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With the ever increasing energy costs, as well as adverse weather conditions, sizeable savings can be made by simple minimising corrosion defects with the easy application of different coatings on fasteners. Contributed by Bossard Engineering Team

To prevent corrosion or keep it in-check, one has to consider the important coatings and the material used in different applications. In other words, if all parameters that causes the defect to take place are known, engineers can provide appropriate protects with different coatings. There will be no out of the control costs for maintenance and repair.

Too glossy, not black enough. Fastening elements must have certain characteristics, which have more to do with design than with their functional characteristics. This requirement is ever more frequently added to traditional characteristics, such as resistance to corrosion, mechanical resistance, and dimensional requirements.

However, the solution cannot be limited to just one colour, or a level of gloss. It must also be taken into account the primary function of the coatings. Resistance to corrosion, dimensional compatibility, preserving the strength of the base materials; these are the principal functions. In applying cost saving design, the correct finishing in a fastener must be taken seriously so that the corrosion resistance is met in the tough environment.

The ‘Design’ Function

Several functions can be involved in coatings, firstly the aesthetic element; the brilliance, the colour, and the roughness, which will allow the fastening element either to be seen, or to blend in with the assembly.

Glossy, Satin, And Matte Coatings

For ages, the aesthetic coatings of choice have been nickel-plating and chrome-plating, which are deposited onto steel and copper-based metals. The iridescence and brilliance allow for coatings that are often compatible with industries that have little to do with mechanics, such as interior design, health management, luggage, etc, and which are used on screws, rivets, and special fasteners.

These coatings are relatively costly as the price of the materials based on Ni and Cr are high; however, the vat based electrolytic process allows one to moderate the manufacturing costs.

To facilitate the adherence of the coatings, a coating of copper plating can sometimes be done on the nickel plating.

Nickel is a silver-white metal with yellow highlights which has a polished shine. It is a malleable metal, which, thanks to its resistance to oxidation and corrosion, is used in the composition of coins, but also several highly resistant metal alloys.

Nickel plating can also give a brilliant look to treated parts and provide protection from oxidation.

Like chrome plating, nickel plating requires a preliminary cleaning and degreasing phase which is followed by stripping. The part is then polished, particularly if the final appearance and aesthetics are important.

There are two procedures for nickel plating: nickel electroplating and chemical nickel plating.

Nickel electroplating is a galvanoplasty application that consists of the electrolytic deposit of aqueous solutions. It can be used on different media:

  • Steel
  • Stainless steel
  • Copper/brass
  • Aluminium
  • Magnesium
  • Zamak
  • Titanium

Chemical nickel plating (‘electroless’) consists of depositing nickel (bonded to phosphorus or boron) without a source of current. The object being coated is submerged in a nickel

bath. This method results in an increased hardness and better resistance over time and with exposure to friction than does nickel electroplating.

Certain people suffer from nickel contact skin allergies. This is why one often finds chrome-plated parts rather than nickel-plated parts in the vast majority of watches.

Mechanical Polishing Of Stainless Steel And Aluminium

Mechanical polishing allows one to obtain extremely varied surface states from practically any point of origin. Mirrored, glossy, matte, satin, brushed, and so on. Mechanical polishing can have a decorative or technical aim, whether in an industrial, commercial, or consumer sector, or for any other entirely different use. For the mechanical polishing of stainless steel, different abrasives are used, from the coarsest to the finest, according to the result sought.

Mechanical polishing also implies the use of different tools according to the profile of the part to be polished. Depending on your objectives – aesthetic appearance, decontamination, protection and reinforcing against oxidation – we can combine mechanical polishing with a chemical treatment such as electrolytic polishing. In this way we are able to meet every request relating to mirrored polish, glazing, brushing for any grain, satin-finishing, to a specific degree of roughness, weld shaving, deburring, etc.

Micro-Peening

Micro-Peening is a procedure consisting of shooting glass or ceramic beads at a surface in order to improve the overall finish of the surface or to eliminate contaminants from it.

It can be used to reduce the tooling marks in a compound, but also to smooth the surface in order to obtain a more regular, uniform finish. This procedure is particularly beneficial when various manufacturing methods have been used to produce a compound or product, each having left the surface in a different state. Micro-Peening can also be used to return a tarnished or patina finish to a clean, new surface.

Advantages And Characteristics

  • Improves surface finish after manufacture.
  • Can be used to decontaminate surfaces.
  • Option of glass or ceramic beads.
  • Smooths the surface after prestress shot blasting.

Zinc Plating And Zinc Electroplating

The most prevalent in the field of fixing are zinc and zinc nickel, where the electroplating is often supplemented by a passivation and/or topcoat, which gives the coating friction properties, corrosion resistance, and of course its appearance.

Naturally, zinc and zinc-nickel without passivation (or with colourless passivation) are respectively, glossy silver and matte grey. If we add acid or alkaline passivation methods, we have iridescence of different colours depending on the formulation.

However, two larger categories of passivation are used, the first of which is the hexavalent group, which generates greenish, yellow, black, and white colourations. The latter is more limited due to the fact that they contain Chromium-6 and that this element is covered by the RoH and Reach prohibitive directives.

Substitution passivation are trivalent passivation, which present iridescences with less notable colour: green, yellow, and grey.

These coatings are sometimes supplemented by a top coat, a black organic film that increases corrosion resistance (approximately 50 to 150 h with a neutral salt spray) and which can have a black satin, very aesthetic, appearance.

Black And Grey Coatings

Zinc Flake Coatings

Zinc lamella or flake coatings are a dry film made up of numerous small flakes, where the basic principle is to protect a multitude of components from corrosion. Thanks to the ‘sacrificial effect’ of zinc, which is less noble than steel, it offers active protection against environmental influences: this form of corrosion protection is called sacrificial cathodic protection. Most of the time, zinc flake coatings are made up of a combination of zinc and aluminium lamella (in compliance with the standards DIN ESO 10683 or DIN EN 13858), included in an inorganic matrix.

Even extremely thin layers – a system typically consists of an underlayer and a finish of 8 to 12 μm – allow for a protective effect against corrosion of the base metal for up to 1,000 hours (red rust) in compliance with the DIN EN ISO 9227 standard.

The matte grey colour will be obtained in the case of standard products with or without a finishing coat because this is the base colour of the under-layer of zinc and aluminium flakes.

This colour perfectly matches hot-galvanised and stainless steel parts, see electrolytically galvanized parts. The number of layers does not affect the tint and the lubrication system that can be included in the two layers or just in one, nor does it affect the grey colour obtained.

Note the need during design itself to perform particularly fine layers to ensure the fit of threaded parts.

The process of applying the coatings does not produce hydrogen and therefore allows you to reduce the risks of it becoming brittle. This is why zinc flake coatings are particularly well adapted to high strength classes. Thanks to their high performance and thin gauge, zinc flake methods have largely taken over in the field of screws and fastening in the automotive industry: one screw in two of the leading manufacturers is coated with zinc flake.

The finishes complement the characteristics of the sub-layer and above all, they can also be used to colour treat parts, knowing that the standard colours are silver and black. Thanks to their versatile properties, they can be used in a wide variety of applications. Depending on the expected use of the treated parts, you can choose organic or inorganic finishes, which can be deposited on the zinc flake, or electroplating.

The zinc flake also exists in a black version, the finish coat called top coat will, in this case, have the double function of determining the coefficient of friction and the colour. Colourants are introduced into the finishing layer, which can be black, although other colours are possible.

This colour, in general, remains matte black, but new formulas are being developed to give them a satin finish. The best known are Geoblack, Deltaprotekt, Zintek and Magni.

Note that from a chemical point of view, the introduction of organic and organo-mineral colourants will reduce the corrosion resistance of the layer concerned, so work is being done to develop two product lines, the matte black coating standard that reaches a performance level equivalent to gray, and coatings with a deeper black somewhat satin finish for which corrosion resistance will be slightly lower. The latter will have more esthetic applications and can be used in association with the new coatings (satin nickel plating, black zinc nickel…) and brilliant composite materials.

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