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How to Assess the Quality of Galvanised Steel

Published by Lucy Thurston on

Galvanising is an economical and maintenance-free corrosion protection system; however, it requires a thorough inspection to guarantee that it complies with the necessary specifications. The galvanised steel sheet properties make the metal ideal for construction, but this also means that a lot of focus needs to be invested into the UK galvanised steel sheet quality assessment. From the weight of galvanised steel sheets to their coating, there are many aspects that need to be assessed when it comes to thick or thin galvanised steel sheets. Rapid Metals is here to walk you through the quality assessment process for magnetic galvanised steel sheets and rods. 

What is Galvanised Steel Sheet?

Before one can assess the quality of galvanised steel, one must be familiar with what galvanised steel actually is. Galvanising refers to the process of applying a protective coating of zinc to steel. It’s for this reason that galvanised steel is so popular in construction, as the metal is resistant to damage. Despite this, not all galvanised steels are of the same quality, which is why it’s important to assess this ahead of use. 

Coating Measurement

When it comes to assessing galvanised steel, the first port of call is always the coating thickness. This is the most scrutinised aspect of galvanised steel, as it’s the zinc coating that provides the metal with protection. As a result, there is a minimum zinc coating requirement that galvanised steel must meet in construction. This can either be measured in terms of the thickness or weight per surface area. 

The thickness of the coating can be measured using a magnetic thickness gauge and optical microscopy. This method exposes the edge of the coating under a microscope, to which the steel must be sectioned, mounted, and polished. Consequently, the microscope is enabled to determine the coating’s thickness. 

The coating weight can be measured in one of two ways; the first is a process that’s referred to as weigh-galvanise-weigh, which measures the weight of steel after it has been cleaned, and again once it has been galvanised. Alternatively, a method known as weigh-strip-weigh can be used, which measures the weight of steel after it has been galvanised, and again once the coating has been stripped off a piece of steel. Each of the weights is then divided by the steel’s surface area to reach the coating weight. 

Finish and Appearance

There are various aspects that can impact the finish and appearance of galvanised coatings, each of which must be visually inspected. This is carried out to guarantee that every specification requirement is being met. Typically, galvanised steel will appear as matte, spangled, shiny and bright, or a combination of each of these. Therefore, there’s no one way in which galvanised steel must appear, and this variation is down to steel chemistry, the cooling rate, and the steel processing. 

The steel chemistry is the predominant reason why galvanised steel has different appearances. This is because of two key elements: phosphorous and silicon. Each of these is a catalyst of coating growth, and a varied thickness of the coating can result in different appearances. 

In order to deoxidise the molten steel, silicon is added during the steel-making process. Quite simply, a higher silicon content results in the steel being more reactive, which leads to a thicker coating with a matte appearance. Similarly, phosphorus also affects the reactiveness between zinc and steel, meaning that a higher level of phosphorus results in a rougher surface with a matte finish. 

Something else that impacts the finish of galvanised steel is the cooling rate of the metal. For instance, steel that presents a mixture of shiny and dull coating can be a result of varying cooling rates. Despite this, as the steel weathers, it will eventually reach a uniform finish. 

The last affecting factor of the finish and appearance of galvanised steel is the processing and fabrication of the steel. Depending on how much pressure is inflicted on the metal and where, the appearance can vary. 

Surface Conditions

When it comes to assessing the surface conditions of galvanised steel, the list is seemingly endless. This is because the production of galvanised steel can affect the surface of the metal in various unforeseen ways. 

During this assessment, you may consider bare spots, blasting damage, chain and wire marks, clogged holes and threads, delamination and peeling, distortion and warpage, drainage spikes, dross inclusions, excess aluminium in bath, flaking, flux inclusions, oxide lines, products in contact, rough surface conditions, runs, rust bleeding, sand embedded in castings, striations and fish boning, surface contaminants, weeping weld, welding blowouts, welding spatter, wet storage stains, zinc skimmings, and zinc splatter. Each of these can impact the appearance of galvanised steel, and some can be controlled whilst others can’t. 

A key specification for galvanised steel is that it must be uniform in appearance; however, this doesn’t mean that metals with the aforementioned conditions can’t be considered. This uniformity is open to interpretation, and the most important aspect to consider is whether the metal will be able to withstand long-term usage. As a result, if the appearance won’t affect the metal’s ability to resist corrosion, this will be deemed acceptable. Despite this, if the surface condition is liable to compromise the coating’s life, it will be rejected. 

Galvanised Steel Sheets for Sale at Rapid Metals

If you’re on the hunt for a competitive galvanised steel sheet price, look no further than Rapid Metals. Browse our collection or get in touch with our team regarding advice and guidance surrounding various metals and their applications. 

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