Mill scale on sheet piling

Why is Shore sheetpile corroded so fast?

Most of the installed shore sheetpiles are applied without the use of a corrosion protection system like: Cathodic protection or a coating. The general assumption is that the shore sheetpile will last long enough with some overdesign (corrosion allowance). Whether this assumption holds, really depends on the environment at which the shore sheetpile is exposed to. The most common type of corrosion is pitting, which results in small holes in the metal surface. Atmospheric corrosion is another common type, which manifests as a uniform thinning of the metal, usually with the formation of a thick corrosion crust (mineral deposition). This can be caused by exposure to pollutants such as sulfur dioxide and industrial emissions. Shore sheetpile are particularly susceptible to this type of corrosion, as they are often exposed to salt spray and other corrosive agents like soil.

As a result, it is important to regularly inspect shore sheetpile for signs of corrosion and take steps to protect them from further damage.

The different types of corrosion and their effects on shore sheetpile

There are many types of corrosion, but the three most common mechanisms behind corrosion that affect shore sheetpile are MIC, ALWC, and Atmospheric Corrosion.

Atmospheric corrosion is simply rusting that occurs due to exposure to the air. Usually, it is a more or less uniform process.

MIC, or microbiologically influenced corrosion, is caused by microorganisms in the water that interact with the iron in the sheetpile. This interaction can generate exceptionally high corrosion rates.

ALWC, or accelerated low water corrosion when the surface area is exposed to a mix of atmospheric conditions with little or no water present and conditions where the metal is submerged. This leaves the steel exposed to mixing conditions with both oxygen, formation of biofouling and other chemicals in the water, that promote biological growth and .

All of these types of corrosion can be damaging to shore sheetpile, and so it is important to be aware of them in order to protect your investment.

Figure 1. close-up from sheet piling corrosion between segments

Mill scale

Mill scale, also known as simply ‘scale,’ is the flaky surface of cold rolled steel. It’s made up of mixed iron oxides like iron(II) oxide (FeO), iron(III) oxide (Fe2O3), and iron(II,III) oxide (Fe3O4, magnetite).

Mill scale is a thin layer of iron oxide that forms on the surface of iron steel when it is exposed to oxygen. Mill scale is blue-black in color and instrinsically formed on cold rolled sheets of metal. It is a thin iron oxide barrier that adheres to the steel surface and protects it from further corrosion, given there are no breaks in the corrosion layer.

Mill scale is beneficial to protect the steel agains further corrosion until something causes its layer to break. Steel is cathodic to mill scale, so if the mill scale encounters any breaks, it will cause fast corrosion of the underlying steel. .

Mill scale is an annoyance when trying to process steel since any paint applied will be pulled off with the scale as moisture-laden air gets underneath it. Therefore, mill scale can be time-consuming and energy intensive to remove from steel surfaces using methods such as flame cleaning, pickling, or abrasive blasting. This is why, in the past, shipbuilders and steel fixers would leave steel and rebar delivered straight from mills out in the open to weather until most of the scale fell off due to atmospheric action. Now, if you want a shop primer on your steel so that it can be welded or painted later, most steel mills can accommodate that.

In the case of shore sheetpiles, most of these sheetpiles are still delivered without a primer and a mill scale still present on both sides of the sheetpile.

Methods to prevent corrosion from forming on shore sheetpile

First and of high importance, is to remove the mill scale. This can be done most effectively at the shore sheetpile factory. However, once installed it becomes a more challenging task. Methods such as abrassive blasting and Bristle blasting can be used. Within our masterclass on Bristle Blasting, we have shown the procedure to remove mill scale. Once this layer has been removed, you can start to create the line of defence against corrosion.

There are many ways to prevent corrosion from occurring on shore sheetpile. One way is to coat the metal with a viscoelastic material. This will create a barrier between the metal and the environment, and will also help to dampen vibrations. Another method is to use cathodic protection. This involves passing an electric current through the metal, which will help to promote the formation of a protective oxide layer. Finally, it is also important to keep the metal clean and free of debris. This will help to reduce the risk of corrosion by preventing the metal from coming into contact with corrosive materials.

In conclusion, it is important to be aware of corrosion and how it can impact shore sheetpile. By understanding the different types of corrosion, their effects and methods of prevention/mitigation, we can ensure that our structures last longer. If you have any further questions or would like to learn more about this topic, please do not hesitate to contact us.

Corrosion on sheet piling in Underground Car parks

We see sheet piling being widely used as ground retaining wall within undergroun car parks. The assumption is that the sheet piling remains dry on the side of the car park and is only subject to atmospheric corrosion. In many cases, however, it is underestimated that in practice the sheet piling remains under much wetter conditions due both ground water penetration through the sheet piling and condensation. The avarage ground temperature in north Western Europe is about 10 to 12 degrees celcius. Both in spring, summer and autumn, the concrete and sheet piling is the coldest surface. As a result, there is considerable condensation and both concrete floors and sheet piling are soaking wet for much of the year.

Engineering requirements

Corrosion at shore sheetpiles is often underestimated. Usually, the sheetpiles are applied at very large surface areas. During the engineering phase of a project it has a major impact on the total required investment, whether one will coat the sheetpiles or leave them exposed to the environmental conditions. Creating a bit of thickness on the sheets and giving them corrosion allowance, is usually much cheaper compared to coating applications. Environmental conditions such as condensation or the anaerobic activity of corrosive micro-organisms are usually not considered at the engineering table.

Our partner Nebest has a lot experience to measure and diagnose sheetpiling and aldo provide recomendations. If you would like to have support for you situation, you could rase a case through our portal (only Dutch).

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