Seacoast environment

For most adhesive bonded metal joints that must see outdoor service, corrosive environments are a more serious problem than the influence of moisture. The degradation mechanism is corrosion of the metal interface, resulting in a weak boundary layer. Surface preparation methods and primers that make the adherend less corrosive are commonly employed to retard the degradation of adhesive joints in these environments. 

FIGURE 15.21 Shear strength of mild steel joints, (a) two-part epoxy, (b) two-part acrylic, (c) anaerobic acrylic, (d) cyanoacrylate, (e) PVC plastisol, (/) one-part heat cured epoxy.  

Resistance of the adhesive joint to salt climates depends not only on the type of adhesive but also on the method of surface preparation and on the type of primer used. The good bond durability in saltwater exposure of anodized surface pretreated joints has been shown 

Room temperature curing, two-part epoxy adhesives are usually considered to have a lower level of outdoor performance than the heat cured adhesives. Their performance, however, can be improved by heat treatment following the room temperature cure. 

Many organic adhesives tend to be susceptible to chemicals and solvents, especially at elevated temperatures. Most standard tests to determine chemical resistance of adhesive joints last only 30 days or so. Unfortunately, exposure tests lasting less than 30 days are not applicable to many service life requirements. Practically all adhesives are resistant to these fluids over short time periods and at room temperatures. Some epoxy adhesives even show an increase in strength during aging in fuel or oil over these time periods. This effect is possibly due to either postcuring or plasticizing of the epoxy by the oil. 

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Type 304 is listed because it Is the least costly of the acceptable materials. Other 300 series SS s may be needed for considerations other than low temperature. For example, low-carbon grades are desirable for seacoast environments to avoid intergranular stress corrosion cracking during the periods when the material is not at cryogenic temperature,... 

Table 10.7 Effect of Surface Treatment and Exposure to Seacoast Environment on the Durability of 6061-T6 Aluminum Alloy Joints Exposed in the Unstressed Condition (Nitrile-Modified Epoxy Paste Adhesives) ...

In a review of previous work, DeLollis concluded that the nitrile-phenolic is the most durable adhesive, surviving all environmental exposures. The nylon-epoxy adhesive performed well on exposure to medium-humidity environments, but was susceptible to high humidities. The nitrile-epoxies showed good resistance to most outdoor exposures except the seacoast environment, where bonds failed after four years. Epoxy/polyamide bonds gave good results after four years exposure to an industrial environment and joints exposed to a tropical climate had retained about 33% of their initial strength after three years. This performance should be compared with that of some heat-cured adhesives, for which the bonds are completely degraded by tropical exposure. 

ASTM D 512 provides a method for measuring the chloride ion concentration of water. The sulfate ion content is measured in much the same way as the chloride ion test. ASTM D 516 provides a method of measuring the sulfate ion concentration in water. Titration methods and water testing kits can also be used. Soil chlorides can be found naturally in soils, or can be introduced by deicing salts and in a seacoast environment. 

The results of studies with vapor-degreased, acid-etched, and sulfuric acid-anodized surface joints exposed to a seacoast environment are given in... 

Although empirical in nature, the threshtdd value provides a valid measure of the relative susceptildities of aluminum alloys to SCC under the ledfic conditions of a particular test or environment. Also, for some alloy/ per combinations, results of accelerated laboratory tests reliably predict stress-corrosion po--formance in service for example, results of an 84 day alternate inunersion test of alloy 7075 and alloy 7178 products correlated well with performance of these products in a seacoast environment. 

Differences in performance are also noted depending on the specific nature of the environment. Differences can be expected in joint durability when bonds are exposed to an aggressive wet-freeze-thaw cycle, marine seacoast, or inland environments. Outdoor weathering conditions are often classified by one of the following exposure conditions ... 

By far, the most detrimental factors influencing adhesives aged in a nonseacoast environment are heat and humidity. The reasons why warm, moist climates degrade many adhesive joints were presented in the last section. Near the seacoast, corrosion due to salt water and salt spray must also be considered when one is designing an adhesive joint. Thermal cycling due to weather, oxygen, ultraviolet radiation, and cold are relatively minor factors with most structural adhesives. 

The economic development of Dobrogea County, particularly of its seacoast area, during the last 4-5 decades had a great impact on the natural terrestrial and aquatic environment. As there has been practically no concern regarding the protection of the environment, this impact has appeared with all the intensity, which has lead to the occurrence of degradation processes that according to their intensity have altered the environment in different ways. 

This area includes interior or exterior locations on or near a seacoast or industrial environments handling brine or other salts. Under these conditions the surfaces are frequently wet by salt water and include condensation, splash, or spray. 

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. 

The stainless steels have excellent corrosion resistance in outdoor environments. They will suffer some rust staining when exposed to chlorides (seacoast, deicing salts) and/or acid industrial fumes. The higher the alloy content of the stainless steel, particularly chromium, molybdenum, and nitrogen, the better the performance. 

Operational Environment Seacoast Tropical environment Humidity Industrial environment Ground-air ground cycle... 

The majority of aluminum products are used in what would be considered an outdoor atmosphere. Outdoor atmospheres are classified as seacoast, urban or industrial, and rural. Corrosion of aluminum is self-limiting in all of these environments and the rate of corrosion becomes very slow within 5 years, except for alloys and tempers that incur exfoliation. 

Fig, 3 ° parison of exfoliation of alloy 2124 (heat treated to be susceptible EXCO EO rating) in various seacoast and industrial environments. Specimens were 13 mm (Vfi in.) plate. 

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