The reason why rusting is such a problem with iron and steel is that the oxide layer that forms does not adhere to the underlying metal, but powders or flakes off, leaving fresh metal surfaces exposed. Aluminium, which is a much more reactive metal than iron, does not corrode because the oxide layer that forms rapidly on an aluminium surface adheres closely to the underlying metal, and prevents further access of oxygen or other environmental oxidants.

Rust converter products are usually based on phosphoric acid; there are other additives as well to aid with flow and adhesion. The black coating that forms is hard, insoluble, ferric phosphate:

Fe2O3 + 2 H3PO4 —-> 2 FePO4 + 3 H2O

Unlike ferric oxide, ferric phosphate does adhere closely to the underlying metal, and protect against further access of oxygen or oxidants. If some excess phosphoric acid remains adsorbed or otherwise trapped in the phosphate layer, it will give some further protection against rusting at the site of small chips or scratches that damage the initial phosphate layer.

In using these products, it is usually best to clean the rust layer back to a fine powdery covering over the surface. The phosphate layer that forms in the presence of bare metal, or of sizeable rust flakes, is not usually quite so well-formed and effective.

Marine Applications

UNDERSTANDING CORROSION IN A MARINE ENVIRONMENT

The careful diagnosis of any formation of rust should be made before treatment.  Most forms of rust on marine craft, however, are a result of two similar actions and, as such, can usually be treated with similar remedies. In both cases there is the common factor of salt air or salt moisture and it is with this factor that the treatment is concerned. The two forms of rust come about by either:

  1. The result of neglect, where the coated surface of the steel is allowed to deteriorate to the point where it can be attacked by air and water.
  2. The flaking action of mill scale which has the same effect of exposing the bare steel to the elements.

In the former case, no material should be allowed to deteriorate to the extent that its protective coating leaves it bare. The latter, mill scale, is usually confined to fairly new steel and can usually be combated with reasonable attention in the early stages of painting. Mill scale is the bluish-grey sheen on new steel. When the sheet steel is rolled or milled, a fine scale forms on the steel that does not adhere too well to the body of the plate. As time goes on the mill scale tends to loosen and fall off, taking with it any protective coating that may have been applied. Obviously, the only treatment for this problem is the removal of the scale at an early stage before painting is undertaken.

Mill scale can be removed in several ways but the best and most common method is wet sand blasting, where a powerful jet of water directs millions of grains of sand or grit on to the plate at high pressure. This blasts away the scale revealing the clean steel beneath. The newly exposed surface will start rusting in a very short period of time. The weather will do a similar job but it is much slower, taking up to six months.  After the mill scale falls off, there is a coating of powdery rust that can then be brushed away and the steel treated and painted. No protective coating will prevent the scale from falling off over a period of time, so new steel work, whether it be fittings or a complete job, needs to be treated if serious rust problems are to be avoided.

If rust is occurring because of the deterioration of the covering paint, and the rust has eaten into the steel plate itself, then the treatment must be more drastic. The flaking rust must be chipped away, and this can be done most effectively by sand blasting or with chipping hammers. It is important to avoid marking the steel underneath the rust with the chipping hammers, as this tends to bruise the cleaned surface, making it harder for an Xtroll penetrative sealer to penetrate and therefore easier for rust to come back after it has been cured. Remaining rust should then be brushed away with a heavy wire brush until there is a sound substrata ready for treatment.

Treatment with anti-corrosives and paint should then begin. Quite often the metal is rusted in a difficult position, such as with fittings or threads and steel bolts, or in an area where access with a chipping hammer or wire brush is impossible. This is where Xtroll Rust Conqueror excels because it can be sprayed into these awkward places where it will penetrate and encapsulate the rust, alleviating the necessity of removing plates and other sections to gain access.

STEEL BOATS

Rust can and will attack almost any part of a boat that has a steel component. Rigging is very susceptible, as are engines, for the heat of the engine tends to blister any paint which may have been applied, leaving the way wide open for rust to move in. Correct treatment, using the appropriate Xtroll system, will also pay off handsomely in cutting maintenance by saving time spent rubbing back and building up rusted steel will.

There are some parts in a boat’s construction or fittings that cannot be accessed to perform maintenance. In a steel boat, the corners of bilges and the stem and stern joints are all virtually inaccessible. Fittings at the top of masts or similarly difficult spots may have to go without maintenance for some time. This is where Xtroll Rust Conqueror is absolutely magic because it can get in and seal these areas without any further problems occurring.

As with all materials, it is essential that steel surfaces be thoroughly cleaned and dried before they are painted. When all the rust has been removed and the metal has been wire brushed to remove flaking rust and dust, it should be cleaned down with a dry rag dampened with mineral turpentine.

This should leave the surface clean of grease and, although some small rust stains may remain to dull the brightness of the metal, provided these are not loose or liable to flake off, they may be ignored because Rust Conqueror will treat them easily.

Steel should be painted as soon after preparation as possible in order to defeat any attempt by rust to gain a foothold on the clean metal.  The first application will be Rust Conqueror and it should be applied as evenly as possible over the entire area of the new clean steel. This should be followed by a heavy-duty rust inhibiting primer (such as Xtroll G1 Primer), then the top coatings (two top coatings are recommended). It is always recommended that 24 hours be allowed between each coat as this gives the undercoatings time to harden and completely adhere.

Where a boat is close to salt water, it is always a good idea to wash over the area before applying the second coat. This will prevent any salt that is contaminating the surface becoming trapped under coatings. Providing the areas to be painted have been treated for rust and if necessary chipped and neutralised as described earlier, touching up spots on steel varies little from painting the whole vessel. Slight pitting may have occurred if the rust has taken a good hold and this should be plugged with at least a stopper (any good body filler), ensuring that the metal surface is treated before the stopper is used.

The stopper can then be rubbed back to match the contours of the surrounding surface and sealed off with the following coatings if required. It is important to ensure that good contact is made between new work and old. The edges of the old paint should be rubbed back with sandpaper and the undercoats carried out over the edges of the old paint. The same applies to subsequent top coats.

When all the areas have been rubbed back and treated and stopped if necessary, the undercoat can be applied. The procedure from this point is as described earlier, with adequate use of wet and dry sandpaper and the cleaning rag to ensure a good smooth finish.  It is a very easy job to make the boat look good again.

BOAT TRAILERS

Boat trailers, like working fittings, need special attention. Many boat trailers are backed into the water when recovering their boats, which brings all the working areas into direct contact with the salt water. The body of the trailer can be treated as if it was norma1 steelwork, while the springs, axles, hubs and winches can all be coated with Rust Conqueror. Rust Conqueror is an ideal coating in these situations as it not only prevents rust from attacking the bare metal but, in some cases, gives a certain amount of lubrication to working parts at the same time.

ALUMINIUM

Painting of an aluminium surface differs very little from painting steel or timber. The only difference is the surface preparation. To provide a footing for the paint the aluminium surface must first be treated with an etching primer. This, as its name denotes, etches a smooth surface of the alloy and provides a base on which later coats may be built up. The preparation of the base is as important on an aluminium boat as it is for a steel boat.

It is essential that the surface to be etched is free from dirt and grease, since it is not uncommon to grease an aluminium surface for protection. Degreasing should be undertaken in every case to remove all grease. Any grease that is left on the aluminium surface will impair the effect of the etching paint.

After it has been applied, the etching primer dries very quickly and therefore speed of application is essential to ensure an even coat before it becomes tacky. Usually the primer takes only 10 – 15 minutes to dry and it leaves a thin film over the surface of the alloy that will readily take any finishing paints. It is essential that a coating is applied over the etch primer within a minimum of 5 hours and a maximum of 12 hours. This will allow no deterioration of the etched primed surface to take place. From here on, the build-up of successive coats of paint is the same as for steel. The paints used are at the customer’s discretion in terms of brand and colour.

CORROSION

With all craft, people talk about ‘electrolysis’ and ‘galvanic corrosion’ as though they are the same processes. However, they are not:

Galvanic Corrosion is the result of two dissimilar metals which are immersed in a solution (salt water) capable of conducting electrical current. These two metals are in contact with one another, either directly, or by means of the conductive solution. This makes an electrical current, which causes galvanic corrosion.

Electrolytic Corrosion results when an electric current from some outside source starts flowing between two dissimilar kinds of metal. The current can come from the boat’s storage battery, from the dock current outlet or from another boat moored alongside. The word ‘electrolysis’ has been used so often that people have picked it up and applied it to all types of boat corrosion.

Al1 metals are made up of molecules, which are electrically charged particles. If you put a strip of pure copper and one of pure zinc into a jar of salt water and either let them touch or connect them with a wire, one metal, the less noble of the two, will slowly disintegrate, while residues from it will collect on the other.

If you moored an aluminium boat to an iron pipe driven into the bottom of the water and used a steel chain to padlock the two together, children will not be able to steal your boat but the external galvanic corrosion will eat the boat’s bottom into a metallic pulp. On the other hand, you can put an anti-fouling bottom paint containing copper on to a fibreglass boat powered with a stern driven engine. The lower unit of the stern drive is immersed in salt water and attached to the engine, which is connected to the battery that is, in turn, connected to the boat’s wiring system. To this, you connect something like a radio, which has a ground plate, to the boat’s bottom. Current will go from that to the copper paint, and then through the seawater to the stern-drive lower unit. Because that is made of aluminium, which is a less noble metal than copper, after some months at the moorings it will also look like a lump of metallic pulp, thanks to electrolysis.

Electrolysis can also occur in fresh water as well as salt water but it is a much slower action.

The difference again is that galvanic action is different metal in contact and electrolysis is stray current you don’t know about until the damage is done.

How To Use XTROLL IN MARINE APPLICATIONS

ASSESSMENT

First, assess existing and potential problems. Is there a problem with:

  • Electrolysis – due to differing metals and moisture?

Stray Current – from your boat or another source?

  • Mill Scale – the grey-blue sheen on new steel?

Has there been an incorrect coating choice for

  • Salt environments?
  • Acid environments?
  • Alkaline environments?

or

  • Poor preparation with a preceding coat?

PREPARATION

Preparation for steel can involve some or all of the following:

  • Chisel, hammer, wire brush, angle grinder.
  • Needle gun
  • Sand blast
  • Wet abrasive blast (preferably with limestone grit)

For steel preparation, the Xtroll-preferred method is wet abrasive blasting with limestone grit. Water leaches the salts and the impurities from the pores in the steel and limestone has a passivating effect on the steel.  Although Xtroll products exhibit a wide tolerance with regard to surface preparation, the best results are obtained by following this procedure.

Surfaces left overnight may collect salt deposits. Also, Xtroll Rust Conqueror tends to bring impurities to the surface. For these reasons, all surfaces should be washed before coating.

APPLICATION OPTIONS

Below the Water Line

  1. Wet sand blast
  2. Two light coats of Xtroll Rust Conqueror
  3. One coat Xtroll 219 Epoxy Primer
  4. Two coats of either an epoxy two-pack paint or two-pack Polyurethane 343.
  5. Use normal anti-fouling coat for the region.

 Above the Water Line

  1. Wet abrasive blast with limestone grit
  2. Two coats of Xtroll Rust Conqueror
  3. Two coats of enamel, containing about 10% Xtroll Rust Conqueror, or two coats of Xtroll Metalguard.

 Internal Hull

  1. Wet blast with fresh water.
  2. Apply Xtroll Rust Conqueror in fine coats until a persisting gloss is achieved.
  3. Apply one coat of Xtroll Metalguard
  • Saturate all pockets, lockers and metal overlaps with Xtroll Rust Conqueror

Differing Metals in Contact

  1. Separate the metals where possible.
  2. Prepare surfaces (minimum is water blasting)
  3. Coat all surfaces, bolts, welds, rivets etc with Xtroll Rust Conqueror and allow to dry.
  4. Coat again with Xtroll Rust Conqueror before re-assembling.