ENGINEERING SOLUTION CENTRE

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Coating Defects

The major defects found in coatings are:

1. Slow or Incomplete Drying which means coating does not become touch or hard dry within specified time.

The causes are conventional coatings have not been properly mixed prior to application, drying additives have not been properly incorporated.
Two pack products have not been mixed in correct ratio, lack of curing agent leads to incomplete cure.

For remedy, leave coating for further 24 hours, observe if full drying occurs.
If coating is still soft or tacky, remove by mechanical means such as scraping until hard dry substrate is reached.
Re-apply coatings, ensuring material is properly stirred and mixed and checking correct thinners are added. Check temperatures.

2. Surface Discoloration.

The coating surface has a different appearance from that when originally applied.

Causes could be dirt settlement on rough coating surface. Resin breakdown as a result of ultra violet or chemical attack, usually yellowing Fading due to breakdown of colouring pigment. Rust staining due to rust water running down from corrosion sites.

Remedy is to remove discoloration by fresh water washing with water based detergent if required or by solvent washing if stains are not removed by above, abrade lightly with fine sandpaper until original colour is exposed. Patch prime and recoat with finish.

3. Corrosion

Appearance of iron corrosion signs on steel surface.

Cause by removal of coatings by detachment of mechanical forces. Low film thickness, inadequate removal of salt contamination before coating application.

Remedy by scraping off rust scale and loose coatings. Wire brush or needle gun corroded areas to remove as much corrosion as possible. Feather edges of intact coatings to give smooth overlap. Patch prime and apply touch up of finish.

4. Defect of sagging or curtaining.

Coating has slumped on vertical surface during drying process.

Cause by coating being applied at excessive wet film thickness.

Remedy by allowing paint to dry to a hard finish. Remove runs with sandpaper to produce smooth surface. Re-apply finish to ensure thin areas above runs are also recoated to film thickness required.

Paint Mixing

Every new tin of paint opened must be mixed thoroughly with a stirrer to disperse the solids. In two pack products, to ensure that the base and hardener can chemically react together and cure, always use a mechanical mixer. Always mix base and hardener in the correct ratio.

Paint Thinning

Use thinners only when necessary. Use the correct thinner for the product. Don’t use more than 5% by volume of thinners. Excessive use will cause solvent entrapment, runs, sags, slumping, retard cure and affect wet film thickness and dry film thickness ratio.

Pot Life

All two pack products have a pot life. That is time period between mixing and curing when the paint is workable and can be applied. Pot life is temperature related. Never mix more than can be used before pot life expires. Exceeding pot life will waste paint and will block up airless spray equipment.

Airless Spray Procedures

Check tip size and output pressure. Hold spray gun 30 cm to 45 cm from the surface. spray gun to be parallel to surface 90 degrees from it. Horizontal pass 50 cm to l meter. Overlap previous pass by 50% if possible to ensure even film thickness. Trigger to be released at the end of each pass. Cross hatching recommended for even film thickness.

Wet film thickness/dry film thickness

Apply paint at the correct wet film thickness (WFT) to give the specified dry film thickness (DFT) (Check product data) Check WFT as soon as paint has been applied – before solvent evaporation, and adjust the spray or application technique until correct WFT is applied. The WFT:DFT ratio depends upon the volume solids. 50% volume solids WFT of 100 microns = DFT of 50 microns. 70% volume solids WFT of 100 microns = DFT of 70 microns. Check dry film thickness with electronic gauge or banana gauge if necessary but remember this will read total film thickness i.e. old paint and new paint. For wet film thickness use wheel or comb gauge.

Paint Losses

Actual loss from container, equipment, overspray, spillage, pot life and windy conditions. Apparent loss from surface roughness and variable film thickness distribution.

Paint Coverage

Coverage = Theoretical spreading rate TSR in m2/ltr
Area to be painted m2 over TSR = Theoretical consumption in litres.
Practical spreading rate PSR = TSR – Loss factor %
Area to be painted m2 over PSR = Practical consumption in litres.

Types of Coatings

Normally coatings are classified as conventional coating systems or high performance systems. The decision on the type of coating to be used depend on the cost and budget one have and also the expected life or duration of the coatings and the required purpose of the coatings, either for the protection, decoration or coding purposes, etc.

For coding and decorative purposes, conventional systems are usually recommended but for protection purposes, high performance systems are recommended. Conventional coatings are usually one pack product whilst high performance coatings come in two packs. Single pack product are usually air drying, two pack products normally cure by chemical reaction on the base by the catalyst or hardener.

For best results, it is best to consult the manufacturers who are specialised in manufacturing certain range of coatings. Some are specialised in decorative coatings for houses, or protective coatings for industrial purposes or marine coatings vessels and ships or automotive coatings for vehicles or special coatings for appliances or aircrafts. Go to the right manufacturer for the right type of coatings needed.

Type of Application

Application can be done by brush, roller, conventional spray or airless spray. Brush and roller are used where condition does not allow air or airless spray. Not suitable for high build coatings or achieve high film thickness. Slow in process and coverage is about 180m² to 380m² per man day. Conventional spray is suitable for decorative and coding as only thin film can be achieved.

For protection purposes, airless spray is recommended and is the quickest method of application. Covers 750m² to 1100m² per man day. Control of application is by tip size and input and output pressure. Can be used for most paint types and can achieve high film thickness.

Application Condition

Paint should not be applied in adverse conditions, like temperature below 5 degrees C and not above 35 degrees C. Not in a windy condition or when surfaces are wet due to rain, snow, ice, fog or condensation. Allow painting only when surface temperature is 3 degrees C above the dew point.

Protective Coatings

Anti corrosive pigments are:-
a) Red Lead
b) Zinc Chromate
c) Zinc Phosphate
d) Zinc Dust

Preventing corrosion involve minimizing the flow of corrosion current so that if negligible current, negligible corrosion result. Resins with highest electrical resistance are, catalyzed epoxy, phenol, vinyl and chlorinated rubber. Addition of pigments such as talc, china clay, mica and iron oxide also assist in increasing the resistance.

The removal of soluble deposits on the steel surface is necessary as their presence will short circuit the resistance of the paint film to stage where rusting can occur. The thicker the coating the higher will be the electrical resistance. Coatings limit the access of oxygen, water and salts and therefore control corrosion in this way by a purely barrier effect. Catalyzed epoxies, coal tar epoxies, polyurethane, vinyl’s and chlorinated rubber coatings have the lowest permeability to water and oxygen.

Alkyd and oil based coatings are more permeable to corrosive agents than high performance coatings specified to ensure all corrosive substances are removed from the steel surface and maximum adhesion is achieved between the coating and steel. The coatings chosen or specified should be conducive to the environment, characteristic and purpose the substrate is subjected.

For purpose to prevent and resist causes by:-
a) Chemical spillage
b) Chemical fumes
c) High temperature
d) Skid resistance
e) Abrasive resistance
f) Oil resistance
g) Fire retardant

Protective Coatings

In the previous chapters we discussed the importance of surface preparation. To prevent and control the surface from further corrosion, the application of protective coatings need to be carried out before oxygen and water and other corrosive agent reactivate the substrate.

Choice of appropriate types of coatings should be made depending on what the substrate is subjected to the type of environment, characteristic and purpose. For instance, they must,

a) develop strong adhesion to the substrate.
b) must inhibit corrosion.
c) act as a barrier to corrosive substrate.
d) provide a good base for subsequent coats.

The last line of defense in combating corrosion is the primer. The performance of a coating system is ultimately dependent upon the ability of the primer to withstand the attack of corrosive agent from the surrounding environment and to inhibit the activitly of these agents where they may be held on the substrate by the overlaying coatings.

The primer is the most important in any coating system. All other coatings are permeable by water and oxygen. In addition some soluble corrosive substance may pass through the coating film. The primer should neutralize these materials before reaching the steel substrate. Certain compounds are able to chemically inhibit and neutralize corrosive agents.

Surface Preparation Standard

Steel Grades

A - Steel surface largely covered with adhering mill scale but little, if any, rust.

B - Steel surface which has begun to rust and from which the mill scale has begun to flake.

C - Steel surface on which the mill scale has rusted away or from which it can be scraped, but with slight pitting visible under normal vision.

D - Steel surface on which the mill scale has rusted away and on which general pitting is visible under normal vision.

St - Hand and power tool cleaning

St2 – Thorough hand and power tool cleaning. When viewed without magnification, the surface shall be free from visible oil, grease, and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter.

St3 – Very thorough hand and power tool cleaning. As for St2, but the surface shall be treated much thoroughly to give a metallic sheen arising from the metallic substrate.

Sa – Blast Cleaning

Sa1 – Light blast cleaning. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter.

Sa2 – Thorough blast cleaning. When viewed without magnification, the surface shall be free from visible oil, grease and dirt and from most of the mill scale, rust, paint coatings and foreign matter. Any residual contamination shall be firmly adhering.

Sa2.5 – Very thorough blast cleaning. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from mill scale, rust, paint coatings and foreign matter. Any remaining traces of contamination shall show only as slight stains in the form of spots or stripes.

Sa3 – Blast-cleaning to visually clean steel. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and shall be free from mill scale, rust, paint coatings and foreign matter. It shall have a uniform metallic color.

Surface Profile

Surface profile is determind by a range of variable which include the following:-

1. Strength and hardness of the surface being cleaned
2. The velocity of the abrasive
3. The hardness and specific gravity of the abrasive
4. The size or mass of the abrasive
5. The angle of abrasive impact
6. The particle shape of the abrasive
7. The time the surface is exposed to blasting

What is Hydro blasting?

A cleaning technique which relies on the energy of water striking a surface to achieve its cleaning effect.
Two different operating pressures are commonly used
1. High pressure hydroblasting operatine at pressures above 680 bar (10,000 psi)
2. Ultra high pressure hydro blasting operating at pressures above 1700 bar (25,000 psi)

What is water cleaning?

1. Low pressure water washing operates at pressure less than 68 bar (1000 psi)
2. High pressure water washing operates at pressure between 68-680 bar (1000-10,000 psi)
3. High pressure hydro blasting operates at pressure between 680-1700 bar
(10,000-25,000 psi)
4. Ultra high pressure hydro blasting operates at pressure above 1700 bar (25,000 psi)
normally in the range 2000-2500 bar (30,000-36,000 psi

Adavantages of Hdro Blasting Over Dry Abrasive Blasting

1. Health and Safety
levels oNo abrasive used, injury potential reduced
No respiration problems for operators from dust
Lower f noise
No spark production
Minimises disruption of other work in vicinity

2. Contamination
No dust contamination on wet paint, delicate machinery parts
No clean-up requirements

3. Surface Salt Reduction
Reduced salt levels on the surface

Soluble Salt Contamination

Salt conntamination of steel and other substrates in ships’ ballast and cargo tanks is a common cause of coating breakdown, via blisstering. There are two well known mechanisms for blister development, osmosis and corrosion.

If the surface is contaminaated before painting with hygroscopic material by more than a critical amount, then osmotic blister failure is likely to occur in service. Hygroscopic material such as salts found in seawater tend to absorb water. They attract this water through the coating, where it’s volume forces blister formation. This type of failure frequently occurs within several weeks of immersion. The critical amount depends on a number of factors including coating characteristics and environmental factors. The more soluble the salts the more powerful the effect. Sodium chloride is much more soluble than sodium sulhate and is therefore a more harmful contaminant.

Breakdown by corrosion is a more gradual process, typically taking many months to appear. The presence of conductive salts under the film promotes corrosion.

The corrosion products which are generated at the cathode have a powerful osmotic effect, giving rise to blisters, while the anode produces voluminous rust which also causes blisters.

The source of contminant salts in ballast and cargo tanks can be from previous cargoes, ballast water, the atmosphere, washing water and also from blasting abrasive used to prepare the steel. Many shipowners are specifying salt contamination levels in their coating and new building contracts.

Surface Preparation

Surface preparation is a very important process before any protective coatings can be applied. New and old metal surfaces should be free from mill scale and rust. They should be cleaned to a certain standard before applying any protective coatings to achieve the best result.

Types of Surface Preparation

1. Solvent Cleaning & Degreasing

a) Removal of oil and grease from all surfaces
b) Solvent or thinnerdegreasing should be used for small areas only.
c) Water rinseable degreasing fluids are most efficient for large areas.

2. High Pressure Fresh Water Washing

a) Removal of soluble saltsdeposited from seawater, poluted environments or formations from rusted steel.
b) Removal of emulsified degreasing fluid.
c) Removal of loose dust and dirt.
d) Removal of plant and slime fouling

3. Manual Preparation

i) Hard Scrape
ii) Chipping hammers orchisels
iii) Wire brushing
a) Removal of loose superficial rust deposit on small localised areas.
b) Standards of St2 and St3 can be achieved

4. Power Tool Preparation

I Rotary Wire Brushing
Care should be taken to ensure the rust is removed and not just
polish.
ll Rotary Discing
lll Impact cleaning
Used on relativellly small and difficult areas
More efficient then hand tool cleaning
Standards of St2 and St3 can be achieved

5. Dry Abrasive Blasting

The most efficient method of surface preparation
Steel shot abd grit, mineral grits, sand and metal slags are used
as abrasives
Does not totally remove soluble salts
Standards of Sa1, Sa2, Sa2.1/2 and Sa3 can be achieved

6. Water Cleaning Processes

Low pressure water washing
High pressure water washing
High pressure Hydro blasting
Ultra high pressure Hydro blasting
Water and abrasive blasting – slurry blasting

Best mehod for removal of soluble salt
Water alone does not produce a surface profile

What is corrosion ?

Corrosion is a process of rusting through oxidation and weathering of the natural elements of rain and sun on all metal surfaces, if not adequately protected with appropriate coatings. Normally starts with rusting which eventually creeps into the metal causing pitting,then forming scales that chips off weakening the metal.

This happens to all things that is built, constructed of steel structures, metal tanks, pumps, steel piles, pipes and ship hulls, etc. are all subjected to corrosion if they are not protected with the right type of coatings and/or cathodic protection.

Other forms of Corrosion

Apart from the corrosion action of oxygen and water on steel as describe above, there are other causes of corrosion.

1. Corrosion through attack by chemical and chemical fumes.
2. Through use of dissimilar metals. Like joining new steel pipes to old steel pipes. Even old metal plates joined to new metal plates by bolting or welding. New steel is anodic to old steel.
3. Highly stressed surfaces are anodic to unstressed surfaces.
4. Steel are also anodic to mill scale. Mill scale which is formed during the high temperature milling of steel, is a hard brittle coating of distinct layers of iron oxides. It cracks and fissures readily, permitting air and moisture to penetrate and initiate rusting to the underlying steel.

There are many more other forms of corrosion but for our purpose, we will dicuss until this section.

Corrosion Prevention

Corrosion prevention is to provide sufficient protection to prevent corrosion taking place for a certain period of time by calculation and inspection, when subsequent maintenance or renewal are needed before the material structures begin to show signs of corrosion.