other precipitation-hardening stainless steels. The general level of corrosion resistance of alloy 15-5PH stainless steel is best in the fully hardened condition, and decreases slightly as the aging temperature is increased. 

Alloy 17-4PH has excellent corrosion resistance. It withstands attacks better than any of the standard hardenable stainless steels and is comparable to t) e 304 in most media. It is equivalent to type 304 when exposed in rural or mild atmospheres. However, when exposed in a seacoast atmosphere it will gradually develop overall light rusting and pitting in all heat-treated conditions. 

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.)... 

B.W. Lifka. 1987. Corrosion of aluminum alloy plate in mral industrial, and seacoast atmospheres. Aluminum, 12 1255-1261. 

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