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Is in-house galvanizing the answer?

Monday 11 December 2017

Over the past two or three decades, several of the larger metal-finishing companies have considered the costs of doing their own in-house galvanising. This was often because they wished to retain full control of the entire coating process from start to finish, and it also gave them added professionalism in that they could 'track' the progress of a customer's order from the time it arrived until it was returned to him both fully protected and painted. Others wished to keep it in-house to avoid additional transportation costs, possible arguments with a galvaniser over collection or delivery dates and a myriad of other potential daily problems.

Zinc coating systems: Zinc plating

To this end, it was far from practical to install a hot-dip galvanising plant with the very high initial attendant costs, labour requirements plus the usual legal and subsequent environmental issues that accompany such an installation. Because of this and also because it is not at all viable to set up a small hot-dip galvanising plant, many people chose to use zinc-plating tanks instead as they are relatively easy to install and maintain by comparison. There are in fact a very large number of companies both in the UK and worldwide who continue to use zinc-plating set-ups and with great ongoing success. Zinc plating is normally only applied to a thickness of around 15 microns, and so it does in fact have its limits, and the service-life at the coast could normally be as short as 5-6 months, unless it is top-coated with a powder or wet paint.

Zinc paints

Some companies have tried using various zinc paints for galvanising purposes, and have found out to their cost that what they have used is indeed a paint in every sense of the word. Once it becomes damaged or breached in any way, the corrosion process begins as usual because the protection is mostly passive. The exception to-this rule will be zinc silicates, which are inorganic zinc paint: based on a silicate matrix. They do not fall under the same category as epoxy, polyurethane and alkyd-based zinc paints as their curing arc adhesion mechanism is quite different to that of normal organic paints. However, zinc silicates are normally only used for heavy industrial projects and are not normally recommended across the board for general use on lightweight metal fabrications. They require specific application equipment and a good understanding of both the surface preparation and the application technique that is required.

Zinganising

This is a galvanising process that is now into its thirty-sixth year of use and is being used daily throughout fifty-two countries world-wide. Zinganising is a totally unique galvanising concept in that whilst it provides exactly the same protection levels of hot-dip galvanising (with a protective voltage of 1.04 volts) it can be applied by spray gun, bringing it into the realms of the average good quality industrial coating company. Another massive bonus is that the coating is always done at ambient temperatures, which completely rules out any distortion or buckling of components.

Galvanising different metals

Zinga is used for the galvanic protection of cast iron and steels, as these types of metals cannot normally be immersed in a hot-dip galvanising tank for several reasons. [caption id="attachment_39278" align="alignleft" width="400"]

Killybegs Pier - Zinganised Steel Supports on August 2001[/caption] Zinga has also been used to galvanise aluminium boat superstructures and has even been used to coat brass propellers to stop 'de-zincification' of the brass in seawater where the zinc element of the alloy dissolves in seawater to protect the stainless steel prop-shaft etc and the remaining copper is naturally soft and begins to break up. On bimetallic welds where mild steel has been welded to stainless steel grades 304 and 316, the whole assembly is galvanised with Zinga and this forces an equilibrium of electrical potentials onto the joined metals. On pipes and steel sections, the blast-cleaning and Zinga application is usually only required to be carried out around 50 – 75 mm across the weld and onto the stainless steel side of the welded joint. [caption id="attachment_39279" align="alignleft" width="663"]

August 2001 – Zinga was applied at 4 x 50µm DFT. On the right – June 2014 – After removal of some barnacles, exposed Zinga layer is still in perfect condition[/caption] Zinga is also used to overcome bimetallic corrosion where a cast steel is welded onto a mild steel, or where there exists the possibility of carbon-migration across a welded joint that leaves one area carbon-rich and therefore more anodic (more active) than the adjacent area. This could normally lead to the anodic areas becoming corroded prematurely where the paint coating breaks down. As a very high percentage of modem welding fabrication is done using MIG welders, it stands to reason that the welding wire reeds to contain constituents Eke manganese to maintain their rigidity when feeding off a reel curing normal welding processes. Hence on steel that is not cathodically protected, the weld-beads automatically become more active (more anodic) than the adjacent metal surfaces and subsequently become the first point of attack by corrosion in very humid or marine atmospheres