Since steel sheet pile are not made of stainless steel, they can rust. This corrosion process does not, naturally, improve the service life of the sheet pile. Especially when the critical thickness of the steel is reached: this means that the sheet pile is exactly still thick and strong enough to absorb the forces for which the sheet pile structure has been calculated. If the corrosion process continues and the sheet pile becomes even thinner, the sheet pile could collapse because the sheet pile structure is no longer sufficiently strong.
An aspect that also plays a role within this context is the fact that the sheet piles have become thinner and thinner during the last 10 to 20 years (attention: not less strong because this has remained the same due to adjustments in the geometry of the sheet pile or has even become greater) and therefore the corrosion speed has become relatively greater (1 mm corrosion of a 6 mm steel thickness has a greater impact on the structure than 1 mm corrosion of a 10 mm steel thickness).
The steel of the sheet pile does not rust as quickly everywhere. The following types of corrosion have been identified:
The following is indicated for sheet piles in the Richtlijnen voor het Ontwerpen van Betonnen Kunstwerken (Guidelines for the Design of Concrete Engineering Works).
The corrosion speed per side of the sheet pile amounts to:
In relation to fresh water
In relation to salt water
1. Atmospheric zone
2. Tidal/splash zone
3. Underwater zone
4. In the soil zone
Higher values for the uniform corrosion are given in CUR publication 211:
1. Atmospheric zone
0.050 to 0.100 mm/year
2. Splash zone
0.150 to 0.400 mm/year
3. Tidal zone
0.100 to 0.250 mm/year
4. Low water zone
0.100 to 0.250 mm/year
5. Underwater zone
0.050 to 0.200 mm/year
0.020 to 0.050 mm/year
Although not explicitly mentioned, the above values refer to the corrosion under salty conditions (sea ports).
[Source: CUR 166 Sheet pile structure 6th edition, Part 2]
This is why methods have been developed in due course to nullify this corrosion process or at least to delay it so that the service life of the sheet pile can be extended and the costs in the long term are lower.
We have identified the following options within this context:
A. Applying a coating.
Experience has taught us that a coating can delay the start of the corrosion process by up to 20 years. A coating is guaranteed in a descending order for a period of 5 years.
A condition within this context is that the steel must be blasted with grit up to the standard purity degree of SA 2.5 before the coating is applied. Different systems are also possible within this context but usually a 2- or 3-layer system is applied. For example:
The best result is obtained when the coating is applied in a workshop that has been especially set up for this where the temperature and humidity are under control. Then only repairs have to be performed at the construction site that have occurred because of sheet pile installation (for example, caused by the clamp of the vibratory hammer) or damage caused during the transport or the handling of the sheet pile.
An additional benefit of a coating is that the image of the sheet pile improves and certainly if the sheet pile is not simply coated in a boring black. It also applies to steel sheet piles: appearances also count!
B. Metallic protection layer
More and more often steel sheet piles are galvanised when the conditions are highly corrosives (a salty environment). This process takes place in a zinc bath: the steel sheet pile is submerged in a hot liquid zinc bath (hot-dip galvanisation). Since these zinc baths only have a limited length, the sheet piles cannot have a very long length. When galvanising, there is a risk of deformation of the sheet pile since the temperature of a zinc bath is around 800 - 900 °C. Too much zinc may also end up in the interlock that can cause too much friction when pile driving. The interlocks must therefore be cleaned as much as possible before galvanising.
C. Cathodic protection
The corrosion under the waterline can be disabled in an electrolytic manner by integrating a cathodic protection structure with an `active voltage` or `sacrificial anode`. This method is often combined with a coating. A steel sheet pile can be protected by applying aluminium in the shape of what is commonly referred to as ingots. The aluminium works as a sacrificial anode within this context. Zinc and magnesium are also used for this.
Cathodic protection is extremely suitable or sheet pile structures with a tidal line and where remediation or replacement is too expensive. Sheet piles with cathodic protection required special structural measures that must therefore already be taken into account during the design phase.
D. Adding alloys
Experience from the past has taught us that adding copper to the steel for the sheet pile sections that are submerged in water does not have a service life increasing function. Adding copper, however, in combination with nickel and chrome as well as phosphorus and silicon can lead to a longer service life in the splash zone and especially in tropical areas where the air is full of salt.
The different sheet pile steel grades as listed in the EN 10248 standard and the steel grades in the EN 10025, EN 10028 and EN 10113 standards do not show different corrosion behaviour. If the higher tensile strength of the steel is achieved by adding niobium, titanium and vanadium, this will have a positive impact on corrosion behaviour.
E. Corrosion protection through overdimensioning
By selecting a section with a higher resistance moment or using a higher steel grade, a longer service life can be realised if we keep the calculated bending moment in mind.
Sections with a greater wall thickness offer a better protection against rusting through. Usually, the place where the maximum moment occurs will not be in the zone that is affected the most by corrosion. This means that in general there is already some spare capacity for corrosion in this zone.