Salt spray test

As indicated in Table 6, salt spray testing is the most popular form of testing for protective coatings. These tests have been used for more than 90 years as accelerated tests in order to determine the degree of protection afforded by both inorganic and organic coatings on a metallic base. Table 5 lists several widely used salt spray tests. 

The acetic acid-salt spray (fog) test (ASTM G 85, Annex Al Former Method B 287) is also used for testing inorganic and organic coatings but is particularly applicable to the study or testing of decorative chromium 

This test can be as brief as 16 h, although it normally ranges from 144 to 240 h or more. As in the neutral salt spray test, a 5% NaCl solution is used, but the solution is adjusted to a pH range of 3.1 to 3.3 by the addition of acetic acid, and again, the temperature of the salt spray cabinet is controlled to maintain 35 -l- 1.1 or —1.7 °C (95 -H 2 or — 3 °F) within the exposure zone of the closed cabinet. 

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Product Utilization. The principal appHcation for chromium phosphate coatings is as a paint base for painted aluminum extmsions and aluminum beverage can stock. In these appHcations, extremely demanding performance criteria are met by the chromium phosphate conversion coatings. As an example, the Architectural Aluminum Manufacturer s Association Voluntary Specification 605.2-92 requires humidity and salt spray testing for 3000 hours and allows only minimal incidence of paint failure after testing (26). 

B287 acetic acid salt spray test... 

Sa.lt Spray Tests. One of the older accelerated corrosion tests is the salt spray test (40). Several modifications of this imperfect test have been proposed, some of which are even specified for particular appHcations. The neutral salt spray test persists, however, especially for coatings that are anodic to the substrate and for coatings that are dissolved or attacked by neutral salt fog. For cathodic coatings, such as nickel on steel, the test becomes a porosity test, because nickel is not attacked by neutral salt fog. Production specifications that call for 1000 hours salt spray resistance are not practical for quahty acceptance tests. In these cases, the neutral salt spray does not qualify as an accelerated test, and faster results from different test methods should be sought. 

The reproducibility of test results between labs using the neutral salt spray tests has not been consistent, but the repeatability, within one lab, is better, and the test has value in comparing variations in coating systems. Correlation of hours of exposure in the salt spray test to actual performance of the plated part in service, even in marine atmospheres, is not consistent and usually avoided. A classic example is that cadmium deposits outlast zinc deposits on steel in salt spray tests and clean marine atmospheres, yet zinc outlasts cadmium when exposed to real, industrial atmospheres, because of the presence of sulfur-bearing corrodents in industrial environments. An important variable in salt spray testing is the position of the surface to be tested. Whereas the surface of test panels is specified to be 15—30° from the vertical (40), when salt spray testing chromated zinc-plated specimens, this range has appeared excessive (41). 

Salt spray tests, humidity tests, and other accelerated tests, some usiag sulfur dioxide and carbon dioxide, have shown favorable results for tin—2inc ia comparison with 2iac, cadmium, and fin deposits. Chromating improves the performance. 

As of this writing the 2inc alloys are too new to have actual corrosion resistance data, except for that based on accelerated tests. Zinc—nickel usually shows better results than 2inc-cobalt in salt spray tests. The reverse is tme when the Kesternich test is used. Tin—2inc performs well in both salt spray and Kesternich tests, but appears only to equal 2inc plating and 2inc—nickel in humidity tests. 

Corrosion-resistance test This can be done with the help of a salt spray test. The test piece is suspended in a salt spray chamber (Figure A13.6) for. seven days in 100% relative humidity (IS 101 and IS 11864). After the test, the surface should have no signs of deterioration or corrosion. 

Ductile and easily buffed chromium deposits having satisfactory corrosion resistance have been produced thus 0.005 mm-thick chromium deposits applied to steel by chemical deposition or by eiectrodeposition gave simiiar results when subjected to a salt-spray test . 

Resistance to corrosion Most authors who compare resistance to corrosion of electroless nickel with that of electrodeposited nickel conclude that the electroless deposit is the superior material when assessed by salt spray testing, seaside exposure or subjection to nitric acid. Also, resistance to corrosion of electroless nickel is said to increase with increasing phosphorus level. However, unpublished results from International Nickel s Birmingham research laboratory showed that electroless nickel-phosphorus and electrolytic nickel deposits were not significantly different on roof exposure or when compared by polarisation data. 

Even small traces of certain corrosion stimulants, notably soluble chlorides and sulphates, can maintain a continuing corrosion process under a paint film because the salts accelerate the initial dissolution of ferrous iron (and other metal ions) but are not immobilised in the hydrated oxide corrosion products. Filiform corrosion is the most spectacular example of this phenomenon, but progressive spread, preceded by blistering, is also observed from scratches or other breaks in a coating, for example during salt spray tests. 

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

The main value of salt-spray tests is in the evaluation of the effectiveness of phosphate coatings in restricting the spread of rust from scratches or other points of damage in a paint film. This feature is of particular interest to the motorcar industry, as vehicles are often exposed to marine atmospheres and to moisture and salt when the latter is used to disperse ice and frost from road surfaces. Great care is needed in the interpretation of a salt-spray test, as it has been found to favour thin iron phosphate coatings more than is justified by experience with natural weathering. In the motorcar industry the present custom is to use zinc phosphate coatings on the car bodies and all other parts exposed to the outside atmosphere. 

Humidity tests are generally of more practical use than salt-spray tests, particularly where painting is employed, as the thoroughness of rinsing may be checked by this means. The use of contaminated water can leave... 

Table 15.13. This specification follows good industrial practice, with additional safeguards in rinsing to remove residues to treatment solutions. Nonaccelerated treatments must be followed by a single rinse which may contain chromate accelerated treatments must be followed by three rinses—cold water, hot water and a final chromate rinse. Table 15.14 shows the salt-spray test requirements for phosphate coatings with various finishes without formation of rust the paints and lacquer have the additional requirement that no rust shall be visible beyond 0-2 in (5 mm) from the deliberate scratches and no blistering, lifting or flaking beyond 0-05 in (1-27 mm) from the original boundaries of the scratches.

The American Aeronautical Material Specification AMS 2480 A calls for 150h salt-spray test without rusting extending more than 0-125 in (3-175 mm) on either side of scratch marks, using a black enamel finish for the phosphate coating. 

Fig. 19.11 Effect of specimen position on corrosion in salt-spray tests specimens of cold-rolled steel, (a) 20% NaCl, (i>) synthetic sea-water...

The results will also be influenced by the concentration of NaCl solution sprayed —some metals are affected more by one concentration than another — for example, zinc is corroded most by a concentrated brine (20%), while iron is corroded most by a dilute brine (3%) synthetic sea-water is less corrosive to these metals than either brine. In view of the many other ways by which the conditions within a salt-spray box differ from those of exposure to a natural sea-coast environment, there seems to be no great advantage in making-up complicated synthetic sea-waters for use in salt-spray testing. However, tablets for this purpose are commercially available. 

The salt spray test has seemed to yield the most consistent results when used to establish the relative merits of different aluminium alloys in resisting attack by marine atmospheres. The best results have been secured when the spray has been interrupted for so many hours each day . 

For additional information on some of the features of the salt spray test and its limitations in respect of certain of the purposes for which it may be used, reference should be made to the book Corrosion Testing for Meta Finishing prepared by the Institute of Metal Finishing. 

Acetic acid salt spray test (ASS test) Copper-accelerated acetic acid salt spray test (CASS test)... 

Corrosion tests in artificial atmospheres salt spray tests... 

The solution was applied to the surface of cold-rolled steel, dip-galvanized steel sheet, and aluminum strip for degreasing and passivating in a single operation at 40°C for 90 s. The surfaces were then lacquered (85-100 pm thickness) and tested in a salt-spray test for 480 h without showing subsurface migration and blistering. The method is especially useful in the automobile industry for coated sheet [191]. 

Salt sensitivity, 22 813 Salt spray test, 9 790 Salt substitutes, 22 819-820 Salt-sulfuric acid metal chloride decomposition process, 73 823 Saltville, Virginia, salt production at, 22 800... 

Determination Test, Weight loss test, Eschke test, Salt spray test and Sulphur dioxide test at 50°C. Mechanism of inhibition of corrosion and surface study of mild steel was also studied by Metallurgical Resesarch Microscopy and Scanning electron Microscopy. 

Simplicity and reliability of operation make AC impedance measurements attractive as a technique in the evaluation of coating integrity. As opposed to classical salt spray test, analysis times are shorter with the AC impedance technique and quantitative data are obtained permitting relevant mechanistic Information to be derived. Impedance test methods are likely to find many applications in the resolution of unsolved practical problems ( .) ... 

Thus it appears that by incorporating parameters such as pore resistance and coating capacitance to the existing theoretical impedance model dealing with metal dissolution one would obtain valuable overall information (14,27). Complemented by results from regular immersion and salt spray tests it should be possible to find satisfactory solutions to corrosion problems of coated metals (9 ). 

Coated specimens were placed in an electrochemical cell. After 4 hours of temperature, open-circuit potentials were measurements were made on duplicate samples, in a salt spray test cabinet (ASTM B117-73) for 1, 17 and 96 hours respectively and their surfaces photographed in order to calculate the percentage of surface covered by corroded spots and blisters (ASTM D610-68). 

Specimen 19 tested for Impedance after exposure to salt spray tests shows a low charge transfer resistance of 4300 0 cm and a fairly high double layer capacitance of 316 pF cm . This shows the Increased tendency of NaCl solution to penetrate the paint film exposed to salt spray test. The capacitance Is even higher for specimen 21 prepared with no phospho-chromic rinse. 

It Is clear on analyzing the results obtained from specimens 18 and 19 as well as from 20 and 21 that film degradation and coating Integrity can be followed more efficiently by Impedance measurements than by salt spray testing (Table II and Figures 1 and 2). 

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