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 performance, 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.
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.
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".
Waterjetting or hydroblasting as a surface preparation technique is being used more and more in shipyards. Commonly used and suitable water pressures range from High pressure (700- 1700 bar) to Ultra High pressure (greater than 1700 bar).
A major advantage of using water pressure as an abrasive is the lower impact on generated than is the case with grit blasting. It also constitutes less of a safety risk caused by sparks and reduces the amount of salt remaining on the surface.
Prior to Waterjetting, water insoluble foreign matter such as oil and grease must be re¬moved. The surface quality then resulting from Waterjetting is divided into three DW grades by the German STG-norm 222-1992. The left picture gives a close up of the steel area after hydro blasted at 2000 bar. The rotating nozzle leaves a circular pattern on the steel. Some flash rust is visible too.
The table below gives an overview and description of surface preparation grades used in this manual and Transocean data sheets.
|Preparation as stated Transocean datasheets||Reference standard||Description|
|HPFWC= high pressure fresh watercleaning (pressure 70-700 bar)||This method is routinously used on ships in drydock to clean the underwater area of fouling, salts, loose adhering paint and other foreign matter.|
|Solvent Cleaning||SSPC-SPl||Foreign matter other tha n oil and grease should be removed by scraping or brushing followed by HPFWC. Removal of oil, grease, dirt, soil, salts and contaminants by cleaning with solvent, alkali, emulsion or steam. After cleaning remove dirt, dust and other contaminants by vacuuming or blowing with clean, dry air.|
|Thorough Hand and Power tool cleaning ISO-St2||SSPC-SP2||When viewed without magnification, the surface must be free from visible oil, grease and dirt and from poorly adhering mill scale, rust, varnish coating and foreign matter.|
|Very Thorough Hand- and Power tool cleaning ISO-St3||SSPC-SP3||Similar to St2 but the surface must appear very thoroughly treated to give a metallic sheen arising from the steel surface.|
|Brush off Blastcleaning ISO-Sal||SSPC-SP7||When viewed without magnification, the surface must be free from visible oil, grease and dirt and from poorly adhering mill scale, rust, varnish coating and foreign matter.|
|Thorough Blastcleaning ISO-Sa2||SSPC-SP6||When viewed without magnification, the surface must be free from visible oil, grease and dirt and from most of the mill scale, rust, varnish coating and foreign matter. Any residual contamination must appear firmly adhering.|
|Very Thorough Blastcleaning ISO-Sa2½||SSPC-SP10||When viewed without magnification, the surface must be free from visible oil, grease and dirt and from most of the mill scale, rust, varnish coating and foreign matter. Any remaining traces of contamination shall show only as light stains in the form of spots or stripes.|
|"White metal" Blastcleaning 150-Sa3||SSPC-SPS||When viewed without magnification, the surface must be free from visible oil, grease and dirt and from mill scale, rust, varnish coating and foreign matter. It shall have an uniform metallic colour.|
|Hydrojetting OW2 (STG-2222)||Loosely adhering mill scale, rust and poorly adhering coatings are removed. Various spots of old coating systems and firmly adhering mill scale is still present. Thin coatings on previously blastcleaned surfaces are predominantly removed. Before drying a weak sheen arises from the metal surface which disappears during drying due to flash rust formation.|
|Hydrojetting OW3 (STG-2222)||As OW2. Firmly adhering mill scale is still present. From firmly adhering rust at most thin dark oxide layers and/or slight residues in the roughness valleys are present. From firmly adhering old coatings residual areas having spots with damages, various scattered small spots and residues in the roughness valleys may be present. Thin coatings on previously blastcleaned surfaces are predominantly removed. Before drying a distinct sheen arises from the metal surface which disappears during drying due to flash rust formation.|
Paint is not a finished product until it has been applied and dried on an appropriate substrate at the designed performance film thickness. Proper application therefore is critical to the performance of the paint system. High performance paint systems are especially sensitive to misapplication and knowledge of the application characteristics and recommended film thickness is vital to obtain optimum result
Low viscosity paints are easily applied by these techniques to yield low applied film thickness. Modern, thixotropic paints are often specified at high film thickness especially where they perform a protective function. Therefore, where brush and roller methods are called upon (especially for "touching up" or "stripe coating") a number of coats may need to be applied in order to achieve the minimum specified dry film thickness. It is in general better to apply high solids paints by brush instead of roller. Although these techniques have largely been replaced by spray application, they may find use in maintenance schedules operated by ship crews. Both methods have the advantage that paint losses are low but on the downside is the slow working speed.
This technique mixes a jet of air with a stream of paint to propel a fan of paint droplets towards a surface. The mix of air with the paint particles gives high turbulence however and considerable "bounce back". Air atomisation of paint can thus results in considerable overspray. Therefore, not only must adjacent areas be protected but also paint applicators must wear protection to avoid paint mist inhalation. The technique particularly suits low viscosity paints and in the marine paint field is most commonly used for the application of conventional decorative paints and zinc silicate coatings.
This technique relies on hydraulic pressure rather than air atomisa¬tion to produce the spray. Paint under very high pressures (1.000 to 6.000 p.s.i., approximately 100 to 400 kg/cm2) is delivered to the spray gun and then forced through a very small orifice to atomise it. Thus more rapid coverage can be achieved with much less over¬spray and considerably higher film thickness can be obtained.
Most paints manufactured by Transocean Marine Paint for application on ships can be applied by airless spray. It has many advantages over conventional application methods such as a high output, reduced spray mist and less need for thinning. Finally, it must be remembered that airless spray ejects under very high pressure. The spray gun should not be directed at people as injury can be easily caused and due precautions taken when the equipment is being cleaned.
The quality of paint application can be seriously affected by weather conditions. Bad weather conditions are a perennial hazard in ship painting operations especially during winter in moderate climates. At low temperatures (below 5°C), the curing of paints such as ordinary epoxies may slow down dramatically and for some paints stop altogether. Others are not seriously affected and chlorinated rubber and vinyl paints may be used at or below 0°c as long as the surface is free from ice. Most paints will become thicker when temperature decreases and this effect may result in poor atomisation, dry spray and poor flow. The problem may be rectified by the addition of thinners but never more than the amount stated in the product datasheet.
Excessively high temperatures too may present problems. Reaction times for two-component paints increase resulting in shortened potlifes and increased curing speed. It is advised not to mix more material than can be used in the time stated as the potlife at the relevant temperature.
Generally, painting should be avoided during extremely hot hours¬where paint operations are carried out in hot climates, the paint should be applied in the morning and early evening. Paint should never be applied on wet surfaces and therefore paint¬ing is to avoid not only in rain, sleet and fog but also when high hu¬midities and low steel temperatures lead to condensation. Condensation is very difficult to detect on surfaces and will occur if the steel temperature is below atmospheric dew point. As a general guide, application should not take place when the steel surface temperature is less than 3°C above the dew point.
Finally, good visibility of the applicator is important to achieve a good control of the paint thickness and the quality of application. Therefore, painting should be preferably carried during daylight and if necessary, under additional lighting.
Application problems can originate from several causes or even from a combination of causes. An overview of typical problems experienced with paint application along with its causes and possible solutions is given in the table below.
|Excessive spray fog||
|Streaks or rattails||
In Transocean datasheets for all products theoretical spreading rates are given. From this figure the theoretical consumption can be calculated as followed;
Theoretical consumption (l/m2) = 1 / Thearetical spreading rate (m2/1)
However, this formula does not take loss factors into account. The choice of application method has a great influence on the total loss factor.
Application by brush or roller typically has loss factors of 10 - 15% whereas ap-plication by conventional spray loss factors of 50% is no exemption.
For airless spray usually a loss factor of 30% is calculated.
Other loss factors are roughness of the substrate (5 - 20% loss), uneven application (5 - 10% loss) and windy conditions (dependent on the wind force may range from 5 to 30% losses).
Introduction of the loss factor in the calculation leads to the terms practical spreading rate (m2/l) and finally to the practical consumption (l/m2). It is clear that the loss factor always is an estimation based on the local conditions and the experience of the painter.
Thinning may be necessary to achieve a good painting result. When thinning one has to realize the volume solids of the paint is lowered. As a result more wet film thickness has to be applied to achieve the necessary dry film thickness.
From the table below it can be deducted how volume solids change when adding a certain amount of thinner.
Suppose a dryfilm thickness of 100 micron has to be achieved with a paint having 50% volume solids.
Unthinned theoretically 200 microns wet has to be applied. However, when the paint is diluted with 10% thinner, new volume solids form table will be 45%.
As a result new wet film thickness to reach 100 micron dry is 100/0,45= 222 microns.