Zinc electroplating: thickness, passivates and hydrogen embrittlement

Process

Parts are cleaned, pickled or activated in acid, and plated as the cathode in a zinc electrolyte. Small parts such as fasteners are plated in rotating barrels; larger parts hang on racks.

Acid chloride baths give a bright deposit at high efficiency. Alkaline baths give more even thickness on complex shapes. After plating, parts are rinsed, baked if required, passivated and dried. A sealer or topcoat can be added for extra corrosion resistance or a set friction coefficient.

A thin coating

Electroplated zinc is much thinner than hot-dip galvanizing. ASTM B633 defines four thickness classes, 5, 8, 12 and 25 µm, matched to service conditions from mild indoor (SC1) to very severe (SC4). ISO 2081 uses a similar notation, such as Fe/Zn 8.

Because zinc corrodes at a rate set by the environment, a thin plated coating lasts far less time outdoors than a hot-dip coating several times thicker. Plating is best suited to indoor, sheltered or short-life service, or as a base for paint.

The thin, even layer is an advantage on threads. Standard 6g/6H metric fits have only a small allowance for coating, so ISO 4042 relates the maximum coating thickness on a thread to its pitch and tolerance position. Thick plating on fine threads may need a thread made undersize before plating.

Passivates: clear, yellow and black; trivalent and hexavalent

A chromate passivate is a thin conversion layer formed on the fresh zinc. It delays white zinc corrosion and sets the colour.

Hexavalent chromium passivates gave the familiar yellow and olive-drab finishes and heal small scratches. Hexavalent chromium is toxic and is restricted in the EU under the End-of-Life Vehicles and RoHS directives, so most plating now uses trivalent chromium passivates.

Trivalent passivates are available as clear or blue, iridescent and black finishes. Thick-film trivalent passivates, often with a sealer, can match or exceed older yellow hexavalent finishes in salt-spray testing. Colour no longer indicates performance, so the required passivate and test hours should be specified.

• Clear or blue: decorative, lowest corrosion protection of the passivate types.
• Yellow or iridescent: traditionally hexavalent; trivalent iridescent versions are now common.
• Black: appearance-driven; usually needs a sealer or topcoat for good corrosion resistance.

Hydrogen embrittlement and baking

Acid cleaning and electroplating release hydrogen, some of which enters the steel. In hard, high-strength steel this hydrogen can cause delayed brittle fracture under load, sometimes hours or days after assembly.

Risk rises with hardness. ISO 4042 sets hydrogen-embrittlement measures by hardness bands at 360 HV and 390 HV. Property class 10.9 bolts have a core hardness of 320–380 HV and class 12.9 bolts 385–435 HV, so both fall in the range where risk must be managed. The UK national foreword to ISO 4042 recommends that class 9.8 and 10.9 fasteners continue to be baked.

Baking heats parts after plating and before passivating, so hydrogen diffuses out of the steel. Time and temperature are chosen by the steel's strength under ASTM B850 or ISO 9588. Baking reduces the risk but does not remove it entirely, so many specifiers avoid electroplating class 12.9 bolts and use non-electrolytic zinc flake coatings instead.