There are two main factors which govern the performance of a protective paints system, mainly the nature of the paint coating and the degree of cleanliness of the surface to which it is applied.
THE OPTIMUM PERFORMANCE OF ANY,PAINT COATING IS DIRECTLY DEPENDENT UPON THE CORRECT AND THOROUGH PREPARATION OF THE SURFACE PRIOR TO COATING.
THSE MOST EXPENSIVE AND TECHNOLOGICALLY ADVANCED COATING SYSTEM WILL FAIL IF THE SURFACE PRE-TREATMENT IS INCORRECT ORINCOMPLETE.
Common Surface Contaminants
- Oil & Grease-Deposit from the working environment.
- Millscale-Oxide formation from the hot rolling process at the steel mill.
- Corrosion products-Rust and scale formed on un protected steel.
- Soluble salts-Deposited from the atmosphere.
- Laitance-On concrete.
- Zinc Salts –On galvanized surface and weathered Zinc Rich Epoxy/Zinc Silicate coating. The removal of such contaminants is essential for optimum performance and Mechanical means like emery scuff or wire brush the followed by thorough washing with fresh water.
1. OIL&GREASEDegreasing : All Oil, Grease, Drilling and Cutting Compounds and other Surface Contaminants
If present even in trace quantities this may impair the adhesion of protective paint systems and lead to premature failure.Removed by solvent swabbing is common; however, it is essential that the deposits are removed and not simply spread over the surface.A number of washes using clean solvent and swabs is essential. A P’s Thinner may be used as per recommendation given in individual Data Sheet.
This layer of Oxides although initially intact readily embrittles and flakes off
bringing with it the paint system.
Numerous methods of scale removal have been used.
1.Natural Weathering : This is an unreliable practice as the surface will remain contaminated with soluble Salts and corrosion products.
2.Hand Tool Cleaning : The degree of cleanliness achieved is largely dependent upon the amount of Weathering to which the steel has been subjected, and the efforts of the
Operators who have difficulty maintaining a constant satisfactory standard. It is impossible to remove all rust and millscale by this method. Generally this Method would be adopted for the following:
- Maintenance Painting.
- Easily Accessible Steelwork in Rural Areas.
- Steelwork inside Building where conditions are Non-corrosive.
- Steelwork which is to be encased in brickwork,concrete,etc.
- Internal Surfaces of enclosed that required Painting.
Methods for Hand Tool Cleaning are described in SSPC-2SP2 and should be to Swedish Standard St.2-B,C or D.
3.Power Tool Cleaning: Although impact tools such as chipping hammers and needle guns are
Reasonably effective in removing rust and scale the time and effort required is
Power rotary wire brushes and grinding tools wear away the unwanted surface layer. This method tends to burnished the surface especially where firmly bonded scale exist. The burnished surface effectively reduced the adhesion properties of the primer.
Other unfavorable factors are excessive noise levels and dust hazard. Generally this method would be adopted on maintenance painting where areas require remedial treatment.4.Flame Cleaning : In flam cleaning method, a high temperature oxyacetylene flame is passed over the
Surface. Scale and rust are dislodged partly by differential expansion of the steel and partly by evolution on steam from moisture within the rust. Scraping and wire brushing is necessary to remove the burnt residues.
The main disadvantages of this method are:
- Fire and Health Hazard.
- Possible damage to adjacent areas.
- Steel must be atleast 16 gauge thick to avoid duckling.
- Steel temperature must not exceed300C.
- Use prohibited on high strength friction grip joints and adjacent areas.
5.Acid Picking : Millscale and rust can be removed by immersion
in acid solution such as sulphuric or
The pickling carrie out its function in two different ways. First, the acid serves to dissolve both scale and rust. Secondly, as the acid creeps into the breaks of the millscale a reaction between the innermost layers of millscale and the acid evolves hydrogen gas.
This gassing result in the millscale popping off. Following a hot water rinse the steel is often depend in a bath containing a solution of phosphoric acid reacts with the steel to form a thin film of iron phosphate which acts as a rust inhibitor.
- Cost of priming paint should be applied as quickly as possible after drying. The main disadvantages to this method are:
- Awet process with effluent and fume control requirements.
- Unsuitable substrate for metal spray and many two pack systems.
- A workshop operation with work limited to size of baths. Generally, pickling is done by specialist firms.
- Process not suitable for structural steel or large objects.
6.Abrasive Blast Cleaning: Prior to blast cleaning any obvious surface defects in
the steel such as
Lamination, shelling, weldspatter, etc. should be removed by chipping or grinding.
Any defects unobserved prior blast cleaning and priming should be treated at the priming stage and touched up as necessary. Where steel has been allowed to rust extensively, longer times for blast cleaning may be required.
It is therefore advisable to blast clean prior to rusting whenever practicable.
2.6.1 :ABRASIVE BLAST CLEANING describes all methods used to project an onto the
e.g. air blasting, centrifugal blasting, or water/sand blasting. During the course of development this type of cleaning has been given several different names, e.g. sand blasting, shot blasting and grit blasting, depending upon the abrasive used.
2.6.2 : VACUM BLASTING:
Ideal for small repair areas, welds, etc. The abrasive is collected for re-use by shrouding the blasting nozzle with a vacuum hood.
The mixed abrasive and debris pass thorough a separating device before the filtered
abrasive is returned to the blasting circuit.
2.6.3 : WET ABRASIVE BLASTING:
The abrasive is carried to the work surface in a stream of water. It is necessary to use non-metallic abrasive.
Corrosion inhibitors may be added to the water to delay re-rusting of the steel. The amount and type of inhibitor addition must be strictly monitored to ensure that the adhesive properties of the paint coating are not impaired. Too much inhibitor may have the same effect as leaving corrosion products on the surface thus reducing the adhesion of the surface.
The prime advantages of this method are:The hazard of dust associate health problems are largely overcome.
PRODUCTS: All cleaning methods as mentioned above should be cater for the more difficultRemoval of rust.
4. SOLUBLE SALTS:
These are commonly as encountered on steel that has been allowed to rust in a Polluted atmosphere. Removal of salts formed in pits is difficult but essential if premature failure is to be avoided. The use of wet blasting is advised. Careful control by visual and chemical means is advocated.
Blast cleaning is strongly recommended in all cases where high quality long life expectancy systems are called for.
5. PHOSPHATING :
Chemical conversion treatment is common in OEM industries where generally CRCA/HRCA sheet of lower thickness are used. Chemical conversion treatment is, commonly known as phosphating, generally done with dipping or spraying.
Dip phosphating is the most commonly adopted method in OEM industry. Spray Phosphating is preferred mostly where space constraint is observed. Spray phosphating is generally coarser in structure & hence less preferred, while dip phosphating has more controlled particle size & structure.
Generally chemical conversion is done with metal ions like Zinc modified with Calcium or Zinc with Nickel & Manganese. The tricatoinic coating with Zinc, Nickel & manganese is done in Automobile Industry. While treatment with iron & alcium modified Zinc is preferred in other OEM industries.
Phosphating includes the stages of:
1. Cleaning of metal degreasing,
2. Removable of rust derusting(in severe cases Acid pickling)
3. Surface activation
The actual work stages may very form 3 to 10 depending upon many factors. The no. of tank generally employed are 7 & hence known as 7-tank process. The phosphating improves the adhesion & corrosion resistance properties of the paint system. There are variety phosohating chemicals available in the market. Chemical which gives compact Coating from a reputed suppliers are to preferred as the process required lots of technicals support for bath maintenance.
Notes: The 3 in 1 solution / cold phosphating without rinsing generally gives very poor performance as they leave left over of acid on the surface which interface in corrosion resistance subsequently and shall be avoided.
B. For Non-ferrous Metals
The following recommendations for non-ferrous metals, including galvanized steel, will provide the proper surfaces on which to apply coating.
Aluminium : Solvent was to remove organic matter. For some systems, either a chromates or phosphate pre-treatment is recommended. For other system, they may be applied directly to aluminium if an anchor pattern is provided by brush-blasting. Application of an etch primer, viz. GP PRIME401 or GP PRIME203 is recommended before painting.
Note : blasting is never used on sheet aluminium because of warping may occur.
Galvanised Metal : New galvanized metal often comes from the mail or supplier with a light, invisible Coating of Cosmo line or other material to keep it bright. This must be removed by solvent washing.
Weatherized galvanized metal may have remnants of the brighter remaining or a light skiff of zinc oxidation products, depending on the length of exposure, solvent washing is removing these potential bond-breakers.
Application of an etch primer,viz.GP PRIME 401/GP PRIME203 is recommended before
Stainless Steel and
Other Alloys : Solvent wash to remove oil and grease. Brush blasting to provide a suitable anchor pattern is most often recommended depending on the mill finish. Bright, polished stainless steel and other polished metals can be a problem unless an anchor pattern is provided.
C. Concrete and Mansonry
Failure of coating on concrete and masonry is often caused not so much from a failure of
Adequate surface preparation as from a construct configuration which: allows moisture to wick into the concrete from behind, above or below and became trapped behind the coated surface. Unless vapour barriers or stops are built-in where concrete is below or where concrete floors are used as the based for laid-up block or bricks,etc., coating performance will be jeopardized.
Expansion joints are present to accommodate the movement of concrete or masonry structure and should not be coated over as part of the single continuous film which covers the adjacent areas.
Methods : The most efficient method of preparing concrete for coating
is by sand blasting. This
Eliminates from release agents, removes surface laitance, reveals air pockets and creates an anchor pattern.
Where sand blasting is not possible or practical, these general recommendations should be followed:
1. Remove dust, direct, oil, grease and from release agents. This can usually be accomplished by scrubbing with a strong alkaline detergent.
2. Check for voids which will often be only small opening in the surface with larger voids beneath. Large voids should be filled with latex concrete group(2 part cement to 1 part fine sand). Voids left in the surface may result in bridging by the coating and subsrquent failure when the trapped air expands.
3. Etch the surface with a solution of 1 part commercial hydrochloric acid in 2 part water. Allow the acid solution to “work” for 5-10 minutes, then thoroughly flush the surface with water and aloe to dry thoroughly. This will neutralize laitance and efflorescence and provide a slight anchore pattern. This treatment is really only practical for floors or horizontal surfaces.
4. Vacuum cleaning of the surface is recommended before painting. Where sand blasting is not possible, alternative mechanical means are scarifier, power grinder with vacuum attachment are employed.