ASTM B556-90 pdf free.Standard Guide for Measurement of Thin Chromium Coatings by Spot Test.
A drop of hydrochloric acid (test solution) is depositedon the surface of the test specimen,and the time required forthe hydrochloric acid to penetrate through the chromiumcoating (penetration time) is measured.The coating thicknessis proportional to this time.
The thickness of a decorative chromium coating is oftencritical to its performance. This procedure is useful for an approximate determina-tion when the best possible accuracy is not required.For morereliable determinations， the following methods are available:Methods B 504, B 568, and B 588.
This test assumes that the rate of dissolution of thechromium by the hydrochloric acid under the specified conditions is always the same.
For chromium on nickel the test solution is reagentgrade hydrochloric acid having a specific gravity at 16°c of1.180 0.002. (This corresponds to 11.5 N  0.2 N, whichmay be checked by titration.) For chromium on stainless steelthe test solution is 20 g/L of antimony trioxide_dissolved inreagent grade hydrochloric acid having a specific gravity at16°C of 1.160.As received，reagent grade hydrochloric acid is normallymore concentrated than 11.5 N.
The test area must be free of foreign material. Clean byrubbing the test area with a paste of magnesium oxide, rinse it,and dry it with a clean cloth or filter paper. Draw a ring with adiameter of about 6 mm on the test area with melted paraffin orwith a wax pencil.
Let the test specimen, the test solution, and the dropperstand long enough to reach room temperature, which should bebetween 16 and 25°C. Temperatures up to 30°C are permis-sible, but the measurements become less reliable at the highertemperatures because of increasing sensitivity to temperature.Thin test specimens should be set on a heavy metal plate toavoid a rapid change in temperature which could be producedin such specimens by the heat of reaction.
To determine the penetration time，t,deposit a drop(0.03 to 0.05 mL) of the test solution inside the ring of wax,and measure the time between the beginning of the gasformation and the first appearance of nickel. Make this measurement to the nearest 1/2 s with a stop watch. If thereaction,that is, the formation of gas bubbles， does not startimmediately,it can be started by touching the test surfacewithin the ring with a fine nickel wire.The end point of thepenetration is characterized by the cessation of the gassing andthe appearance of the yellow color of the nickel surface. Theremay be an uncertainty of 2 or 3 s in determining when the endpoint has been reached.ASTM B556 pdf download.