Corrosion is a natural reaction where steel under the influence of water and air transforms to rust. The speed of corrosion is enhanced by the presence of salt. It will be clear that water, air and salt perfectly describe a marine environment.
Corrosion can also occur when steel comes in contact with corrosive chemicals even when at first sight this is not expected. For instance coal in its basic form is harmless but coal ore may contain sulphur impurities, which in combination with moisture forms sulphuric acid, a strong corrosive chemical.
Another type of corrosion that deserves mention is biological corrosion caused by organisms such as the sulphate reducing bacteria (SRB). SRB are widespread over the world and start growing when oxygen is absent. Such conditions can be found on submerged- or buried structures but also in water ballast tanks where often a layer of mud is deposited on the steel.
The exact process has not been elucidated yet but basically SRB use sulphate as their source of oxygen and in turn produce sulphide ions. Sulphide ions are highly corrosive and as result steel corrodes to typical terrace-shaped craters with black iron sulphide on the crater bottom.
In seawater, the corrosion rate of unpainted steel is quite low, 50-175 micron per year, and corrosion is only dangerous when accelerated. The danger arises from the presence of cathodes whose area is large relative to the anodic areas - the conditions are then right for a high current density at the anodes and rapid metal loss (i.e. pitting). Chemical and biological corrosion are two good examples of such conditions. Pitting is the most dangerous form of corrosion, because in its worst form it might cause total breakdown of the steel structure. However, overall light corrosion can also contribute to deterioration in ship per¬formance, particularly when it affects the smoothness of the underwater hull.
By now it will be clear that protecting steel against corrosion requires a strategy where factors as steel exposure conditions and intended functional use of steel structures have to be considered.
PROTECTION AGAINST CORROSION
Painting steel is an efficient method of preventing corrosion. By doing so, a barrier is formed against two factors needed for initiating the corrosion process: air (containing oxygen) and moisture. However, not all coatings can be used on steel or can withstand the harsh marine environment. It is also important to realise that where a corrosion cell exists, conditions at the cathode become alkaline and therefore protective paints should resist alkali. For some applications other requirements such as acid- and chemical resis-tance are important too. Therefore, marine coatings are specially developed and tested in order to protect ships against corrosion.
Marine coatings can be divided into 3 groups according to their function in the coating system.
Primer contain inhibitive pigments that give protection against corrosion during the service life. Furthermore, they provide a good adhesion on sufficiently prepared steel and cleaned old coatings. Primers should be easily recoatable with suitable build coats or finishes.
Build coats are used as an intermediate coat in a coatings system in order to enhance the overall protection and to provide a good intercoat adhesion. Contents of a build coat depend on the part of the ship on which it is used. In most cases they contain pigments, which reduce moisture penetration and decrease oxygen permeability.
Although in some applications they are left uncoated, they are normally designed to be easily recoatable with topcoats.
As a finishing layer, a topcoat gives the required colour and gloss and provides protection against various influences such a sunlight, weather, abrasion and chemical attack.
The "Properties Comparison Table" shows a review of the properties of each paints type.
One should bear in mind that the mentioned qualifications shown are general indications. Total paint system and surface preparation are for instance two major influences that can affect the properties and performance of paint in a positive but also in a negative way. Of course, your local Transocean Company will be more than happy to advise and assist you in selecting an appropriate paint system. However, when looking for a specific paint system, this table might help in selecting the most suitable one.
The single most important function that can influence paint performance is the quality of surface preparation. For optimum service life, the surface must be completely free of all contaminants that might impair performance and should be treated as such to assure good and permanent adhesion of the paint system. The quality of surface preparation has a direct relation with the lifetime of a system. Even when using surface tolerant paints it cannot be emphasized enough that better surface preparation always result in longer lifetimes.
The most important methods employed to prepare surfaces for paint application in modern shipyards are power tool cleaning, blast cleaning and hydro jetting.
Common Powertools are wire brushes, scrapers, chipping hammers etc. Examples of mechanical tools are rotary wire brushes, sanding disc and needle guns.
Preparation grades with powertool cleaning are specified according to International Standards method ISO 8501/1: 1988 and relevant preparation grades are St2 and St3. Preparation grade St2 is in general not recommended for underwater areas.
Blast Cleaning is based on the principle of an abrasive jet of particles in a compressed air stream impinging on the surface, removing impurities, millscale, rust and old paint. Abrasive blast cleaning is the most thorough and widely used method of surface preparation in the shipbuilding and repair industry. Different degrees of surface cleanliness are possible and depend in part on the surface condition prior to treatment and also to the length of time for which the surface is exposed to the abrasive jet. In addition to cleaning the surface, the abrasive particles will impart a surface roughness to the steel.
|This so-called "profile" roughness can be a very important "key" for anchoring of paint systems. Mineral slag blasting grit generally gives faster rates of cleaning and lower health risk (from shattered grit) than does sand. Grit also gives more effective cleaning, especially for pitted substrates, and some grades can be recycled.||
|Spot blasting is localised abrasive cleaning often carried out in ship repair, especially on the outside hull, where patchy corrosion or damage has occurred. It can be used to yield surfaces that are cleaned to Sa 2 or better but often surrounding intact areas are peppered with stray grit. Always mark areas to be spot blasted and mechanically "feather" the damage round the area using rotary disc or sander.||
The surface appearance resulting from blast cleaning has been defined by several bodies - American (ASTM D 2200 and SSPC VIS. 1 & 2), British (Standard BS 4232), German (Standard DIN 18364) and Japanese (JSRA SPSS, 1975). The most widely used was the Swedish Standard (SIS 05 5900 "Pictorial surface preparation standard for paint steel surfaces") which also sought to define the initial condition of the steel. This standard was taken over by International Standard ISO 8501/1: "Rust grades and preparation grades of uncoated steel substrates after overall removal of previous coatings".