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Corrosion resistance of metals

Considering why metallic items deteriorate and how their resistance to corrosion can be assessed.

Many items in today’s society are produced from metals. These are very versatile materials of varying strengths, allowing for a wide range of products and applications. However, all metallic items are susceptible to deterioration (corrosion) to some degree. Therefore, it is important that they are treated correctly to ensure that their durability is compatible with their intended function.

Metals corrode over time due to chemical reactions which occur between the metal and its environment. This deterioration normally results in visual changes, and can also have an adverse effect on the item’s overall strength. An example of corrosion, which is the most common, is iron oxide (rust), which occurs as a result of a chemical reaction between the iron and oxygen, normally from the air. Corrosion can occur at different rates, depending on the metal, how it is coated and the environmental conditions in which it is placed.

There are different types of metallic corrosion. Some of these include the following:

General corrosion is most common form, which affects the entire exposed surface of the metal.

Image © D3j4vu

A stainless steel cable ladder with mild steel bolts subject to galvanic corrosion

Localised corrosion targets one area of the metal structure. Because the area corrodes at a much faster rate than the rest of the item, the result is more noticeable. This is normally due to a defect on the surface or due to water breaching the coating.

Galvanic corrosion occurs when two different metals are in close contact and are exposed to an electrolyte. One metal becomes the anode and corrodes at a faster rate than the other metal, which becomes the cathode where deposits will collect. Moving parts can also become fused together.

Environmental cracking can occur from exposure to severe environmental conditions, such as by aggressive chemicals, high temperatures and stress on the metal.

High temperature corrosion can occur as the result of certain compounds forming during the combustion of fuels. These compounds can be very corrosive towards metal alloys, including stainless steel.

Why corrosion is a problem

Figure 1: Metallic items in SATRA’s corrosion chamber being tested for resistance to deterioration

The corrosion of metallic items may not only cause a financial problem – it can also impact on the health and safety of individuals. It is, therefore, important to make sure that an effective prevention system is in place at the design stage in order to eliminate the issue as much as possible. SATRA regularly carries out corrosion testing of personal protective equipment (PPE) to ensure that any metallic materials incorporated do not deteriorate to the point where the item fails to protect the user within its normal operating limitations. An example of this is fall protection PPE, where corrosion build-up can lead to a failure of a product’s function or reduce its overall strength. If this were to happen, there is the potential for the product (such as a karabiner) to fail, resulting in the user falling to the ground, possibly causing serious injury or death.

There are several ways to guard against corrosion. The most common of these is to apply an additional coating, which protects the metal from being exposed to the environment. This is normally done by painting the device or plating – using, for instance, zinc or cadmium. Another common method, normally used on aluminium alloys is anodising. This surface treatment gives the metal a harder surface layer, making it much more resilient to weathering and corrosion. Even with this added protection, it is advisable to perform regular inspections of metal to ensure that any problem is identified at an early stage.

Testing for resistance to corrosion

SATRA offers corrosion testing of metallic components and products to several standards, including EN ISO 9227 and ASTM B117. Both of these methods require samples to be placed in a chamber for a set period of time, either as detailed in the specific product standard or for a length of time as agreed with the customer. The test chamber is then heated to 35ºC and filled with a 5 per cent salt water mist. This atmosphere, along with breaks in the open air at set intervals, speeds up the corrosion process, thus determining if corrosion-related failure is likely to occur (figure 1). In real use, it is dependent on the atmosphere and location of the item as to how fast corrosion takes place, so these corrosion tests are just a simulation of what could happen over time.

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