A shop manager once asked me about an issue with rack-zinc plates for a customer per ASTM B633 Type lll Class FeZn8.
Frank AltmayerAccording to their customer’s lab, the shop says they were having problems passing the 12-hour salt spray test performed per ASTM B117. The shop’s outside lab didn’t agree with the results, and they wanted to know if there are written guidelines for determining whether a white spot is corrosion or a stain.
Another concern they had was that the parts were too long for the salt spray cabinet; their customer was cutting a section of the part off and using tape to cover the un-plated area for the salt spray test. They wondered if the un-plated edge could cause problems for the zinc-plated areas.
White Corrosion Spot vs. Stain
There are no written guidelines about a white corrosion spot versus a stain. As chromate conversion coatings are exposed to the corrosive environment of a salt spray chamber, it is not unusual for the chromate film to initially discolor, producing variations in coloration that may be interpreted as “stains.” Stains typically are not considered corrosion.
Let’s first discuss the mechanism for corrosion in a salt spray chamber. The salt spray test takes advantage of the phenomenon called “Oxygen Concentration Cell Corrosion.” Suppose a drop of water rests on a metal surface (see figure). In that case, there is a difference in the concentration of oxygen available to the specimen relative to the position within the drop. At the drop's center, the metal is in contact with the dissolved oxygen in the droplet and the oxygen in the hydroxide ions due to water dissociation. At the edge of the drop, additional oxygen from the air is available; thus, an oxygen concentration gradient is produced from the edge of the drop to the center.
Since oxygen may diffuse from the air outside the drop into the water at the edge of the drop, the difference in oxygen contact is greatest at the area furthest from the edge of the drop, which is the center.
Difference in Oxygen Content Creates Oxidation
According to Dr. Harold Read, a well-known expert in corrosion, this difference in oxygen content creates an oxidation potential that can be measured and is approximately 0.3 volts. The metal at the drop's center dissolves, producing a pit and electrons, which flow to the edge of the drop, where corrosion products are precipitated. It produces the white zinc oxide/hydroxide you see when examining the parts, which yields salt spray failure.
The above corrosion mechanism is accelerated in four ways in the salt spray test. First, the use of salt (sodium chloride) results in a higher solubility for metal ions in the water droplet, extending the life of each cell. Second, electrochemical reactions are accelerated by elevating the temperature of the corrosion cell. Third, by inclining the test specimen, the corrosion cell is continuously formed and replenished with fresh electrolytes; therefore, the reaction is never slowed by the accumulation of metal ions and hydroxides within the corrosion droplet. Fourth, the sprayed fine mist results in the formation of thousands of corrosion cells, which are set up and continuously replenished during the exposure period.
In summary, when the chromate conversion coating fails, the zinc under the chromate film reacts with the alkalinity within the saltwater drop to produce zinc hydroxide/oxide. If this zinc hydroxide/ oxide is not present on the surface of the test sample, there is no failure, even if the chromate is discolored.
Proving the Absence of Corrosion
If you need to prove the absence of corrosion products, wipe the stained area with a cotton swab soaked with deionized water. Transfer the deionized water to a small glass dish and acidify with a drop of any reagent-grade acid. Next, test the acidified water for the presence of zinc using an atomic absorption spectrophotometer. (Be sure to run a blank for comparison purposes. Do this by wiping and testing a chromate sample not subjected to the salt spray test.) The absence of zinc or the presence in the same concentration as the blank will confirm the absence of a corrosion product.
As for exposure of parts that are too large for the chamber, the first rule is never to use a chamber that is less than 15 cubic feet. Assuming your chamber is larger than that, and you still need to cut the specimen, you need something in the ASTM specification to prevent you from cutting it. Parts may be cut to fit the cabinet as long as what is left after cutting truly represents the sample. The cut edge must be protected from corrosion and face the bottom of the cabinet. Plastic tape (no paper) may be used, but wax is a far better mask for the cut edge. Any corrosion that appears within 1/4” from the cut edge is ignored.
Frank Altmayer is a Master Surface Finisher and an AESF Fellow, 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 received 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.
Photo courtesy https://testlabs.sirris.be
