How to prevent ships corrosion by Application of Marine paints

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How to prevent ships corrosion by Application of Marine paints

Protection by Means of Paints : It is often assumed that all paint coatings prevent attack on the metal covered simply by excluding the corrosive agency, whether air or water. This is often the main and sometimes the only form of protection; however there are many paints which afford protection even though they present a porous surface or contain various discontinuities.

For example certain pigments in paints confer protection on steel even where it is exposed at a discontinuity. If the reactions at the anode and cathode of the corrosion cell which form positive and negative ions respectively, are inhibited, protection is afforded. Good examples of pigments of this type are red lead and zinc chromate, red lead being an anodic inhibitor, and zinc chromate a cathodic inhibitor. A second mode of protection occurs at gaps where the paint is richly pigmented with a metal anodic to the basis metal. Zinc dust is a commercially available pigment which fulfils this requirement for coating steel in a salt water environment. The zinc dust is the sacrificial anode with respect to the steel.

Anti-fouling paints offer protection against vegetable and animal growth which can lead to increased resistance requiring additional power, hence fuel, to maintain the same speed. The greater the time spent at sea the less the fouling; but areas of operation and seasons also decide the amount of fouling, and with modern anti-fouling compounds the problem today is less important.

Marine Paints

The paint system applied to any part of a ship will be dictated by the environment to which that part of the structure is exposed. Traditionally the painting of the external ship structure was divided into three regions.

(i) Below the water-line where the plates are continually immersed in sea water.

(ii) The water-line or boot topping region where immersion is intermittent and a lot of abrasion occurs.

(iii) The topsides and superstructure exposed to an atmosphere laden with salt spray, and subject to damage through cargo handling.

However now that tougher paints are used for the ship’s bottom the distinction between regions need not be so well defined, one scheme covering the bottom and water-line regions.

Internally by far the greatest problem is the provision of coatings for various liquid cargo and salt water ballast tanks.

(a) Below the Water-line The ship’s bottom has priming coats of corrosioninhibiting paint applied which are followed by an anti-fouling paint. Paints used for steels immersed in sea water are required to resist alkaline conditions. The reason for this is that an iron alloy immersed in a sodium chloride solution having the necessary supply of dissolved oxygen gives rise to corrosion cells with caustic soda produced at the cathodes.

Further the paint should have a good electrical resistance so that the flow of corrosion currents between the steel and sea water is limited. These requirements make the standard non-marine structural steel primer red lead in linseed oil unsuitable for ship use below the water-line. Suitable corrosion-inhibiting paints for ships’ bottoms are pitch or bitumen types, chlorinated rubber, coal tar/ epoxy resin, or vinyl resin paints. The anti-fouling paints may be applied after the corrosion-inhibiting coatings and should not come into direct contact with the steel hull, since the toxic compounds present may cause corrosion.

(b) Water-line or Boot Topping Region Generally modern practice requires a complete paint system for the hull above the water-line. This may be based on vinyl and alkyd resins or on polyurethane resin paints.

(c) Superstructures Red lead or zinc chromate based primers are commonly used. White finishing paints are then used extensively for superstructures. These are usually oleo-resinous or alkyd paints which may be based on ‘nonyellowing’ oils, linseed oil-based paints which yellow on exposure being avoided on modern ships.

Where aluminium superstructures are fitted, under no circumstance should lead based paints be applied; zinc chromate paints are generally supplied for application to aluminium.

Temporary paint application during shipbuilding

After the steel is blast cleaned it may be several months before it is built into the ship and finally painted. It is desirable to protect the material against rusting in this period as the final paint will offer the best protection when applied over perfectly clean steel.

The formulation of a prefabrication primer for immediate application after blasting must meet a number of requirements. It should dry rapidly to permit handling of the plates within a few minutes, it should be non-toxic, and it should not produce harmful porosity in welds nor give off obnoxious fumes during welding or cutting. It must also be compatible with any subsequent paint finishes to be applied. Satisfactory formulations are available, for example a primer consisting of zinc dust in an epoxy resin.

Protection by Means of Paints

Cargo ships passage

Good surface preparation is essential to successful painting, the primary cause of many paint failures being the inadequacy of the initial material preparation. It is particularly important before painting new steel that any millscale should be removed. Millscale is a thin layer of iron oxides which forms on the steel surface during hot rolling of the plates and sections. Not only does the non-uniform millscale set up corrosion cells as illustrated previously, but it may also come away from the surface removing any paint film applied over it.

The most common methods employed to prepare steel surfaces for painting are:

i) Blast cleaning

ii) Pickling

iii) Flame cleaning

iv) Preparation by hand

Blast cleaning is the most efficient method for preparing the surface. Following the blast cleaning it is desirable to brush the surface, and apply a coat of priming paint as soon as possible since the metal is liable to rust rapidly.

There are two main types of blasting equipment available, an impeller wheel plant where the abrasive is thrown at high velocity against the metal surface, and a nozzle type where a jet of abrasive impinges on the metal surface. The latter type should preferably be fitted with vacuum recovery equipment, rather than allow the spent abrasive and dust to be discharged to atmosphere, as is often the case in ship repair work. Impeller wheel plants which are self-contained and collect the dust and re-circulate the clean abrasive are generally fitted within the shipbuilding shops. Cast iron and steel grit, or steel shot which is preferred, may be used for the abrasive, but non-metallic abrasives are also available. The use of sand is prohibited in the United Kingdom because the fine dust produced may cause silicosis.

Pickling involves the immersion of the metal in an acid solution, usually hydrochloric or sulphuric acid in order to remove the millscale and rust from the surface. After immersion in these acids the metal will require a thorough hot water rinse. It is preferable that the treatment is followed by application of a priming coat.

Using an oxy-acetylene flame the millscale and rust may be removed from a steel surface. The process does not entirely remove the millscale and rust, but it can be quite useful for cleaning plates under inclement weather conditions, the flame drying out the plate.

Hand cleaning by various forms of wire brush is often not very satisfactory, and would only be used where the millscale has been loosened by weathering, i.e. exposure to atmosphere over a long period.

Blast cleaning is preferred for best results and economy in shipbuilding; pickling which also gives good results can be expensive and less applicable to production schemes; flame cleaning is much less effective; and hand cleaning gives the worst results.

Paint hazards

Marine Paints may contain toxic or irritant substances, and the solvents may give rise to flammable and potentially explosive vapours, which may also be toxic. Personnel using such paints should be warned of the particular risks arising from their use. Paints containing organic pesticides can be particularly dangerous. If the manufacturer's instructions are not given on the container, information should be obtained at the time of supply about any special hazards, and also whether special methods of application should be followed. Such advice should be readily available at the time of use but the following precautions should always be taken.

Preparation and Precautions

Painted surfaces should always be rubbed down wet to reduce dust from the old paint, which may be toxic if inhaled. Where the dust is known to contain lead, other dust treating methods should be used. Dust masks should be worn as protection against other dusts.

If the surface to be rubbed down is known to contain lead, then methods that do not create dust should be adopted. It is safer to avoid or minimise dust creation than to try to clean up the dust afterwards. Sanding or abrasive blasting should be avoided. Lead based paint should never be burnt off as fumes will contain metallic lead in a readily absorbed form.

Rust removers are acids and contact with unprotected skin should be avoided. Eye protection should be worn against splashes . If painting aloft or otherwise near ropes, care should be taken to avoid splashes on ropes, safety harness, lines etc.

Interior and enclosed spaces should be well ventilated, both while painting is in progress and until the paint has dried.

There should be no smoking or use of naked lights in interior spaces during painting or until the paint has dried hard. Some vapours even in low concentrations may decompose into more harmful substances when passing through burning tobacco.

When painting is done in the vicinity of machinery or from an overhead crane gantry, the power supply should be isolated and the machine immobilised in such a way that it cannot be moved or started up inadvertently. Appropriate warning notices should be posted . Close-fitting clothing should be worn.

Use of Paint Spraying Equipment

As there are many different types of paint spraying equipment in use, operatives should comply with the manufacturer's instructions for use.

Airless spray-painting equipment is particularly hazardous since the paint is ejected at a very high pressure and can penetrate the skin or cause serious eye injuries. Spray should not be allowed to come into contact with the face or unprotected skin.

Suitable protective clothing such as a combination suit, gloves, cloth hood, and eye protection should be worn during spraying.

Paints containing lead, mercury or similarly toxic compounds should not be sprayed in interiors.

A suitable respirator should be worn according to the nature of the paint being sprayed. In exceptional circumstances it may be necessary to use breathing apparatus .

If a spray nozzle clogs, the trigger of the gun should be locked in a closed position before any attempt is made to clear the blockage.

Before a blocked spray nozzle is removed or any other dismantling is attempted, pressure should be relieved from the system.

When blowing through a reversible nozzle to remove a blockage, all parts of the body should be kept clear of the nozzle mouth.

The pressure in the system should not exceed the recommended working pressure of the hose. The system should be regularly inspected for defects.

As an additional precaution against the hazards of a hose bursting, a loose sleeve, for example a length of 2 to 3 meters (6 to 10 feet) of old air hose, may be slipped over that portion of the line adjacent to the gun and paint container.

To minimise risk of fire and to a lesser extent pollution, paint lockers are supposed to be kept clean. Areas of spilled paint must be cleaned immediately and shelves should be neatly stacked. Insofar as is possible, similar paint-types should be stored together with bars firmly in place, to prevent drums from jumping in bad weather.

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