Controlling corrosion on a ship
Coatings are the most visible sign of a shipowner’s battle to keep his asset in top condition. Except where they fail, any corrosion will very likely not be visible. While coatings are the first line of defence they are not the only means of fighting corrosion and in some aspects they are not even the most suitable.
Corrosion of ship structures and machinery has multiple causes including with seawater or with salt-laden air, microbial action, corrosive cargoes and even the sulphur within the fuel used in main and auxiliary engines. Describing the science of corrosion is beyond the scope of this guide but some of the various methods used to combat corrosion are covered below.
Corrosion by sea water is known as aqueous corrosion and is an electrochemical process where ions are exchanged between metals on the ship’s hull and propulsion and manoeuvring systems. Some metals are corrosion resistant and the hull coating also gives an element of protection to the steel which is likely the most corrosion prone of the underwater parts. Most corrosion resistant metals including stainless steel rely on an oxide film to provide protection against corrosion. Combatting sea water corrosion can be done in many ways including:
• Protection by coatings
• Changing the potential of the metal to a point where corrosion ceases - by impressed voltage or coupling to a sacrificial anode.
• Use of corrosion inhibitors
• Changing the pH of the local environment by chemical dosing.
• Avoiding materials which corrode easily where possible.
Coatings must be effective if they are to prevent corrosion. Performance can be affected by poor preparation or application, mechanical damage caused by any of several means and also the action of marine organisms. In ballast tanks in particular sulphate reducing bacteria, left undisturbed in marine silt or mud deposits, will produce concentrations of hydrogen sulphide which are particularly aggressive to steel.
Some damages such as those caused by the action of anchors and anchor chains against the hull and above waterline mechanical damage caused by contact with objects at sea, quays and tug operations are usually visible and should be repaired at the earliest opportunity. This is necessary because under the right conditions corrosion can take hold and spread rapidly.
However, often the corrosion will be hidden from view in areas where access is difficult or only allowed at certain times or where it has begun to take hold under a coating. Discovering corrosion underneath apparently intact coatings anywhere on the ship can be done using an ultrasonic thickness gauge. Damage below the waterline is less easy to identify but can be addressed by cathodic protection.
In the simplest form of cathodic protection, sacrificial anodes – usually made from zinc – are attached to the hull and the surrounding seawater behaves as an electrolyte allowing a natural electro-chemical reaction to take place. The anodes gradually wear away and become spent but in doing so protect the main hull or other area they from corroding.
An improved version of cathodic protection known as impressed current cathodic protection (ICCP) system uses an arrangement of hull mounted anodes and reference cells connected to a control panel, the system produces a more powerful external current to suppress the natural electro-chemical activity on the wetted surface of the hull and so eliminates the formation of aggressive corrosion cells on the hull and avoids the problems which can exist where dissimilar metals are introduced through welding or brought into proximity by other components such as propellers. ICCP systems are designed to automate the current output while the voltage output is varied allowing protection to be maintained at all levels of seawater resistivity. In a sacrificial anode system, increases in the seawater resistivity can cause a decrease in the anode output and a decrease in the amount of protection provided. With ICCP systems protection does not decrease in the range of standard seawater.
An essential feature of ICCP systems is that they constantly monitor the electrical potential at the seawater/hull interface and carefully adjust the output to the anodes in relation to this. As well as hulls, ICCP systems have been developed to protect bow and stern thruster tunnels. Alongside this has gone the design of more sophisticated shaft earthing systems and rudder bonding equipment to provide reliable protection for propulsion and steering systems.
The anodes in an ICCP system are far fewer than for a sacrificial anode system and are designed for longevity lasting for several drydockings. Where sacrificial anodes are used, scrimping on their number and location and extending periods between dry-docks will result in the anodes becoming ‘spent’ and ceasing to function before they are due for replacement.
Corrosion inhibitors added to bilges and other places where seawater collects will provide protection inside the ship and on superstructure. Procedures need to be put in place to ensure that the corrosion inhibitor is replenished when necessary and also not used over liberally otherwise costs will escalate.
Choice of materials can cause or prevent corrosion. Where different metals are in contact with each other galvanic corrosion can take place. This can extend beyond the different structures to include bolts and fixings or even welds. It is not unknown for some metal alloy components to be subjected to galvanic corrosion within their own structure. This is sometimes found to be the cause for premature failure of alloy components. Special attention must be given to the problem at the design stage or when modifying vessels. In future new materials and composites may be a way around this phenomenon.
Beating the bug
Microbial corrosion of fuel tanks is an issue that has only come to light within the last decade or so. It is a problem in some cases where fuel tanks contain quantities of water and certain organisms are also present. The microbes live in the fuel/water interface and draw their nutrients from the fuel. Tank systems are usually designed to allow the water to be drained off, but where the design is deficient or the drains blocked or damaged, the water content in the fuel can easily rise above a critical level allowing the colonies to form. As the various organisms feed on the fuel they produce a sludge-like waste that can block filters which is a problem in itself but not the direct cause of corrosion. The corrosion are fungal by-products of bacterial metabolism. The corrosive attack caused by fungi follows the same general pattern: the fungi excrete organic and inorganic acids along with other metabolism by-products, and these trigger the corrosion process. Alkaline biocides will cure the problem but their use requires expert assistance as will proper diagnosis of the problem.