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Home , Salt spray test

Advice & Counsel

Frank Altmayer, MSF, AESF Fellow AESF Foundation Technical Education Director Scientifi c Control Labs, Inc. 3158 Kolin Ave., Chicago, IL 60623-4889 E-mail: [email protected]

Sample Prep for Testing

Dear Advice & Counsel, so as to expose the underlying before ducting the and the customer In the recent course on salt spray testing, testing. The conditions of making the is essential to make sure that the cleaning is the text indicates that for salt spray test- scratch shall be as defined in Test Method done properly. ing, parts should be cleaned using a sol- D1654, unless otherwise agreed upon Now let’s take a look at the CASS test vent wipe (alcohol or acetone), and hard between the purchaser and the seller. specimen preparation language. ASTM plated parts should be washed B368 states: with a magnesium slurry. ASTM 6.5. Unless otherwise specified, the cut B368 calls out for MgO slurry washing edges of plated, coated or duplex materials “8.1. Preparation of Test Specimens only. We have been using MgO. Is there a and areas containing identification marks - Clean metallic and metallic coated problem with what we are doing? or in contact with the racks or supports specimens. Unless otherwise agreed upon, shall be protected with a suitable clean decorative /nickel/chromium Signed, stable under the conditions of the practice. or nickel/chromium immediately Inquiring Student before testing by wiping significant sur- NOTE 1—Should it be desirable to cut test faces with a cotton pad saturated with a Dear Student: specimens from parts or from preplated, slurry containing 10 g of pure magnesium The quick answer is “no,” but that would painted, or otherwise coated sheet, oxide powder (ACS reagent grade) in 100 make for a very short article, so let’s dis- the cut edges shall be protected by coating mL of distilled water. Upon rinsing in warm cuss sample preparation and exposure. them with paint, wax, tape or other effec- running water, be sure that the clean sur- ASTM B117 has the following language tive media so that the development of a face is free of water break.” for sample preparation: galvanic effect between such edges and the adjacent plated or otherwise coated metal You are definitely in compliance with “6. Preparation of Test Specimens surfaces, is prevented.” ASTM B368, assuming you are scrubbing 6.1. Specimens shall be suitably cleaned. the parts to a water break free state. For The cleaning method shall be optional Based on the above, your cleaning of deco- other types of coatings, the CASS speci- depending on the nature of the surface and rative chromium plated parts with magne- fi cation reads: the contaminants. Care shall be taken that sium oxide meets the “suitably cleaned” specimens are not re-contaminated after language. However, if you are testing dec- “Anodized aluminum parts may be cleaned cleaning by excessive or careless handling. orative chromium plated parts using a salt with inhibited 1,1,1-trichloroethane spray chamber, then I would assume that or other suitable organic solvent (see 6.2. Specimens for evaluation of paints and the nature of the plating is such that it has Warning). other organic coatings shall be prepared in a low chromium plating thickness. While accordance with applicable specification(s) magnesium oxide is very low in abrasion, Do not clean organic and other nonmetallic for the material(s) being exposed, or as I would recommend using a less abrasive coated specimens. Other methods of clean- agreed upon between the purchaser and the method of cleaning such as the solvent ing, such as the use of a nitric-acid solution supplier. Otherwise, the test specimens shall wipe recommended in the class text. for the chemical cleaning or passivation of consist of steel meeting the requirements of For hard chromium plating, the magne- stainless steel specimens, are permissible Practice D 609 and shall be cleaned and sium oxide scrub is recommended as the when agreed upon between the purchaser prepared for coating in accordance with the scrub will have no impact on the surface and the supplier. Take care that the speci- applicable procedure of Practice D 609. condition of the plated part. The scrub mens after cleaning are not recontami- does tend to remove any impregnated nated by excessive or careless handling. 6.3. Specimens coated with paints or non- oil, wax and possibly some impregnated Protect the cut edges of plated, coated or metallic coatings shall not be cleaned or resin. If the plater has used this type of multilayered materials and areas contain- handled excessively prior to test. surface impregnation, it is critical that ing identification marks or in contact with such information be supplied to the testing the racks or supports with a coating that is 6.4. Whenever it is desired to determine laboratory. Assuming such impregnation stable under the conditions of the test, such the development of corrosion from an is allowed by the customer’s specifi cation as wax, stop-off lacquer or pressure-sensi- abraded area in the paint or organic coat- for hard chromium, I would recommend tive tape. (Warning - 1,1,1-trichloroethane ing, a scratch or scribed line shall be made avoiding the magnesium oxide scrub. should be used in a well-ventilated area through the coating with a sharp instrument Communication between the person con- away from open flames.)”

March 2008 • Plating & Surface Finishing 25

Advice 3/08 25 2/29/08, 11:55:32 AM While the text of the CASS specifi cation 7.1.4. Salt solution from one specimen shall significant surfaces at the general level of indicates that wax is a suitable maskant not drip on any other specimen.” the condensate collectors. If the position on for cut edges, past experience with this test the automobile is vertical, place the part in in our laboratory and at GM’s laboratory To the above, I would add the following an incline position 15° from vertical to has found wax unsuitable, as it tends to additional guidance: allow surface wetting by the condensate. decompose and leave residuals beyond the If the position on the automobile is facing original border. A thick layer of high qual- • Expose test panels that have drilled down, rotate the part approximately 180° ity stop-off lacquer is far more suitable. holes so that the holes are at the bottom to test the significant surface. If there are (prevents runs of corrosion residuals several significant surfaces at different Exposure of test specimen over the test surface). angles, expose each surface of one or more Practices that contribute to reproducible • Expose test panels parallel to the fog specimens. salt spray results include the following: fl ow pattern. This allows free fog fl ow between panels in accordance with para- 8.2.2. Make sure the specimens do not Salt Spray Test graph 7.1.3. come in contact with each other or any ASTM B117 has the following exposure • Support the test specimen from the other metallic material or any material guidelnies for the salt spray test: bottom whenever possible. Use plastic capable of acting as a wick. hooks or plastic coated string only when “7.1.1. Unless otherwise specified, the necessary. 8.2.3. Place each specimen so as to permit specimens shall be supported or suspended • Do not allow test specimens to touch free settling of fog on all specimens. between 15 and 30° from the vertical and each other. preferably parallel to the principal direc- • Do not permit spray to impinge directly 8.2.4. Make sure the salt solution from tion of flow of fog through the chamber, on the test specimen. one specimen does not drip on any other based upon the dominant surface being specimen. tested. CASS Test ASTM B368 indicates: 8.2.5. Place the specimens in the chamber 7.1.2. The specimens shall not contact each just prior to bringing the test chamber to other or any metallic material or any mate- “ 8.2.1 Support or suspend the specimens the required temperature. rial capable of acting as a wick. 15 ± 2° from the vertical and prefer- ably parallel to the principal direction of Note that the CASS specifi cation is much 7.1.3. Each specimen shall be placed to horizontal flow of fog through the cham- stricter on exposure angle. Unless you are permit unencumbered exposure to the fog. ber, based upon the dominant surface unusually talented and have the eyes of an being tested. Support or suspend automo- eagle, the 15 ± 2° requirement can only be bile parts, however, so as to expose all met through the use of a protractor. P&SF

Finisher’s Think Tank Continued from page 24 Electrocleaning operating parameters

• Plated deposits blister. There could be Basis Metal NaOH (oz/gal) A/ft2 (anodic) Temp. (°F) Time (min) several factors. But if the electrocleaner is Steel 4-8 50-120 140-195 1-5 considered, hexavalent chromium could be Stainless steel 5-9 40-80 140-195 1-5 the culprit. The basis metal surface is pas- sivated by the chromium contaminant. Add Brass 1-1.5 10-30 130-160 1-3 a hexavalent chromium reducing agent or Copper 4-8 40-60 140-195 1-5 switch to a formulated electrocleaner that 1-1.5 15-35 130-160 1-3 contains this additive. If the electrocleaner solution turns yellow accompanied by lack White metal 0.75-1.0 10-20 120-140 1-3 of foaming, it is usually a positive sign of hexavalent chromium contamination. The anodic current densities are given for of conductivity and the brown fi lm is iron rack processes. In barrel treatments the hydroxide not dissolved due to low reserve • Dezincifi cation of brass. Electrocleaner anodic current densities would be much alkalinity. buffering is out of balance or one is using less, of a range of 10 to 20% of the values the wrong electrocleaner formulation for given in the table. NaOH denotes sodium • Corrosive pitting. This usually occurs the in tank requirement. hydroxide. The other source of caustic, if an acid pickle containing hydrochloric potassium hydroxide, can also be used. acid is dragged in, such as the second • White pits and corrosion of zinc. The electrocleaner in a double cleaning cycle. electrocleaner concentration is out of bal- Typical electrocleaning problems Chloride is oxidized to chlorine gas on the ance or one is using wrong formulation. • Etching of steel with brown fi lming. The parts forming the corrosive pits. Change to Use a formulation with correct caustic to a non-chloride containing acid or use an electrocleaner concentration may be low, inhibitor balance. P&SF or a formulation modifi cation to a higher electrocleaner with a specialized inhibitor caustic level is required. The etch appear- to address the problem. ance would resemble burning, due to lack

26 Plating & Surface Finishing • March 2008

Advice 3/08 26 2/29/08, 11:55:34 AM