Protecting a steel vessel from corrosion, requires absolute dedication to correct preparation procedures if the selected system is to have any chance at all of succeeding. To get good adhesion the surface must be cleaned to white metal and then prime coated immediately, The most successful way to achieve this is sandblasting, which not only cleans large areas quickly, but leaves an uncontaminated, and slightly roughened profile for the primer to bond to.
The sandblasting procedure is a professional job and should only be carried out by qualified applicators. These people are experienced in the various degrees (or standards) of clean metal states, and can produce the level specified by the paint manufacturer. Usually the minimum requirement is S.A. 2 112 near white metal, or S.A.3 white metal, Alternatively, A.S. 1627.4 should be considered as the minimum level. The white metal state is seen as a uniform white metal surface without patches of grey. Once this state is achieved, it is essential that the primer coat be applied immediately, to seal off the steel from atmospheric moisture (which will start the corrosion process all over again). Co-ordination of the blasting and painting will require careful planning and optimum weather conditions.
This can be confusing and needs some careful evaluation of:
a. The areas to be coated,
b. The type of use the surface will be put to,
c. What degree of finish will be required,
d. What maintenance cycles are contemplated and
e. What geographical environment will be involved.
These offer the simplicity of easy recoating and generally are more tolerant of application temperatures. On the
other hand, they are usually solvent sensitive and not as abrasive resistant. They can only be used successfully as above
water coatings because they are only water resistant, not waterproof, and will swell up and blister under constant immersion.
2 -Pack Paint systems
They offer higher film build protection on a coat for coat basis, but can give inter-coat adhesion problems if incorrectly applied. Recoating also requires more preparation to obtain a satisfactory bond, but offers considerably less maintenance. To compound the problem of paint selection, each area on a steel boat usually has a different performance requirement. This requires close analysis of the coatings ability to provide value for money, in the longer term. For example, a yacht may require considerable freeboard fairing with epoxies to eliminate hollows in plate welding whereas a commercial steel trawler will ignore variations in hull smoothness and paint the area in a low sheen finish to mask the surface irregularities.
As this surface has the highest heat absorption value, the coatings should be capable of distorting to a similar co-efficient of expansion to that of the steel. If the paint fails to flex in a similar manner, it will crack and allow corrosion to develop.
If the paint is too soft, it will not withstand traffic. Therefore, it should be hard enough to withstand abrasion but still be sufficiently flexible to cope with the steel distortion.
Although the distortion factor will be less than the deck area, the colour, quantity of fairing filler to be used, and abrasion
resistance will also dictate the selected finish.
The use of an anti-fouling paint over the protective primers also provides another dimension due to the fact that these metals will cause an electrical current to flow through the Electrolyte (seawater) causing the dissolution of one of the metals (unless adequately insulated). To overcome this problem cathodic protection principles are employed to maintain the integrity of the hull, and this requires professional advice to obtain the best results.
The basic principles of corrosion and cathodic protection are listed below for your consideration. CATHODIC PROTECTION
When two different metals are in contact and immersed in an electrolyte (i.e. an electrically conducting solution such as seawater), an electric current flows from one metal to the other through the electrolyte. Associated with this flow of current is the dissolution of one of the metals. This phenomenon we recognise as corrosion.
The composition of the electrolyte plays only a minor part and consequently it is possible to arrange metals in a table such that for any pair, the one higher in the table will corrode in preference to the other. Because this is an electrical phenomenon, a certain voltage or potential difference exits between any pair of metals. If one metal is taken as a reference standard for the scale, it is then possible to ascribe a specific voltage to each metal in the table. In practice, hydrogen is taken as zero and after standardising a number of variables, the 'electro-chemical series of metals' has been evolved. The following have been extracted from that table.
7 Copper + 0.34 volts
8. Silver + 0.80 volts c,..-t
Taking iron and copper as an example, the iron being higher in the table will corrode at the expense of the copper. The potential of the iron relative to the copper is: -O.44V -(+0.34)v = -0.78V or minus 780 millivolts. This latter
figure will v~ry SO~ewhat in pra~tice depending. on the ~atu~e of the electrolyte and its temperature. 1
So far, consideration has been given only to pairs of quite different metals. It happens that there are t frequently minor potential differences over the surface of a single metal, steel for example, and hence it will corrode in the absence of a second metal. A great deal of work has been carried out on steel and millscale and the potential of the former to be approximately -450mV, a quite substantial value.
When corrosion occurs, the flow of electric current is accompanied by chemical reactions. The phenomenon is
1therefore more than an electrical one, it is electro-chemical and some of the terms that are used have been borrowed from that science. The words anode and cathode are applied to the two metals of a cell. The metal which corrodes or has the lower potential or is nearer the top of the above table is the anode, the other is the cathode.
The electro-chemical process of corrosion.
In the simple case of steel sheet partially covered with millscale and immersed in seawater, iron ions carrying two positive charges (Fe++) pass into solution at the anodic areas and electrons (-) migrate within the steel from the anodic to the cathodic areas thereby establishing an electric current.
1. Remove surface contamination (grease, oil etc.) with NORGLASS WAX & GREASE REMOVER, and hose off salt deposits.
2. Abrasive blast clean to AS1627.4 or S.A. Class 21/2 to 3 (near white or white metal). Remove all sand and dust, and apply
the first coat within 6 hours or as soon as possible. If the surface is contaminated by rain, spray or mist, reblasting will be necessary.
INTERIOR Unpainted Surfaces
1. As above where possible.
2. If abrasive blast cleaning is not carried out, power sanding to a clean metal state may be acceptable as an alternative.
Remove all dust and apply the first coat within 6 hours or as soon as possible.
Previously Painted Surfaces (in good condition)
1. Remove grease and oil with NORGLASS WAX & GREASE REMOVER and wash with fresh water to remove salt deposits. Allow to dry
2. Sand to a smooth, flat surface and remove dust
Previously Painted Surfaces (in poor condition)
1. Where the adhesion of previous coats is poor, removal by blast cleaning is recommended. If the bare steel has not been penetrated, then scraping, wire brushing, and sanding, should be sufficient.
2. Sand to a smooth flat surface and remove dust.
*HULL EXTERIOR: (Above and Below waterline)
Apply a minimum 2 coats of NORGLASS EPOXY TAR as directed on the Data sheet and label instructions.
*HULL EXTERIOR: (Alternative above waterline system. (From the top of the boot-topping line).
Immediately after sandblasting, apply 3 coats of NORGLASS NoRUST Primer as directed. After the final coat allow 2 hours to cure and re-mask the waterline from the top of the boot-topping. Below waterline areas require NORGLASS EPOXY TAR.
STRUCTURE: Same NoRUST recommendation as for above waterline area.
*INTERIOR: As per Decks/Superstructure, continue using NoRUST as specified. UNDERCOATING *HULL EXTERIOR: Above and below the waterline)
Apply a minimum 2 coats of NORGLASS SHIPSHAPE PRIMER UNDERCOAT.
As per Hull exterior undercoating. Where practical the use of WEATHERFAST UNDERCOAT can be used as an alternative to SHIPSHAPE.
*INTERIOR: (Bilge area to waterline). Use only SHIPSHAPE PRIMER-UNDERCOAT.
*INTERIOR: (Above waterline)
Apply 2 coasts of NORGLASS WEATHERFAST UNDERCOAT.
*HULL EXTERIOR: (Above waterline area)
Where required NORGLASS NORFLEX EPOXY FILLER can be applied by trowel or rubber squeegee. Mixing ratio 2:1 by volume. Note: Additional coats of NORGLASS SHIPSHAPE will be required to cover the NORFLEX FILLER after fairing and sanding has been completed.
STRUCTURE: As above.
*HULL EXTERIOR: (Below waterline and including boot-topping).
.Apply 2 coats of NORGLASS TOPFLIGHT ANTI-FOULING as directed on Data sheet and label instructions.
*HULL EXTERIOR: (Above waterline options)
(a) Apply 2 coats of WEATHERFAST GLOSS ENAMEL.
(b) Apply 2 coats of WEA THERFAST GLOSS ENAMEL white blended on a 50:50 ratio with WEATHERFAST UNDERCOAT.
This will produce a free chalking white semi- gloss finish that will mask plate distortions. (c) Apply 2-3 coats of NORTHANE Gloss.
*DECKS: Apply 2 coats of WEATHERFAST DECK PAINT. Note: Where smooth perimeters are desired pre-coat these areas with WEATHERFAST ENAMEL first then mask out the SLIP RESISTANT areas.
*SUPERSTRUCTURE: Apply 2 coats of WEATHERFAST MARINE ENAMEL. *INTERIOR: (Bilge area) No further treatment required. *INTERIOR: (Above waterline).
Apply 2 coats of WEATHERFAST ENAMEL.
*ALLOY MASTS & SPARS:
Refer to the NORGLASS guide on Painting Aluminium for options.
*BRIGHTWORK: Refer to the NORGLASS data sheets on Weatherfast Poly Clear or Marine Varnish for options.
*ESTIMATING MATERIAL REQUIREMENTS: All coverage rates expressed are theoretical and do not al)ow for losses or spillage.
Where a 2 pack product expresses a coverage rate this relates to the mixed product. Do not include spraying thinners in these calculations and remember to multiply areas by the recommended number to be applied.
Norglass, provides information data sheets on the following products for your assistance.
NoRUST PRIMER* NORFLEX EPOXY FILLER* SHIPSHAPE PRIMER/UNDERCOAT* NORGLASS EPOXY
TAR* WEATHERFAST ENAMEL * WEATHERFP.ST DECK PAINT* TOPFLIGHT ANTI-FOULING* SOFT COPPER ANTI-FOULING* NORTHANE GLOSS.
NORGLASS ADVISORY SERVICE
Ph: (02) 9708 2200 or Fax: (02)9796 3069. -Email: email@example.com www.norglass.com.au