How do these standards help with corrosion mitigation? There are a number of standards used. For example, maintenance standards for coatings cover areas such as surface preparation, coating application, coating inspection, and how to determine coating deterioration.
The standards clarify what is required and make it possible for all parties involved with ship construction and maintenance to have a comparable understanding of the requirements. What are your expectations for a coating system in terms of application, performance, service life, and return on investment?
We follow maintenance regulations and guidelines that define the requirements for the condition of the coating.
In some areas, more work may need to be done to maintain the specified coating condition than in other areas. As discussed previously, ballast tank coatings also have a target life of 25 years and the coatings must meet certain regulated conditions throughout this time period.
If the coating condition falls short of this rating earlier than its desired service life, then remedial action needs to be taken. Although there are many facets of ship design and maintenance that need to be considered when battling corrosion while at sea, there are many corrosion standards, guidelines, and corrosion protection technologies and products available to assist the maritime industry in economically achieving the desired service life of its vessels.
The article originally appeared in the Spring issue of Maritime News, www. Become a Member Sign In. Become a Member Sing In. Photo courtesy of Stolt Tankers. What types of corrosion issues could be encountered on tankers? Many factors can cause corrosion on tankers. Painting the galvanised items can very much extend the lifetime of the zinc coating past this point.
Smaller items or sheet steel can be galvanised using electro-deposition processes. When special anticorrosive properties are required from a surface, then specialist processes such as flame spraying may be an economic choice over more corrosion resistant alloys. Sacrificial cathodic protection uses the principle of galvanic corrosion to provide protection to the chosen structure by the dissolution of another metal. Zinc or aluminium anodes are usually used to protect steel structures, whilst zinc anodes can be used to protect aluminium hulls.
Sacrificial anodes can be used in tanks and holds and on exterior surfaces such as hulls and jetties. Passive cathodic protection using anodes is a very common as a backup to unimpressed current system on the outer hull.
Anodes need to be carefully sited around the steering gear and the propellers to give a good current distribution and to work in harmony with the coating system.
Passive cathodic protection systems are very useful in cargo holds, cargo tanks, ballast tanks and in areas where galvanic couples can occur and cause high corrosion rates. Anodes are commonly used in mixed metal situation in heat exchangers and inter-coolers associated with propulsion and air-conditioning systems.
ICCP systems use inert non-dissolving anodes together with reference electrodes as part of a feedback system. This allows manual or automatic control over the protection provided.
These systems are only used on exterior surfaces as they generate gases that could be dangerous in confined spaces. It is important that the cathodic protection system works together with a coating scheme and does not cause paint breakdown.
Careful choices and balances of the both elements are required for optimum performance of both systems. In military applications cathodic protection systems on the outer hull can lead to undesirable electrical fields in the water surrounding the vessel that can lead to it being detected more easily. Special cathodic protection systems are designed that have a number of external anodes distributed along a length of the hull. They are also used in conjunction with electronic systems that reduce the electrochemical field produced by the turning propeller shafts and the propellers themselves.
The amount of current drawn from such systems depends on the quality of the coating on the hull and it's through lifetime integrity. Should the integrity of the coating become reduced then the amount of current taken from the ICCP system will increase and increase the likelihood of the vessel being detected. Good coating quality management at the outset is the best way to reduce this risk. When dealing with designing out corrosion at the new building and procurement stage very careful consideration should be given to balancing the needs of cathodic protection systems and organic coatings.
Before construction begins, it is essential that all parties involved agree on the methods, conditions and time scales involved in construction. Adequate inspection by trained personnel and good record keeping are also necessary, in case of future disputes.
Incoming raw material checks. Surface Preparation Standards. QC procedures. Shop primer line. Sub assembly. Block assembly.
Weld and Edge Preparation. Partial fitting out. Erection on dock or slipway. Sea trials. Each process presents its own corrosion challenge both during construction and in service.
When considering coatings as an anticorrosive strategy for new warships, it is extremely important to operate a "get it right first time" policy. Coatings applied at the new construction stage can be applied under much more favourable conditions than coatings applied as part of a maintenance and repair procedure. This is because substrate surface condition in terms of surface cleanliness, surface contamination, surface profile and initial steel quality can often show far more long term effects than the quality of the coating applied.
Throughout the build cycle of the vessel, the conditions under which the coatings are applied are crucial in determining the service life and cost effectiveness of the coatings scheme. Weather conditions such as fog, rain and other high humidity conditions can lead to coating delamination between coats. Low temperatures during coating application can lead to poor adhesion between coats and airborne contamination can lead to inter coat blistering.
Welding and other forms of rework during the construction process can lead to coating damage especially with warships that have an extensive fitting out period. Clear planning of this period in order to minimise all forms of coating damage and corrosion damage due to exposure to the elements and the effects of mechanical abrasion is essential to give good through life performance without inconvenient and expensive repairs being necessary at a later stage.
Good planning of a coatings maintenance procedure starts at the design stage as it is often areas that are physically difficult to access that tend to break down first. It is often far more economic to apply a very good coating on to a well-prepared surface when that surface is easily accessible than to try to affect repairs when the surface has been covered by insulation, wiring or pipe work.
Choosing mechanically strong or abrasion resistant coatings for areas subject to wear and impact damage should be considered for all surfaces that will be in contact with personnel, mooring ropes and in contact with jetties and tugs. Often corrosion problems arise with the new materials supplied to the shipyard.
Many materials pick up surface contaminants during their manufacture and subsequent transportation. Care should be taken to remove surface contamination prior to installation or preparation and painting. Stainless steel components will often need thoroughly cleaning and passivating prior to installation otherwise severe surface pitting can occur. Copper and copper based alloys commonly arrive with carbon films on the surface that can provide good cathodes and lead to high corrosion rates.
This is particularly common with pipework, valves and pumps. Steelwork can arrive in a heavily pitted condition that will cause subsequent problems with the blasting and painting operations. The shop primer line is often referred to as primary surface preparation. It is at this point that the quality of the coatings applied to the block sections is set.
If material that is contaminated with oil or grease passes through the shop primer line without being removed properly, then the coatings applied on top of it will be severely compromised. Close attention should be applied to inspection of the plates as they emerge from the automated blasting process and also to the thickness and quality of the shop primer applied. At the sub-assembly stage the hull components are coated with shop primer which can then become contaminated or damaged.
Welds and edges need careful attention with regard to smoothness and lack of porosity or other irregularities. Often the welding and cutting processes can introduce surface contamination in the form of oils, weld fume or footprints. This area of the ship sits below the waterline, so it experiences more exposure to damaging salts and minerals.
Applying two-part coal-tar epoxy and a vinyl tar coat can help to protect it. The deck is also susceptible to corrosion, especially during adverse weather conditions.
During storms or strong tides, seawater can wash up on the deck and lead to surface damage. Corrosion in cargo tanks is also common.
This happens when the sulfur and water in crude oil mixes with water vapor. Two types of corrosion particularly relevant for the marine industry are pitting corrosion and bacterial corrosion.
Preventing corrosion requires elimination or suppression using two principal methods, cathodic protection and coatings. Generally, cathodic protection systems are used in conjunction with coating systems. The objective with cathodic protection is to suppress the electrochemical reaction taking place.
Under normal corrosive conditions, current flow from the anode results in a loss of metal at the anodic site which results in the protection of the metal at the cathodic site.
Protection can be provided by making the structure you wish to protect cathodic, using two methods:. When two dissimilar metals are immersed in seawater, the metal with the lowest electrical potential will suffer the greatest corrosion. For example, the corrosion rate of mild steel can be controlled by connecting it to zinc as it will then become the anode and corrode. In this example, the zinc anode is referred to as a sacrificial anode because it is slowly consumed corrodes during the protection process.
Another use of zinc as a sacrificial anode is when steel is coated with the zinc; either in the form of galvanization or metalisation or in a paint which contains high levels of active zinc. A vessel hull can be made cathodic by using a direct current source. An impressed current is applied in the opposite direction to cancel out the corrosion current and convert the corroding metal from anode to cathode.
In this example, the negative terminal of DC is connected to a pipeline to be protected. The anode is kept in to increase the electrical contact with its surrounding environment.
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