Steel
Steel is very suitable for the construction of marine structures, particularly where there are high design loads however it readily corrodes in a marine environment where it is exposed to salt water and oxygen.
It is also used for machinery and parts such as lock hinges, bollards, handrails and access ladders.
For machinery, the manufacturer will supply the maintenance procedures.
Components such as bollards will have been installed with a design working load however with age this should be revisited either through testing or the use of a civil engineer to survey the structure to ensure the foundations and fixtures such as bolts are still suitable.
Provided the steel has been effectively protected by a suitable system the effect of corrosion can be minimized but there is a need for a planned maintenance system to ensure this.
There are two principal protection systems: coating and cathodic protection.
Coatings will normally be a paint system that depends upon the quality of the product, surface preparation, and application and can give a life of 10 to 20 years (dependent on the manufacturer’s recommendations and intermediate maintenance).
The issues with maintaining coatings are access, preparation, and application of the new product in situ.
Cathodic Protection, such as sacrificial anodes or impressed current cathodic protection (ICCP) systems using DC power and long-life anodes protect the surrounding steelwork.
ICCP will have a greater initial cost than paint or sacrificial anodes.
Either cathodic system only protects the wetted areas.
Areas that may dry out still require a coating for the steel’s protection from corrosion.
Referring to Fig. 1, the “traditional” corrosion zone for steel is the splash zone; that point around the high-water mark where a protective coating can be damaged by contact with floating material or a vessel.
The atmospheric zone is above the splash zone and below the top of the structure and as such is in a salt-laden environment where any bare steel will corrode.
This sort of corrosion will have an average rate of 0.1 to 1.4mm over 5 years2.
A relatively new identified problem for carbon steel in port environments is Accelerated Low Water Corrosion (ALWC) caused by microbially influenced corrosion, ALWC is also known as biocorrosion.
In reality, the problem has been around for many years, identified in the 80s but was generally thought to be down to local conditions until the number of cases resulted in a revised opinion.
Carbon steel has been used in sheet piles for many years and ALWC is a widespread problem that particularly affects steel between mean low water springs and the lowest astronomical tide.
ALWC can be very aggressive, greatly reducing the strength of a structure in a vertically concentrated section along its whole length; Fig 2 shows a severe case where there is a significant reduction in strength.
ALWC corrosion rates of up to 4mm per year have been reported although 1mm per year appears to be the norm3.
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