Most metal coated fasteners look bright and attractive when installed but often have thin coatings and even in a mild environment will rust vigorously after only a short period of time. It is important to ensure that you have selected the right coating for the required environment.
There are a variety of metal coatings that can be used to protect small steel components. Taking the example of using zinc, there are at least three different ways of using it in such instances, all of which offer totally different coating characteristics and performance.
These three systems are:
1. Centrifuge or spin hot dip galvanizing
2. Metallic Coating with zinc dust – sherardizing
3. Zinc electroplating
Centrifuge or spin galvanizing
The terms ‘spin galvanizing’ or ‘centrifuge galvanizing’ are used to describe the process for hot dip galvanizing threaded components and other small parts. The process is very similar to galvanizing larger steel sections. Small components however are placed in perforated containers, allowing items down to 8mm in diameter to be processed. After immersion in the galvanizing bath the container is centrifuged at high speed so that the spinning action throws off any surplus zinc and ensures a clean profile. When the articles are immersed in the galvanizing bath, a series of zinc-iron alloy layers are formed with a metallurgical bond to the steel surface. These alloys are harder than mild steel and are normally covered by an outer layer of comparatively soft zinc which remains after the spinning operation. This structure is unique and gives the galvanized coating very good resistance to rough treatment – direct blows are cushioned by the outer layer of zinc and the hard alloy layers resist abrasion. As a general rule, nuts, bolts and washers down to M8 can readily be galvanized and threaded components up to 2m long can be processed. Special arrangements can often be made to centrifuge galvanized components outside this range.
Metallic Coating with Zinc Dust – Sherardizing
Articles to be sherardized are first chemically cleaned or blasted and need to be thoroughly dried. Any remaining water can cause problems when mixed with zinc powder. The clean, dry work is then placed in the sherardizing barrel with the appropriate amount of zinc dust. In some cases fine sand has to be added to aid even distribution of the dust and the temperature raised, prior to tumbling, until the desired coating is achieved. The dust should be very fine as coarser powders take longer to form the coating. The need to ensure even temperature within the barrels imposes a size limitation on the process, which is generally used for small and fairly intricate components.
A zinc salt solution is used to electrolytically deposit a layer of zinc on a cleaned steel surface. Acid or alkaline electrolytes can be used. After cleaning the work is transferred, as quickly as possible, to the plating solution. Good adhesion of the plated layer to the substrate depends on a very close physical conformity of the plating with the base metal. To achieve this, the base metal must be scrupulously clean immediately before plating. The table below summarises the coating characteristics and general coating thicknesses that are available via each process. It is crucial from a corrosion protection point of view that the correct system is specified for the relevant environment to achieve the desired performance. One difficulty that arises is that all of these processes use zinc and their different properties, coating thicknesses and performance may be confused into an overarching description of ‘galvanizing’. In fact zinc electroplating is often known – misleadingly – as electrogalvanizing. It is not therefore enough to merely specify ‘galvanizing’ if long life is required. If hot dip galvanizing is to be used, the specification should clearly state that the fastener coating should conform to BS 7371 : Part 6 + A1 and to be galvanized by a member of Galvanizers Association.