Salt spray testing seawater

Carter also quotes Le Que on the unsuitability of salt spray tests for sacriflcial coatings and comments on the failure of the latter investigator to place cadmium and zinc in the same order as in outdoor exposure. He makes the further point that the comparative performance of zinc and cadmium varies with the composition of the salt solution used for spraying. For example, the superiority of cadmium is lost when natural seawater (containing, e.g., magnesium salts also) is substituted for a pure sodium chloride solution. It is suggested that variations in relative performance may be explained by the nature of the corrosion products formed and the extent to which condensation occurs on the surface of the specimens. 

Annex 4, Salt/SO, Spray (Fog) Testing, may be performed using either a 5 % salt solution or synthetic seawater solution. Like the salt spray test, it is conducted at 35°C. The fog may be continuous or intermittent. Depending upon the chosen cycle, the sulfur dioxide is injected into the chamber four times a day for 1 h or within a cycle of VS-h salt fog, A-h SO injection, and 2-h soak [4]. The test was developed by the Navy to simulate exfoliation corrosion on aircraft carriers [6]. 

The most widely used cabinet test is the neutral salt spray (Fog) test (ASTM B 117), which consists of a fog of 5 % sodium chloride within the chamber at 35 C [46. Controversy exists over the validity of B 117 as a performance test because corrosion mechanisms are not always the same as those observed in automobile service. Also, not all materials can be successfully evaluated in the test. However, the value of the salt spray test as a quality assurance test is well documented [46]. Several modifications to the salt spray test have been developed including acetic acid salt spray (ASTM G 85, Annex 1), copper accelerated acetic acid salt spray (ASTM B 368), acidified synthetic seawater fog (ASTM G 43, Method of Acidified Synthetic Seawater (Fog) Testing), and modified salt spray (ASTM G 85). ASTM G 85 also includes cyclic tests. 

In either the solution treated or aged condition, it is corrosion resistant to seawater, salt emd other natural environments, oxidizing media, inhibited reducing acids, alkalies, and metallic chlorides at room temperature. In salt-spray tests, aged Ti-13V-llCr-3Al exhibits no pitting and experiences no general corrosion or degradation in mechanical... 

Immersion tests in distilled water, tap water, and seawater have shown that chromized steels exhibit high corrosion resistance to these solntions. After 8 h testing in a salt spray fog at 35°C, chromized steel was less affected than alnminized steel, galvanized steel, or a 17% Cr steel. 

Since many inhibitors occur naturally in water, corrosion in practice is often less than in the laboratory where solutions of pure chemicals are used. Perhaps the best example is the use of tests in sodium chloride solution or spray as a guide to behavior in seawater. Seawater contains many other substances, of which magnesium salts in particular are inhibitors of zinc corrosion. Additions that can be made to water to reduce its corrosivity are given in relation to specific waters later. 

Annex 3, Acidified Synthetic Sea Water (Fog) Testing, is commonly referred to as the SWAAT test and is often used to test aluminum alloys. Rather than a 5 % salt solution, a synthetic seawater solution is used with the pH adjusted to 2.8-3.0. This aimex includes recommendations regarding the chamber operating temperature for different substrates. The chamber must be equipped with a device to conduct the following cycle 30-min seawater spray (fog), followed by a 90-min period at a relative humidity greater than 98 %. The high relative humidity is achieved by maintaining approximately 1 in. of water in the bottom of the chamber [4]. 

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