Design for Galvanizing
The Galvanizing Process
Coating Formation in Galvanizing
Technical support team
As part of our focus on service Joseph Ash offers its customers a high level of technical assistance by way of its Technical Support Department, staffed full time by qualified personnel who have a wealth of experience in the galvanizing industry. The department is always available to discuss customers’ technical and/or quality requirements and queries, particularly with regard to design for galvanizing, venting, and the steel chemistry and grade necessary to give optimum finish quality. They have access to a technical library and a large database of galvanizing information, much of which can be made available to our customers. Our technical support team is also available to answer customers’ queries after galvanizing has been completed and when necessary will attend meetings, carry out site inspections and provide detailed advice.
If there is anything you have ever wanted to know about metal finishing processes, as well as how to prepare and treat your metal projects before delivering them to a galvanizing plant, click on the links below or contact our Technical Support Team.
Request a copy of the Joseph Ash brochure for even more technical advice.
For detailed information about 'designing steel for galvanizing' please watch our new short film.
You can also download our Venting Tips for Hot Dip Galvanizing poster.
A copy of the Galvanizers Association 'Engineers and Architects Guide to Hot Dip Galvanizing' may be viewed here.
Please also read this interesting article about salt spray called ‘An Article on the Limitations of Accelerated Corrosion Testing'. The article originally appeared in Issue 2 of Hot Dip Galvanising.
Hot dip galvanizing »
Applying a hot dip galvanizing process to steelwork provides fabrications with a robust, durable and corrosion protective finish that under normal conditions will last for many years without maintenance of any sort. The process itself has a number of stages that are required to achieve the final finish. These are all by immersion and they are summarised as follows (rinse stages omitted):
- Degreasing – This may be carried out using either acid or alkaline based proprietory products and they may be heated or used at ambient temperatures. The target is to produce a surface, which is not contaminated with oil, or grease based products.
- Pickling – This is carried out in dilute hydrochloric acid which dissolves rust and scale and produces a ‘chemically clean’ surface which will react with the molten zinc.
- Fluxing – A mixture of zinc chloride and ammonium chloride in solution is the standard fluxing agent of choice. This is normally used at between 50°C and 70°C, which helps the steelwork to dry after it is withdrawn. Drying is important as it helps prevent zinc splash and a separate drying stage is sometimes employed.
- Zinc Immersion – This ‘final’ stage utilises a special bath holding molten zinc at 450°C. The clean steel is immersed in the zinc and while it is submerged it alloys with the iron in the steel to form zinc/iron alloy layers. These layers form the basis of the coating, which is then overlain with free zinc, as it is withdrawn from the galvanizing bath. The end result is a coating that not only has great abrasion resistance but one that according to the most recent exposure tests* lasts between 34 and 170 years before the base steel is exposed. With S02 levels in the atmosphere reported as continuing to decline this should further increase the life of galvanized coatings in the years to come.
Spin galvanizing »
Small items such as nuts, bolts, chains and screws need to be as rust resistant as the steel structures they support, however they are too small to galvanize in a hot dip galvanizing bath. With spin galvanizing small items can be placed in a perforrated basket and immersed into molten zinc. Once the zinc/alloy layers form, the basket is centrifuged at high speed so that the spinning action removes the excess zinc and creates a clean profile.
Powder coating »
Powder coating is a type of paint that is mainly applied to metals to provide a harder finish than a normal paint. The paint is applied as a powder electrostatically from a spray gun and is cured in an oven under a high heat to form a tough, durable skin. Powder coating paints come in many colours and can be used to create different finishes such as textured or metallic. If you would like a copy of the RAL colour chart that we use to determine powder coating colours, please get in touch.
Shot blasting »
Before metal is galvanized it needs to be cleaned to give it a smooth surface. One of the most effective ways to create this surface is to shot blast it. Shot blasting fires a high pressure spray of abrasive steel materials onto a rough surface to create the smooth surface.
Minimising distortion »
Fabrications should ideally be symmetrical, suitable for single-dipping and incorporate sections of as near equal thickness as possible at the joints, perfectly fitted components to avoid the use of force or restraint during joining, continuously welded joints using balanced welding techniques to reduce uneven thermal stresses and the largest possible radii on all curved members.
The importance of venting and drainage »
To achieve the best quality zinc coating, it is important to ensure that the molten zinc can flow freely over all surfaces, external and internal. So, where there are sealed hollow sections or cavities, vent and drain holes are essential.
Size, weight and shape matter »
To ensure a good galvanizing finish, a design should ideally incorporate easily-handled components or fabrications which can be single dipped or subsequently assembled by bolting with galvanized fastenings or welding. Weld areas must then be protected with zinc rich paint, by zinc spraying or with low melting-point alloy repair rods. Double dipping is a possible alternative. Seek advice before proceeding.
Suspension holes or lifting lugs may be needed if there are no suitable points for locating hooks or wires. Once again, lifting points should be positioned to maximise venting and drainage.