Paint and Protective Coatings The purpose of this instruction is to specify standard methods of improving the appearance and minimise corrosion of steelwork and pipework. Protection against corrosion is particularly important on all hot gas lines, especially those of systems with reversed cycle defrosting. This document has been revised to correct the micron thickness and add the thickness and colour of final coat to section 3 para. f.
1.0
INTRODUCTION Attention to the selection and application of protective coatings is important. The standard of finish of an item of equipment is the first and often lasting impression that a customer gets of the company’s attitude to quality. This standard sets out information on the different surface protection systems which are in general use by Star Refrigeration. It is arranged in four parts:
2.0
Section 2.0
Surface preparation
Section 3.0
Protective coatings
Section 4.0
Recommended protective coatings for various applications
Section 5.0
Application
SURFACE PREPARATION The selection of a surface preparation has to be related to the protective coating to be applied and the suitability of the component to be prepared. In general, the most common and practical surface preparation for a brush-applied paint system will be scraping and wire brushing. For components where a high performance paint coating is to be applied, abrasive cleaning is recommended subject to practical considerations. Both types of surface preparation are graded using the ISO 8501-1 standard. (Note: BS 7079 Part A1 “Preparation of steel substrates before application of paints and related products”, is identical with ISO8501–1:1988). Brief descriptions of these grades are given below.
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2.1
Scraping and Wire Brushing Loose rust, loose mill scale, and deteriorated coatings can be removed by effective use of hand and power tools. Prior to scraping and wire brushing, remove grease, oil and other contaminants. Chip, scrape, and wire brush rusted surfaces thoroughly to produce a sound surface that is clean and free of foreign matter to assure good coating adhesion. Care must be taken with power tools to avoid polishing a metal surface – or abrading it too deeply. Two grades of preparation by scraping and wire brushing have been internationally adopted with the ISO 8501-1 standard: Grade St 2 Thorough scraping and wire brushing – machine brushing – grinding - etc. The treatment shall remove loose mill scale, rust and foreign matter. Finally, the surface is cleaned with a dry clean brush. It should then have a faint metallic sheen. Grade St 3 Very thorough scraping and wire brushing – machine brushing – grinding – etc. Surface preparation as for St 2, but much more thorough. After removal of dust, the surface shall have a pronounced metallic sheen. For the majority of applications where scraping and wire brushing is required, Grade St 2 is specified.
2.2
Abrasive Cleaning Abrasive cleaning or blasting is the most effective method of cleaning steel surfaces prior to coating. It is mandatory for certain specialised applications and is recommended for removal of mill scale, heavy rust scale, and previous coatings from large areas. For optimum results the sequence is as follows: i) ii) iii) iv)
Prior to blasting, remove grease, oil, dust and similar contaminants. Abrasive blast to the required grade. After blasting, remove all sand, dust and grit. Blasted surfaces should be coated as soon as possible – before the cleaned surface can become contaminated.
Four grades of preparation of steel by abrasive blasting have been internationally adopted with the ISO 8501 – 1 standard. Sa3: Blast Cleaning to White Metal Completely remove all mill scale, rust, previous coatings etc, leaving a uniform grey-white metallic colour.
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2.2
cont. Sa2½: Very Thorough Blast Cleaning Remove all rust, rust scale, mill scale. At least 95% of each square inch of surface area is to be free of all visible residues, and the remainder shall be limited to slight discolouration. Sa2: Thorough Blast Cleaning Completely remove all dirt, rust scale, mill scale, foreign matter and previous coatings etc. At least 2/3 of each square inch of surface area is to be free of all visible residues, except slight discolouration. Sa1: Light Blast Cleaning (Sweep Blasting) Remove rust scale, loose mill scale, loose rust and loose coatings, leaving only tightly bonded mill scale, rust and previous coatings. Results are comparable to those achieved by thorough chipping, scraping and wire brushing (St3). For the majority of applications, where abrasive cleaning is required, Grade Sa2½ is normally specified.
2.3
Chemical Cleaning (Pickling and Passivating) For components where the internal surface requires a high degree of cleanliness (lubricated pipework) or components where an abrasive method of preparation is not practical, cleaning may be carried out chemically. This form of cleaning is also used on items which are to be hot dip galvanised. The purpose of the system is to remove all scale and rust. A brief description of the process follows: i) ii) iii)
Degreasing in a sodium hydroxide solution for 2 hours. Acid clean in a 10% hydrochloric acid bath for 6 – 8 hours. Passivate in a citric acid, sodium nitrade and ammonia solution for 4 hours (minimum).
NOTE The above specification relates to carbon steel components only. For other materials we must check material compatibility with the cleaning agents before proceeding.
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3.0
PROTECTIVE COATINGS The protective coatings listed are in use by Star Refrigeration. They range from basic prime painting for initial protection to hot dip galvanising. In selecting a protective coating the working conditions and degree of exposure must be considered, as well as the suitability of the protective system and the item to be treated. The Star preferred supplier is given where appropriate: a.
Quick Drying Primer (Rustoleum 569) This is a fast drying, metal primer which is lead and chromate free but it should not be considered for long-term protection. It is a compatible base for most finish top coats.
b.
Damp Proof Red Primer (Rustoleum 769) This primer, which is lead and chromate free, should be used on steelwork which will be subjected to prolonged damp or wet conditions. Enhanced protection can be obtained by repeat coatings. It is a compatible base for most finish top coats.
c.
Finish Coat (Rustoleum Combi Colour – National Blue) This is a rust-impeding primer and top coat combination paint which is lead and chromate free. It is a very durable paint and has a film thickness approximately twice that of other finish paints.
d.
Standard Black Bitumastic (Bitumastic plc, Hebburn, Tyne & Wear NE31 1EY) This is a heavy-duty, anti-corrosive solution based on coal tar pitch. This paint is designed for the protection of metal in arduous conditions. It is not suitable for overcoating.
e.
Zinc-Rich Primer There are a number of zinc-rich primers available on the market. These primers offer protection against corrosion but, in general, are susceptible to damage if not sealed or overcoated immediately after application.
f.
High Performance Epoxy Systems These are normally two-component coatings with give a hard durable finish. Surfaces shall be externally shot blasted to Sa2½ followed by one coat epoxy zinc phosphate to dry film thickness (DFT) of 50 microns followed by one general coat of epoxy polyurethane to DFT of 100 microns. To achieve the full potential of this type of finish a specialist contractor should be considered. Minimum final DFT 150 microns, finish colour RAL 5010.
g.
Chlorinated Rubber Paint This is a high build rubber-based paint with a flexible film and is used on Armaflex insulation.
h.
Etch Primer (Rustoleum Surfa-Etch 108) This is an acid-based etching solution which should be used on galvanised surfaces which require to be painted. The purpose of this treatment is to produce an etched surface to which a paint system will adhere.
i.
Hot Dip Galvanising This is the most durable of all anti-corrosion treatments but is only suitable for a limited range of products. Because of the high temperatures involved in the dipping process, distortion and damage can occur.
j.
Zinc or Aluminium Spraying
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This is a hot metal sprayed coating applied to a shot blasted surface. It offers a high degree of protection from corrosion but requires to be sealed with a primer or special sealant immediately after application. There is a small risk of distortion.
4.0
RECOMMENDED PROTECTIVE COATINGS There is a wide range of types of equipment that require a protective and/or finish coating. To enable selection of a suitable coating Table 1 of this standard has been created, based on environmental requirements and type of component and this table should be used as a guide to selection. In a number of cases there are alternatives; the first listed method is a preferred option. On some contracts the customer or his consultant will ask for specific requirements. It is important that, when this situation arises, the specific application standard is recognised and made available at the outset of the contract.
5.0
APPLICATION Protective coatings shall be applied by an appropriate method, e.g. brush or spray, by individuals who have been properly instructed. Finish coatings shall be free of obvious runs and brush marks. Parts not to be painted, e.g. nameplates or sight glasses, shall be carefully protected by masking tape or grease. Primer shall be re-applied to weld and heat affected areas prior to finish coating. Special care shall be taken when protecting surfaces which are alternately wet and dry, e.g. liquid lines and hot gas lines of reverse cycle systems and areas where the metal is light gauge e.g. proprietary driers, float chambers, valves etc.
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Standard Specification No: 103
Exposed
Protected
Environment
Engine rooms, interior etc
Open to weather Exterior
High Humidity
Cold Stores
Chills Unventilated Areas
Sub-zero Conditions
Surface preparation
Surface preparation
Protective coating
Surface preparation
Protective coating
Surface preparation
Protective coating
Baseplates
St2
a–c
St2 Sa2½ Sa2½
b-c j–c f i *
St2 Sa2½ Sa2½ Sa2½
b–c b–c j–c f
Brackets, supports
St2
a–c
St 2
b–c i
St2
b–c i
St 2
Uninsulated pipework including valve sets
St2
a–c
St2
b–c
St 2 Sa2½
b–c f
Pressure vessels
St2
a–c
St2 St2
a–c b–c
St2 Sa2½
Insulated pipework including valve sets Insulated pressure vessels **
St2
d
St2
d
St2
Galvanised steelwork Galvanised sheet metal *** Tubesheets
h-c
St2
h–c h–f
Surface preparation
Protective coating
Sa2½
f
b–c i
Sa2½
f
St 2 St2½
b –c f
Sa2½
a–c f
St2 Sa2½
b –c f
Sa2½
d
St2
d
St2
h–c h–f
d
Protective coating
Petrochem
h–c h–f
f
f d
h–c h-f St2
d
SS 103 TABLE 1 - Recommended Protective Coatings The number and subscripts relate to the surface preparation and protective coatings detailed in this standard. * Hot dip galvanising is only appropriate for unmachined baseplates, e.g. for belt drive compressors. **
Where a vessel is insulated with Armaflex the vessel will be prepared to St 2. All nozzles and attachments which penetrate the insulation will be painted with Standard Black Bitumen, together with an adjacent collar, to a minimum width of 50mm on the shell.
***
As a general rule, galvanised finishes are not painted.
Where several options are listed the first is Star’s preferred standard practice.
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