A shop manager wrote to me about an issue with salt spray testing.
Frank Altmayer“In the recent course on salt spray testing, the text indicates that for salt spray testing, parts should be cleaned using a solvent wipe (alcohol or acetone), and hard chromium-plated parts should be washed with a magnesium oxide slurry. ASTM B368 calls out for MgO slurry washing only. We have been using MgO. Is there a problem with what we are doing?”
The quick answer is “no,” but that would make for a very short article, so let’s discuss sample preparation and exposure.
Editor’s Note: This paper was published in 2008
ASTM B117 has the following language for sample preparation:
6. Preparation of Test Specimens
6.1. Specimens shall be suitably cleaned. The cleaning method shall be optional depending on the nature of the surface and the contaminants. Take care that specimens are not recontaminated after cleaning by excessive or careless handling.
6.2. Specimens for evaluation of paints and other organic coatings shall be prepared in accordance with applicable specification(s) for the material(s) being exposed, or as agreed upon between the purchaser and the supplier. Otherwise, the test specimens shall consist of steel meeting the requirements of Practice D 609 and shall be cleaned and prepared for coating in accordance with the applicable procedure of Practice D 609.
6.3. Specimens coated with paints or nonmetallic coatings shall not be cleaned or handled excessively before test.
6.4. Whenever it is desired to determine the development of corrosion from an abraded area in the paint or organic coating, a scratch or scribed line shall be made through the coating with a sharp instrument to expose the underlying metal before testing. The conditions of making the scratch shall be as defined in Test Method D1654, unless otherwise agreed upon between the purchaser and the seller.
6.5. Unless otherwise specified, the cut edges of plated, coated or duplex materials and areas containing identification marks or in contact with the racks or supports shall be protected with a suitable coating stable under the conditions of the practice.
Note 1: Should it be desirable to cut test specimens from parts or from preplated, painted, or otherwise coated steel sheet, the cut edges shall be protected by coating them with paint, wax, tape, or other effective media so that the development of a galvanic effect between such edges and the adjacent plated or otherwise coated metal surfaces is prevented.
Based on the above, your cleaning of decorative chromium plated parts with magnesium oxide meets the “suitably cleaned” language. However, if you are testing decorative chromium-plated parts using a salt spray chamber, I would assume the plating is thin. While magnesium oxide is very low in abrasion, I recommend a less abrasive cleaning method, such as the solvent wipe recommended in the class text.
For hard chromium plating, the magnesium oxide scrub is recommended, as it will not affect the surface condition of the plated part. The scrub does tend to remove any impregnated oil, wax, and possibly some impregnated resin. If the plater has used this type of surface impregnation, it is critical that the testing laboratory be supplied with this information. Assuming the customer’s specification allows this impregnation, I recommend avoiding the magnesium oxide scrub. Communication between the person conducting the salt spray test and the customer is essential to ensure the cleaning is done properly.
Now let’s take a look at the CASS test specimen preparation language. ASTM B368 states:
8.1. Preparation of Test Specimens
Clean metallic and metallic-coated specimens. Unless otherwise agreed upon, clean decorative copper/nickel/chromium or nickel/chromium coatings immediately before testing by wiping significant surfaces with a cotton pad saturated with a slurry containing 10 g of pure magnesium oxide powder (ACS reagent grade) in 100 mL of distilled water. Upon rinsing in warm running water, be sure that the clean surface is free of water break.
You are in compliance with ASTM B368, assuming you scrub the parts to a water break-free state. For other types of coatings, the CASS specification reads:
Anodized aluminum parts may be cleaned with inhibited 1,1,1-trichloroethane or other suitable organic solvent (see Warning).
Do not clean organic and other nonmetallic coated specimens. Other cleaning methods, such as using a nitric acid solution for chemical cleaning or passivation of stainless steel specimens, are permissible when agreed upon between the purchaser and the supplier. Ensure that the specimens are not recontaminated after cleaning by excessive or careless handling. Protect the cut edges of plated, coated, or multilayered materials, and areas containing identification marks or in contact with the racks or supports, with a coating that is stable under the conditions of the test, such as wax, stopoff lacquer, or pressure-sensitive tape. (Warning: 1,1,1-trichloroethane should be used in a well-ventilated area away from open flames.)
While the text of the CASS specification indicates that wax is a suitable maskant for cut edges, experience with this test in our laboratory and at GM’s laboratory has found wax unsuitable, as it tends to decompose and leave residuals beyond the original border. A thick layer of high-quality stop-off lacquer is far more suitable.
Exposure of Test Specimen
Practices that contribute to reproducible salt spray results include the following:
Salt Spray Test
ASTM B117 has the following exposure guidelines for the salt spray test:
7.1.1. Unless otherwise specified, the specimens shall be supported or suspended between 15 and 30° from the vertical and preferably parallel to the principal direction of flow of fog through the chamber, based upon the dominant surface being tested.
7.1.2. The specimens shall not contact each other or any metallic material or any material capable of acting as a wick.
7.1.3. Each specimen shall be placed to permit unencumbered exposure to the fog.
7.1.4. Salt solution from one specimen shall not drip on any other specimen.
To the above, I would add the following additional guidance:
- Expose test panels with drilled holes so the holes are at the bottom (prevents runs of corrosion residuals over the test surface).
- Expose test panels parallel to the fog flow pattern. This allows free fog flow between panels in accordance with paragraph 7.1.3.
- Support the test specimen from the bottom whenever possible. Use plastic hooks or plastic-coated string only when necessary.
- Do not allow test specimens to touch each other.
- Do not permit spray to impinge directly on the test specimen.
CASS Test
ASTM B368 indicates:
8.2.1 Support or suspend the specimens 15 ± 2° from the vertical and preferably parallel to the principal direction of horizontal flow of fog through the chamber, based upon the dominant surface being tested. Support or suspend automobile parts, however, to expose all significant surfaces at the general level of the condensate collectors. If the position on the automobile is vertical, place the part at a 15° incline to allow the condensate to wet the surface. If the position on the automobile is facing down, rotate the part approximately 180° to test the significant surface. If several significant surfaces are at different angles, expose each surface of one or more specimens.
8.2.2. Make sure the specimens do not come in contact with each other or any other metallic material or any material capable of acting as a wick.
8.2.3. Place each specimen to permit free settling of fog on all specimens.
8.2.4. Make sure the salt solution from one specimen does not drip on any other specimen.
8.2.5. Place the specimens in the chamber just before bringing the test chamber to the required temperature.
Note that the CASS specification is much stricter on exposure angle. Unless you are unusually talented and have the eyes of an eagle, the 15 ± 2° requirement can only be met through the use of a protractor.
Frank Altmayer is a Master Surface Finisher and an AESF Fellow and the technical education director of the AESF Foundation and NASF. He owned Scientific Control Laboratories from 1986 to 2007 and has over 50 years of experience in metal finishing. He was the recipient of the AESF Past Presidents Award, NAMF Award of Special Recognition, AESF Leadership Award, AESF Fellowship Award, Chicago Branch AESF Geldzahler Service Award, and NASF Award of Special Recognition.
