Comparative Properties of various Zinc Metal Coating

Characteristic of the coating

Hot Dip Galvanizing

Electro / Zinc Plating

Mechanical Coating

Zinc Spraying

Zinc Dust Painting

Adhesion

Coating is integral with the steel because the for-mation process produces zinc/iron alloy layers over coated with zinc.

Good,  comparable with other electroplated coat-ings.

Good, comparable with electroplated coatings.

Good mechanical interlock-ing provided the abrasive grit blasting pretreatment is carried out correctly.

Good - abrasive grit blasting of the steel gives best results

Continuity and Uniformity

Good - any discontinuities are readily visible as "black spots". Some excess zinc at drainage points on products.

Uniform within limita-tions of "throwing power" ot bath. Pores not a problem, as ex-posed steel protected by adjacent zinc

Thin at corners - the the opposite of hot-dip galvanized coat-ings.

Depends on operator skill Coating are porous but The pores soon fill with zinc corrosion products and are thereafter impermeable.

Good- any pores fill with reaction products. Thin at corners

Thickness

Normally about 50-125 µm on tube and products; thick- er coatings up to 250 µm obtained by silicon killed steel or grit blasting before galvanizing. Coatings 10 -30 µm applied to continuous wire and sheet

Thickness variable at will; generally 3 - 15 µm. Thicker layers are possi-ble but generally uneco-nomical.

Variable at will, usually between 10 - 80 µm.

Thickness variable at will generally 100 - 150 µm but coatings of up to 500 µm can be applied

Up to 40 µm of paint (and more with special formulations) can be applied in one coat

Formability and Mechanical Properties

Conventional coatings ap-plied to finished articles, not formidable; alloy layer is abrasive resistant but brittle on bending. Special coatings with little or no alloy layer readily formed (e.g. on sheet) and resistance welded.

Electroplated steel has excellent formability and can be spot welded. Small components are usually finished before plating.

Good. Does not cause em-brittlement of high strength steels.

When applied to finished articles, forming not re-quired. Can weld through thin coating if necessary but preferable to mask edges to be welded and spray these afterwards

Abrasion resistance better than conventional paints. Painted sheet can be formed and resistance welded with minor damage.

Extra Treatments

Conversion coatings -chromate's prevent wet storage stain; phosphates good on new sheet as a base for paints. Weathered coatings often painted (after 10 - 30 years) for longer service.

Conversion coatings (e.g. chromate's used to prevent wet storage stain) Frequently used as a base for paints.

Can have conversion coatings applied.

Coating with sealant that can provide a base for paints to give long life structures.

Can be used alone or as primer under conven-tional paints.

Other Considerations

Size of bath available. Parts up to about 25 meters long can dipped at some works. Care required at design stage for best results. Continuous wire and sheet avail-able.

Size of bath available. Process normally used for simple, fairly small components suitable for barrel plating or for con- continuous sheet and wire. No heating except for hydrogen embrittlement relief on high strength steels.

Ideal for small parts including washers and springs (e.g. up to 15 cm or 250 g). Access diffi-culties (e.g. inside tubes).

No size or shape limitations very economical for work with high weight to area ra-tio. Uneconomical on open mesh. Access difficulties may limit application (e.g. inside tubes). Best method of applying very thick coatings. Little heating on the steel.

Suitable for anything that can be painted though there may be difficulties of access in narrow tubular struc-tures. Can be brush, spray or dip applied. No heating involved. Performance varies with media used and percentage of zinc dust

Microstructures of various zinc coating