The standard "Volvo STD 5711-102 Corrosion testing in artificial atmospheres - Salt spray tests - Inorganic coatings" developed by Volvo Cars describes the apparatus, reagents and a procedure to be used when performing neutral salt spray, acetic acid salt spray and cuprous acetic acid salt spray tests for the evaluation of the corrosion resistance of metallic materials with or without permanent corrosion protection or temporary corrosion protection. This standard also provides a method to evaluate the corrosiveness of the test chamber environment.
This standard does not specify the dimensions of test pieces, the exposure time to be used for a particular product, or the interpretation of the results. Such details are provided in the appropriate product specifications.
Salt spray tests are particularly useful for detecting discontinuities such as pores and other defects in certain metallic, anodic oxide, and conversion coatings.
Neutral salt spray (NSS) testing is applied to:
The acetic acid salt spray (AASS) test is particularly useful for testing decorative coatings composed of copper + nickel + chromium or nickel + chromium. It has also been found suitable for testing anodic oxide coatings on aluminum.
The cuproacetic acid salt spray (CASS) test is useful for testing decorative coatings composed of copper + nickel + chromium or nickel + chromium. It has also been found suitable for testing anodic coatings on aluminum.
When preparing sodium chloride solution for use in tests, a sufficient amount of sodium chloride is dissolved in distilled or deionized water having a conductivity of not more than 25 µS/cm at 2 degrees plus/minus 20 degrees to obtain a concentration of 50 g/l plus/minus 5 g/l. The sodium chloride concentration of the sprayed solution collected from the downflowing fog should be 50 g/l plus/minus 5 g/l. The density of a 50 g/l plus/minus 5 g/l solution at 25 degrees is between 1.0255 and 1.0400.
Sodium chloride shall contain less than 0,001 percent (m/m) copper and less than 0,001 percent (m/m) nickel as determined by atomic absorption spectrophotometry or another analytical method of comparable sensitivity. It shall not contain more than 0,1 percent (m/m) sodium iodide or 0,5 percent (m/m) total impurities calculated for dry salt.
If the pH value of the prepared solution measured at 25 degrees plus/minus 2 degrees is outside the range of 6,0 to 7,0, it is investigated whether there are any unwanted impurities in the salt or water. The pH value of the salt solution is adjusted to the desired value according to the pH value of the collected sprayed solution.
When performing the NSS test, the pH value of the salt solution is adjusted so that the pH value of the sprayed solution collected in the test chamber is between 6,5 and 7,2. The pH value is checked using an electrometric measurement at 25 degrees plus/minus 2 degrees or using a short-range pH paper that can be read in increments of 0,3 pH value or less in routine checks. Necessary corrections are made by adding analytical grade hydrochloric acid or sodium hydroxide solution. Possible changes in the pH value may be due to the loss of carbon dioxide in the solution when it is sprayed. Such changes can be prevented, for example, by heating the solution to a temperature above 35 degrees before placing it in the apparatus or by reducing the carbon dioxide content of the solution by making the solution from freshly boiled water.
When performing the AASS test, a sufficient amount of glacial acetic acid is added to the salt solution and the pH of the spray solution samples collected in the test chamber is ensured to be between 3,1 and 3,3. If the pH of the initially prepared solution is between 3,0 and 3,1, the pH of the spray solution is likely to be within the specified limits. The pH is checked using electrometric measurements at 25 degrees plus/minus 2 degrees or using short-range pH paper that can be read in increments of 0,1 pH or less in routine checks. Necessary corrections are made by adding glacial acetic acid or analytical grade sodium hydroxide.
When performing the CASS test, a sufficient amount of copper(II) chloride dihydrate is dissolved in the salt solution to obtain a concentration of 0,26 g/l plus/minus 0,02 g/l (equivalent to 0,205 g plus/minus 0,015 g copper(II) chloride dihydrate per litre). The pH value is adjusted using the AASS test procedure.
If necessary, the solution is filtered before being introduced into the device's chamber to remove any solids that may clog the sprayer's holes.
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