Unbrako Strengths - Mechanical Dip Galvanizing


	Strengths

Mechanical Galvanizing

General
Mechanical Galvanizing is similar to Hot-Dip Galvanizing in that they both apply a thick coating of zinc metal which provides sacrificial or cathodic protection to the steel. Because zinc is more reactive than iron, the zinc galvanized coating corrodes first, protecting the steel substrate. Mechanical Galvanizing results in a very uniform coating thickness reducing thread fit issues at assembly, making it a preferable coating for structural applications. Not only does Mechanical Galvanizing provide excellent coating uniformity, but the process is consistent from batch to batch and within each batch.

The Mechanical Galvanizing Process
Mechanical Galvanizing is a room temperature process in which zinc coatings are applied to fasteners without electricity (which is used for electroplating) and without heat (which is used for Hot-Dip Galvanizing). Fasteners are placed in a large rotary barrel along with zinc powder, special promoters, and glass impact media. The mechanical energy generated from the barrel’s rotation is transmitted through the glass impact media and causes the zinc powder to be mechanically welded to the surface of the fasteners. With a proper glass media size mix, all exposed surfaces can be coated very uniformly, and the buoyancy of the glass media cushions the fasteners in the rotating barrel to minimize thread nicking. The room temperature process ensures no chance of re-tempering or softening high strength fasteners and guards against hydrogen embrittlement because the fasteners are also never exposed to acid pickling in the process.

Application
Mechanical Galvanizing may be specified on a wide range of structural products including ASTM A325 Structural Bolts, A563 Grade DH Heavy Hex Nuts, A307 Bolts & Studs, A449 Hex Cap Screws, F1852 Tension Control Assemblies, F436 Hardened-Steel Washers, DIN 6914, DIN 6915, DIN 6916 and for Small Diameter Bolts, as well as fine thread Bolts.

10 Good Reasons for Mechanical Galvanizing/Plating

	Excellent Adhesion & controlled uniformity of Coating thickness.
	No hydrogen embrittlement.
	No detempering.
	Eliminate retapping of units.
	No Galling.
	No Sticking.
	Excellent Corrosion Protection.
	Pos-Plate Treatment.
	Batch to batch conformity.
	Overall Appearance.

The below table provides comparative properties of five well known zinc coating methods.

Characteristic of the Coating	Hot Dip Galvanizing	Electroplating	Mechanical Coatings	Zinc Spraying	Zinc Dust Painting
Adhesion	Coating is integral with the steel because the formation process produces zinc/iron alloy layers over coated with zinc.	Good, comparable with other electroplated coatings.	Good, comparable with electroplated coatings.	Good mechanical interlocking 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 limitations of "throwing power" of bath. Pores not a problem, as exposed steel protected by adjacent zinc.	Thin at corners - the opposite of hot-dip galvanized coat-ings.	Depends on operator skill Coatings 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; thicker 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 - 5 µm. Thicker layers are possible but generally uneconomical.	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
Form ability and Mechanical Properties	Conventional coatings applied to finished articles, not formable; alloy layer is abrasive resistant but brittle on bending. Special coatings with little or no alloy layer readily formed (e.g. on sheet) & resistance welded.	Electroplated steel has excellent formability and can be spot welded. Small components are usually finished before plating.	Good. Does not cause embrittlement of high strength steels.	When applied to finished articles, forming not required. 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 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. Chromates used to prevent wet storage stain) Frequently used as a base for paints.	Can have conversion coatings applied.	Coating with sealants that can provide a base for paints to give long life structures.	Can be used alone or as primer under conventional 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. Conti-nuous wire and sheet available.	Size of bath available. Process normally used for simple, fairly small components suitable for barrel plating or for conti-nuous sheet & 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 difficulties (e.g. inside tubes).	No size or shape limitations very economi-cal for work with high weight to area ratio. Uneconomical on open mesh. Access difficulties may limit application (e.g. inside tubes). Best method of applying very thick coatings. Little heatings on the steel.	Suitable for anything that can be painted though there may be difficulties of access in narrow tubular structures. Can be brush, spray or dip applied. No heating involved. Performance varies with media used & percentage of zinc dust.