At accelerated corrosion rates, piles will lose steel cross-section at anything between 2 to 8 times faster than ‘normal marine corrosion rates’ (typically quoted at 0.125mm per year in temperate waters). This loss could lead to structure life being reduced proportionately, say by 30 years, with a consequent considerable loss in value See image 1.

On steel sections calculations of the structural effect and considerations of the reduced life expectancy of the structure will depend on which is the critical steel section. The designer can consider the highly stressed sections and make a life estimate, however, ALWC often acts preferentially on different parts of the steel sections, leading to unexpected results.

The attack is also not uniform. Not all king piles or sheet piles will be affected in a structure, but those that are may have very similar attack. It is quite typical to find a random 50%, say, of a line of piles affected.

Larssen piles
Larssen and LX sheet piles of ‘U’ section are normally preferentially attacked on the outpan face See image 2.

Depending on the level of high bending moments in the structure this can be a critical position See image 3.

The outpan being the thickest part of the pile cross section, a ‘U’ section pile normally presents thicker steel to the corrosion attack than other pile sections See image 4.

When the attack is at a low stressed area, the pile effectively gives an increased corrosion allowance. Therefore it would be wrong to consider only thickness loss, when analysing inspection results.

There have been examples of piles being attacked on the inpan when timber fendering has been attached to, and partially covers, the outpan. Neither this nor the typical preferential attack has been explained See image 5.

These remarks refer to initial attack conditions. It is to be hoped that no structures are being allowed to proceed into advanced stages when inevitably the holes expand See image 6.

Also see: Hamburg Case History

Frodingham piles
Frodingham sheet piles of ‘Z’ section are normally preferentially attacked on the side pan (web) and the outpan corner (See Image 6).

In most design cases this is not initially structurally critical.

The sidepan being the thinnest part of the pile cross section, a ‘Z’ section pile gives lower resistance to perforation than other pile sections. The maximum web thickness for ‘Z’ piles is 12mm, at a reasonable estimate of corrosion rate of 0.5mm per year this would be holed in 24 years. However smaller sections have thickness of 9mm, which would allow perforation in 18 years or even 10 years, which has unfortunately been some owner’s experience.

Also see: Case Histories

Other Steel Sections
ALWC, although known to attack sheet piles in preferential ways, does not exclude other pile or structural steel sections.

The symmetrical nature of tubes would seem to preclude a preferential attack, but although attack can be evenly circumferential it would normally be one particular site which is attacked establishing an anodic role. Structural sections, if there is a preference, are attacked at the flange ends.

At this point it can be seen that this is a ‘two-sided’ attack, which reminds us that the notional corrosion rates generally quoted are per side.

This is particularly applicable to structural section constructions, fendering and the like. It is not generally the case with tubes which are often open only to outside attack and sheet piles which usually restrain backfill effectively excluding the marine environment.

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Also see: Case Histories