Preserving steel sheet piles can add to their lifespan. Since sheet piles are made of steel, they can rust over time. This corrosion process obviously does not benefit the life of the sheet piling. A sheet pile wall must be thick enough and strong enough to withstand the forces for which the structure is designed. This minimum thickness is called the “critical thickness”. Should the corrosion process continue and the sheet piling become even thinner, the sheet piling could fail.
Although the strength of the sheet piles has not changed (it has remained the same or even increased due to adjustments in the geometry of the sheet pile), sheet piles have become thinner and thinner over the last few years. This also plays a major role in the corrosion process and the lifespan of the sheet piling. In fact, due to thinning, the corrosion rate of the sheet piles has become relatively greater. After all, 1 mm of corrosion when steel is 6 mm thick has a greater impact on the structure than 1 mm of corrosion when the steel is 10 mm thick.
The steel of the sheet piling does not rust at the same rate everywhere. The following zones of corrosion are distinguished:
1. atmospheric zone
2. tidal/splash zone
3. underwater zone
4. soil zone
for fresh water
for salt water
1. atmospheric zone
2. splash zone
3. tidal zone
4. low water zone
5. underwater zone
6. soil zone
0,050 à 0,100 mm/year
0,150 à 0,400 mm/year
0,100 à 0,250 mm/year
0,100 à 0,250 mm/year
0,050 à 0,200 mm/year
0,020 à 0,050 mm/year
Although not explicitly stated, the above values relate to corrosion in salt water conditions (seaports).
[source: CUR166 Damwandconstructie 6e druk, deel 2]
To prevent this corrosion process as much as possible, methods have been developed over time that can slow down corrosion. With these methods, the lifespan of the sheet piling can be extended, which also reduces the long-term costs.
We distinguish the following possibilities:
A. Applying coating
Experience has taught us that coating can delay the onset of the corrosion process for up to 20 years. Coating is guaranteed for a duration of 5 years.
One condition is that the steel must be shot blasted with grit to the standard cleanliness value SA 2.5 before applying the coating. Again, different systems are possible, but usually a 2- or 3-layer system is applied, for example:
1 layer of primer
60 – 70 mu
– phosphate primer
– Zinc dust primer (good adhesion to the steel)
1 intermediate coat of coating
– based on epoxy resin steel)
1 intermediate layer of coating
– based on DTM coating
– on the basis of polyurethane resin
– both available in all RAL colors
NOT used anymore: tar or bitumen products (due to environmental impact)
The best results are obtained if the coating is applied in a specially equipped workshop where temperature and humidity are controlled. After this, any damage caused during transport or insertion can be fixed at the construction site.
An additional advantage of a coating is that the appearance of the sheet piling is improved, especially if the sheet piling is not just coated in dull black. Even with steel sheet piling, the eye wants something too!
B. Metallic protective coating
In highly corrosive conditions (salty environment), the steel sheet pile can also be hot-dip galvanized. The steel sheet pile is immersed in a hot, liquid zinc bath (hot dip galvanizing). Since these zinc baths have limited length, the sheet piles cannot be too long either. When galvanizing thin cold-rolled steel sheet piles, there may be a danger of them deforming as the temperature of a zinc bath is about 800 – 900°C. Moreover, the zinc may enter the lock in excessive amounts, which may cause excessive friction during application. Therefore, locks should be cleaned as thoroughly as possible before galvanizing.
C. Cathodic protection
Corrosion below the waterline can be eliminated in an electrolytic manner, by incorporating a cathodic protection structure with `extraneous current` or a `sacrificial anode`. Often this method is combined with a coating. A steel sheet pile wall can be protected by applying aluminum in the form of so-called ‘bars’. In this process, the aluminum acts as a sacrificial anode. Zinc and magnesium are also used for this purpose.
Cathodic protection is particularly suitable for tidal sheet pile structures and for structures where rehabilitation or renewal is too costly. Sheet piles with cathodic protection require special structural measures, which must be taken into account in the design phase.
D. Adding alloys
Experience has taught us that adding copper to the steel for the sheet pile sections that are submerged in water has no life span-enhancing function. However, the addition of copper in combination with nickel, chromium, phosphorus and silicon can lead to longer life in the splash zone, especially in tropical areas with salt-rich air.
The different sheet pile steels listed in EN 10248 and the steels in EN 10025, EN 10028 and EN 10113 do not show different corrosion behavior. If the higher tensile strength of the steel is realized by adding niobium, titanium and vanadium, it has a positive effect on corrosion behavior.
E. Corrosion protection by over-dimensioning
By choosing a profile with a higher moment of resistance or using a higher grade of steel, a longer service life can be realized. Profiles with greater wall thickness also offer better protection against rusting through. Usually, the place where the maximum moment occurs will not be in the zone most affected by corrosion. This means that some reserve for corrosion is generally already present in this zone.