about corrosion against zinc molten

[denghans]

i have done some tests about the corrosion resistance in zinc molten for HVOF WC-12Co low carbon coating for 3 times recently.
first time: the HVOF WC-12Co low carbon was turned at a low speed, aluminum was added into zinc molten inorder to control the amount ( 0.2%), but in fact we found there was only slight aluminum found in zinc molten. the coating thickness was about 150micron. after testing for about 360h, most of the coating was observed, and testing for 720h, there was coating on the specimen still.
second and third time: the specimen was immersed into zinc without rotation, and aluminum was not added because we found that al could not be added into zinc molten. the coating thickness was about 100micron. unfortunately we found serious corrosion was occurred for all the specimens for only 240h. coating was not observed on the specimens and the diameter of 316L substrate diminished from 25mm to 23mm.
all the as sprayed coating had very dense microstructure and almost no Cobalt was found from XRD analysis. all the specimens were seale using a special sealing reagent. the temperature was ranged from 450-490 oC for zinc molten.
I think it may be connected with the al addition, but i do not know the reason.
so, from the description shown above, could you help me find the key problems .
TKS

[Gordon]

Hi denghans

to the Surface Engineering Forum.

This thread may be worth reading.

[djewell]

We have developed a thermal spray coating for protecting rolls in a molten a zinc bath. It works much better than WC-12Co. It does not dissolve in the bath, and it prevents formation and sticking of dross. The coating is applied with HVAF, so it is quite economical. It is based on molybdenum boride. Contact me off-line for more details. I have a paper which I can send you about the test.

[denghans]

thank you for your kind instruction