Hardide is particularly effective at protecting against mineral acids, including HCl, Nitric acid and H2SO4. It can even resist Aqua Regia at room temperature; particularly notable as this mixture of hydrochloric and nitric acids is capable of dissolving noble gold. The coating was tested alongside a WC/Co detonation coating for resistance to nitric acid. The image below shows two Hardide coated samples: one untested and one tested for 113 hours in 20% nitric acid. The Hardide sample is a yellowish colour due to slight surface oxidation, meanwhile its dimensions have not changed, the weight loss was not measurable - less than 0.001 g and its surface roughness remained the same as before testing - 0.10 micron Ra - which all indicates that the coating had not been attacked.
Hardide-coated 316 stainless steel samples: untested (left) and tested for 113 hours in 20% Nitric acid (right).
In the same test, the detonation coated sample has changed colour to dark grey, while the acid solution became rose coloured due to Cobalt leaching from the sample. The weight loss of the WC/Co sample after 46 hours 40 min was approx. 0.3 g, The roughness of the detonation sample before testing was 0.10 microns Ra. After testing in 20% acid this increased to 0.41 microns Ra due to metal binder leaching. As a result of the increase in roughness the detonation coating can become extremely abrasive for seals and packing when exposed to aggressive media.
The image below shows a 4140 steel test ring with 50 microns of Hardide coating after immersion into uninhibited 28% Hydrochloric acid for 24 hours. The appearance of this sample has not changed. No measurable weight loss or surface roughness change were detected.
4140 steel test ring with 50 microns Hardide-T coating after 24-hours immersion in uninhibited 28% HCl - no appearance change or weight loss were detected
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