Schichtsysteme

Black Spots in the Salt Spray Test

Black spots occuring during the salt spray test are a known phenomenon. Even the DIN 50961 (from june 1987) mentions on page 4, chapter 10.2.1.2:

"... Applying the neutral salt spray test, the corrosion resistance of passivation layers on galvanic zinc depositions shall be tested. The beginning of the reduced protecting potential is defined with the first occurence of corrosion products of the deposited metal on the passivated surface. Colour changes of the passivation layers and the occurence of dark spots are not to evaluate, if not declared else for decorative applications."

Hence, black spots only represent a reduced corrosion protection, if they are corrosion products of the zinc layer beneath the passivation. However, in fact and depending on the type of coating, the dark spots consist of the following:

chromium(III) chromate in case of hexavalent passivations on pure zinc (extremely rare)

iron chromate in case of hexavalent passivations on zinc, which is contaminated with iron (often occures when zinc electrolytes are controlled badly)

iron(II)iron(III) oxide (Fe3O4) in case of trivalent passivations on zinc, which is contaminated with iron

In any case, it is a matter of a modification of the passivation layer, which is - according to the definition - not subject of evaluating the beginning of reduced corrosion protecting. Black spots are not corrosion products of the zinc, which are known to be white.

According the DIN, varying agreements between customer and producer concerning the evaluation of the neutral salt spray test and optical changes of the surface are possible.

Black Spots in the Salt Spray Test

Black spots occuring during the salt spray test are a known phenomenon. Even the DIN 50961 (from june 1987) mentions on page 4, chapter 10.2.1.2:
"... Applying the neutral salt spray test, the corrosion resistance of passivation layers on galvanic zinc depositions shall be tested. The beginning of the reduced protecting potential is defined with the first occurence of corrosion products of the deposited metal on the passivated surface. Colour changes of the passivation layers and the occurence of dark spots are not to evaluate, if not declared else for decorative applications."
Hence, black spots only represent a reduced corrosion protection, if they are corrosion products of the zinc layer beneath the passivation. However, in fact and depending on the type of coating, the dark spots consist of the following:
chromium(III) chromate in case of hexavalent passivations on pure zinc (extremely rare)
iron chromate in case of hexavalent passivations on zinc, which is contaminated with iron (often occures when zinc electrolytes are controlled badly)
iron(II)iron(III) oxide (Fe3O4) in case of trivalent passivations on zinc, which is contaminated with iron
In any case, it is a matter of a modification of the passivation layer, which is - according to the definition - not subject of evaluating the beginning of reduced corrosion protecting. Black spots are not corrosion products of the zinc, which are known to be white.
According the DIN, varying agreements between customer and producer concerning the evaluation of the neutral salt spray test and optical changes of the surface are possible.