Zinc flake coating to ISO 10683

A zinc flake coating consists of many ultra-thin zinc and aluminium flakes that overlap like roof tiles in an inorganic binder matrix. The coating is applied as a dispersion by dip-spinning or spraying and cured at around 200 to 300 °C. No current flows and there is no pickling, so the steel does not absorb any hydrogen. This makes zinc flake the standard coating for high-strength bolts in 10.9 and 12.9, for which zinc electroplating is ruled out because of hydrogen embrittlement. It is standardised in ISO 10683; in enquiries it usually appears under the brand names of the coating systems.

What the designations mean

A designation such as flZn/nc/480h means: zinc flake, without chromium(VI), 480 hours without base metal corrosion in the neutral salt spray test. Variants with a top coat achieve 720 or 1,000 hours; top coats also set the friction coefficient, usually within a window of 0.09 to 0.14, and provide the colour, for example silver, black or blue. The coating thickness is usually between 8 and 15 µm and therefore so small that standard 6g threads usually remain within gauge; for fine threads and tight tolerances, the allowance is planned in.

Advantages and limitations

High corrosion protection with a thin coating, no hydrogen embrittlement, cathodic protection from the zinc, defined friction coefficient for assembly by torque. Limitations: the coating is less abrasion-resistant than zinc, is not suitable for continuous temperatures above about 300 °C and relies on even application in the thread root and hexagon socket; internal threads of nuts are coated to suit or tapped oversize.

How RS Randack goes about it

Zinc flake is applied by partner companies that work to ISO 9001 and are approved for the common systems; parts are inspected before being sent to the coater and, after return, checked for appearance, coating thickness and thread gauging. The marking is stamped deep enough beforehand. On request, RS Randack supplies the partner’s coating certificate with the inspection certificate.

What you should specify

System or designation to ISO 10683, required hours in neutral salt spray, friction coefficient range and colour, whether nuts are to be coated as well, tolerance class of the thread after coating.