By Chris R. Bates, CHEMETALL PLC, Milton Keynes, UK

Has the Critical Art of Surface Preparation face preparation. This is most relevant in relation to the hi g h - p e r f ormance coatings such as fusion-bonded epoxy (FBE), epoxy-primed (FBE or liquid) 3-layer polyethylene or polypropylene (3LP) coatings, and other multi-layer coat- ings that use “epoxy” as the primer for corrosion protec- tion. The operative word here is “epoxy,” a coating that is sensitive to poor surface preparation and that benefits extensively from good surface preparation. ver the past few years, people Sur f ace preparation is doing what is necessary to a sur- in the pipeline industry face to accept a coating and to allow that coating to perfor m from around the world as well as possible. Different coating systems may require have described failed or different degrees or types of surface preparation, but in all failing coatings on pi p e l i n e s cases, the key needs are that have notO been in the ground long. In many cases, these • to clean the surface to a satisfactory level, removing all early failures have been attributed to poor surface prepara- contaminants that can affect coating adhesion, and tion. But why? Surface preparation for pipecoating is • to provide a surface that the coating can adhere to not difficult, is usually well understood, and has co r r e c t l y , effectively, and durably, whether by long been detailed in many key specifications. mechanical or chemical bonding or both. Perhaps excessive familiarity with such a Ar g u a b l y , for those coating systems that s t r a i g h t forward task has caused the rely predominantly on mechanical bond- pipecoating industry to forget the details. ing (e.g., asphalts/bitumens, coal tar Coming from the metal finishing indus- enamels, tape wraps), the level of surfac e try, which relies almost exclusively on Forgotten cleanliness is less important than for cleaning, deoxidising, and conversion chemically bonding epoxy-based coatings. coating chemical processes, I find it hard A good anchor pattern, properly pre- to believe that the aggressive surface prepa- pared, is usually considered sufficient. ration in pipecoating can go wrong. But it does Other systems require a higher level of seem to at times. cleanliness for long-term performance and might This article will examine the key features of surface ? benefit from advances in surface treatment chem- preparation; identify and resolve problem areas; show how istry and developments in coatings technology over the to achieve the right result consistently, cost-effectively, and last 20 years in industrial metal finishing. For instance, more easily; and dispel misconceptions and even some tra- cars can have 6-, 10- or 12-year anticorrosion warranties, ditional beliefs about surface preparation that may be the and aluminium-framed or clad buildings have a 25-year root cause of some of the problems seen recently. warranty on coatings 80 to 150 mm (3 to 6 mils) thick. Translating such advances in surface preparation and What Is the Purpose of Surface Prep a r a t i o n ? coating technologies to pipecoating is difficult and not It appears that many in this industry have forgotten the pur- always possible or even relevant. To its advantage, the po s e — o r , more correctly, the specific requirements—of sur- pipecoating industry utilises the mass and shape of the

26 JPCL / August 2004 / PCE www.paintsquare.com Opposite page Fig. 1: Inside the acid wash applicator—acid washing is an important step in surface preparation of new pipe. Photos and illustrations courtesy of the author.

steel pipe to provide fast and effective condition of the pipe affect our think- sure water-based neutral or alkali su r f ace preparation and coating on rela- ing on and operation of the surface cleaners or by flame cleaning. tively simple, moveable line that is rea- preparation procedure. Fresh, new, Removing general soiling, organic or oil sonably easy to operate and control. high-grade API steels may be free of residues, and general inorganic Ad d i t i o n a l l y , the pipecoating indus- most surface contaminants, but the residues is necessary to protect the try utilises, as its first step, the one tightly bonded mill scale will be the abrasive from cross-contamination and method of surface preparation not suit- most difficult to remove by abrasive re-deposition of contaminants. ed to the light gauge or non-ferrous blasting. Too new and too cold steel and Before abrasive blasting, the pipe metal finishers—abrasive blast clean- the problem of weld gassing can cause surface must be dry and, preferably, ing, often referred to as shot and grit major pinholing in heat-cured epoxies. reasonably hot. Induction heating or blasting. But is abrasive blasting always Older, weathered pipe with an even direct flame heating (e.g., propane, pe r f ormed properly, effectively, and to layer of rust and broken down mill LPG) that does NOT leave combustion specification? More importantly, what scale will be the easiest to blast clean residues on the surface is advised. The can it do and what problems does it but may be heavily contaminated from surface must be at least 3 degrees C (5 solve or cause? These questions will be weathering, storage, or transportation. degrees F) above the dew point, and addressed later in this article. Another problem, chlorides, will be dis- the relative humidity (RH) during cussed later. abrasive blasting must be less than Typical Pipecoating Plant 85%. At high RHs, the dew point will and Proc e s s Initial Cleaning and Drying often be quite high. Surface tempera- Design and layouts of typical pipecoat- New pipe rarely needs initial cleaning tures of 65–85 C (149–185 F) are ing plants for the production of FBE - if its provenance is known. Older, often needed or specified before abra- or 3LP-coated pipe may vary, but such weathered pipe or pipe of uncertain sive blasting. Scales and oxides are plants follow most or all of the follow- provenance should be assumed to have removed faster and more easily from ing steps. unbound surface contamination, much heated steel than from cold steel. 1. Pipe in of which can be removed by high-pres- 2. Initial cleaning and drying 3. Shot/grit blasting—a mixture of shot and grit 4. Shot/grit blasting—mostly grit 5. Decontamination/chloride removal— acid washing between or after the abra- sive blast cleaning steps, as required 6. Inspection 7. Chromate treatment, as required 8. Pre-heat 9. Coating application 10. Inspection and testing 11. Pipe out The steps from pipe in to chromate treatment will be described below.

Pipe in—Received State Be f ore surface preparation begins, sev- eral variables could create difficulties or reduce quality. What is the grade of steel, its wall thickness and degree of variation? How old is it? Where did it Fig. 2: Typical schematic of a complete acid wash facility, including automixing and dosing, come from and how was it pure water generation, and waste water treatment stored/shipped? How will the surfac e www.paintsquare.com JPCL / August 2004 / PCE 27 Shot and Grit Blast Cleaning right surface profile for the coating. Are the qualitative profile tape Centrifugal wheelblasting with steel For FBE and 3LP coatings, a dense, impression and 30X microscopic assess- shot and grit is the principal surfac e angular anchor pattern provides the ment of the profile sufficient for accu- preparation procedure that many most cleaned surface area for maximum rately defining profile sharpness, angu- would argue is sufficient to meet speci- ad h e s i o n . la r i t y , and density? Or do we need an fication needs. Poor management of In some ways, “anchor pattern” is addition to the ISO 8503 standard on these units and the abrasives, overesti- misleading; it implies that the coating is profile density? mating their capability, and relying too anchored or bonded to the surface by Shot and grit blasting is a destructive much on their perceived effects can be virtue of its roughness. Only in severe process that generates dust residues on disastrous. Two wheelblast units are de f ormation conditions could such ter- the steel surface. Dust levels assessed in preferable to one. Surface contamina- minology be relevant. The key phrase accordance with ISO 8502-3 must not tion is minimised, and the ability to pro- here is “surface area” and its maximisa- exceed level 3 on the scale provided. duce the right surface profile is more tion. For coatings that bond mechanical- Even this level of particulate residues easily achieved. ly , chemically, or both to a surface, the on the surface can cause coating applica- A single wheelblast unit has to do greater the (micro)surface area is, the tion or adhesion problems. Remove all everything. Management of the more bonding sites there are, and thus such residues by vacuuming (not brush- shot/grit ratios, abrasive contamination the better the adhesion is. A rounded or ing) or by blowing with clean, hot, dry and replacement, and the overall unit dished profile is not acceptable. high-pressure air. operation requires closer control and Su r f ace profile is measured in accor- For individual or even most of the more frequent adjustments. Two units dance with ISO 8503-1 C1 & C2: clean and properly profiled pipes, the allow separation of the “cleaning” to be Su r f ace Profile Comparators. But only level of cleaning achieved by abrasive done mainly by the first unit (mostly the peak-to-trough height of the profile blasting may meet the average pipecoat- shot) and the generation of the profile (the surface rugosity, RA value, profile ing specification. But this level of clean- by the second unit (mostly grit). amplitude, surface roughness, or other ing is not sufficient on all the pipes The type and grade of abrasive depend descriptor) is actually measured or throughout a coating contract or for largely on the type and grade of steel, its specified. very demanding coating specifications. age, the degree of mill scale, the amount Profile density—the number of peaks For instance, if the abrasive blasting and type of corrosion products, and the per unit area (e.g., per mm2 or cm2) —is procedure magnetises the steel surfac e , efficiency of the units. Throughout a at least as important as, or more impor- only chemical cleaning can remove the lengthy coating contract, these variables tant than, the profile amplitude and dust residue magnetically bound to the can change, often significantly. Changes should also be measured as a true su r f ace together with any other particu- to the abrasive mix, its management, and assessment of profile quality and sur- lates. Thus, a traditional belief about its type may be needed. face area increase. abrasive blast cleaning should be chal- To achieve the required level of

(apparent) surface cleanliness (as mea- Averaged Cathodic Disbonment Test Performance (30 Days, ambient) sured in accordance with ISO 8501 1 Part A1: minimum Sa 2 ⁄2 to Sa 3 [SSPC-SP 10 to SP 5]) and the required profile, the correct hardness and size of abrasive must be used. ISO 8501 Group E: Metallic Cleaning Abrasives, Shot/Grit blast only (SB) ISO 11124-1: Specification for Chilled SB + “Acid Wash” Iron Grit, and ISO 11124-2: Specification for Cast Steel Shot and Grit are the relevant standards. Shot and grit blasting, therefore, has two functions: to clean (descale/deoxi- Fig. 3: Comparison of CD performance—Shot/Grit blast (SB) and SB + acid wash dise) the surface and to provide the

28 JPCL / August 2004 / PCE www.paintsquare.com lenged and two key misconceptions dis- The level of non-visible and potential- become loosely adhered, too hydrated, pe l l e d . ly damaging contaminants is impossible and easily contaminated. As noted earlier, the traditional to control with such surface prepara- The fou r -hour maximum delay phrase, “abrasive blast cleaning,” is tion techniques, and, unless the whole between abrasive blasting and coating is incorrect; it should be “abrasive blast su r f ace of every pipe is tested, we do usually considered practical and reason- de o x i d i s i n g . ” not know what the contamination lev- ably safe in pipecoating but depends The first misconception is that els are and what long-term effect they heavily on the atmospheric conditions. mechanical surface preparation will might have. RHs above 65% will dramatically short- fully clean a surface. It will remove, We rely on the pipecoating industry’s en this supposed safe period. almost completely, one of the key sur- extensive knowledge, experience, and What is needed for proper coating face contaminants affecting coating prediction ability to prepare the surfac e pe r f ormance, therefore, is a means by application and perfor m a n c e , which a well-prepared surfac e i.e., oxidation—scale and rust. can be properly cleaned of all But it cannot remove soluble potentially damaging contami- salts, trace organics, and surfac e nants and changed to provide a bound or chemically reacted stable, non-degradeable surfac e re s i d u e s . condition. 1 The Sa 2 ⁄2 minimum measure The bonus, with measurable of (apparent) surface cleanliness benefits, is coating quality and is generally accepted as suffi- pe r f ormance that easily meet or cient for the application of most exceed the toughest coating heavy-duty or functional coat- specifications. Quality and per- ing systems, but it can be formance translate into extend- improved considerably. Only ed durability and long service pe r f ormance testing shows how life. Such surface treatment and clean such a prepared surfac e finishing, and their effects and Fig. 4: Application of chromate treatment to profiled pipe really is. Different types or lev- benefits can only be achieved els of residual surface contami- with specialised chemical clean- nation or lack of complete cleaning will of steel pipe using these techniques in a ing and conversion coating technology, affect coating adhesion, durability, and way that minimises the impact of poten- which is well established and under- corrosion protection to varying degrees. tial problems. The quality and condition stood. For pipecoating, the process is not But unless tests for cleanliness, dura- of the achieved surface finish and ofte n difficult or hazardous to use once the bi l i t y , and corrosion protection can real- the received pipe surface condition are equipment is installed and operated cor- istically predict in-service coating life, constantly monitored. But cleaned sur- re c t l y . sl o w e r -acting surface contaminants or face is very fragile, degrading as we less than ideal surface preparation does watch. It must be coated as quickly as Chemical Surface Trea t m e n t s not immediately show up. The stories of possible—perhaps in minutes—to get for Pipecoating field failures of coated pipe only a few the best results from it. This is because Acid washing is effective for surfac e years old may well be the result. cleaned ferrous surfaces immediately cleaning, decontamination, and chloride The second, potentially more damag- and continuously try to revert to their removal. Chloride contamination, which ing misconception is that mechanical natural state of iron oxide. In normal dry abrasive blasting cannot remove, su r f ace preparation, such as shot and atmospheric conditions, a thin, non-visi- can seriously damage coating adhesion grit blasting, provides a consistent fi n - ble oxide layer approximately 1 micron and performance on steel. If chloride ish quality. The achieved surface condi- thick will form quickly. For chemically were present in simple free soluble salt tion after shot and grit blasting bonding coatings, re-oxidation is advan- form only, a thorough wash with clean depends on the received surface condi- tageous provided the re-oxidised layer is water (hot and at high pressure) would tion of the pipe, which can vary from strongly adherent and uncontaminated. seem to be sufficient to remove it. pipe to pipe. Le f t too long, the re-oxidation layer can Un fo r t u n a t e l y , chloride is present in www.paintsquare.com JPCL / August 2004 / PCE 29 both soluble and surface-bound for m s , When, How, and Why Is Acid Was h i n g applied at the prescribed concentration especially as the ferritic salt. Used in a Pipecoating Plant? to the pipe’s heated steel surface by low- For many years, the evidence The “when” is determined by its func- volume, low-pressure spray or flood to against chloride contamination has tion: Acid washing is a means of clean- fully wet the pipe surface within 1–2 been building, and it is now generally ing profiled steel so the operation needs revolutions. accepted globally that if there is more to be scheduled after abrasive blasting, 2. A short period of solution contact than 2 µg/cm2 (20 mg/m2) of measur- usually before transfer to the coating (reaction) time is determined by the com- able chloride on a blast-cleaned (pro- line. It has little or no effect or benefit if bination of surface temperature, solution filed) steel surface, subsequent coating used before abrasive blasting. concentration, and line speed—typically performance or durability problems Al t e r n a t i v e l y , it can be used between 20 (15–30) seconds, but without the for - may be expected. It is difficult to mea- the first (shot) blast cleaning and the mation of any visible passive layer. sure chloride levels on profiled 3. High-pressure, clean water steel surfaces, and they can be rinsing removes all traces of difficult to remove to such a the acid wash chemical befor e low level. the formation of any visible The only way to remove such passive layer and provides a contamination is by chemical pH-neutral surfac e . cleaning; for chlorides, this The next part of the ques- means strong acid-based sys- tion is “why” use acid wash- tems with specialised wetting ing. Its use is now considered agents and sequestering power almost mandatory if measur- (Figs. 1 and 2 on pp. 26 and 27). able chlorides are present For the last 25 years, the af ter abrasive blasting. But pipecoating industry has had this is only part of the story; access to the metal finishing the real benefits of acid wash- industry’s second best kept Fig. 5: Another view of application of chromate pret r eatment to profiled pipe ing are in providing a truly secret: Phosphoric acid, com- clean surface for coating, and bined with a special detergent, one that is more stable and quickly and effectively cleans and second (grit) blast cleaning if the plant consistent than a mechanically cleaned decontaminates steel surfaces, remov- layout allows. su r f ace. The result is a slight but notice- ing all inorganic salts such as chlorides Decontamination of the steel surfac e able improvement in coating quality and and sulphates (soluble or bound), trace af ter blast cleaning is often considered pe r f ormance achieved at low cost and oils and greases, light oxidation, dust easier and faster than if the full profile little effor t . and carbon residues, even magnetically has been developed—provided the pipe Regular but not excessive assessment bound ferritic particulates. is fully descaled and the second blast of surface cleanliness afte r acid washing Acid washing must be carried out cleaning does not cause any recontami- will be all that is needed to confirm the correctly to gain the benefits. The nation of the surfac e . elimination of all surface contamination detail of how to employ the definitive There is a secondary benefit to this in- problems—real or assumed. acid wash technology would fill anoth- between operation. If the acid wash is To dispel another misconception: Acid er article. allowed to react with the steel surfac e washing is strictly a surface cleaning Users must be provided with all the too long, a passive phosphate layer process, not a surface treatment and expert advice, documentation, method might form on the steel surface (visible coating process, as some would have it. statements, operating manuals, and as a light blue colouration of the surfac e Extensive performance testing in the health and safety information by the or trace white powder deposits). The past has shown that an iron phosphate process supplier as well as advice and second (grit) blasting will remove this. conversion coating does not benefit the recommendations in preparing specifi- The “how” of acid wash treatment has cathodic disbondment (CD) perfor- cations that include or recommend three steps. mance of FBE coatings. In fact, it can acid washing. 1. The diluted acid wash chemical is actually make the FBE coating’s CD

30 JPCL / August 2004 / PCE www.paintsquare.com performance worse. mance of any applied coating. The application method is low-volume, Af ter acid washing, the profiled pipe For pipecoating, however, the rarest drip feed to start to wet the surfac e . su r f ace is thoroughly clean, more stable and most unusual chromate treatment Spray application is not permissible on (but still somewhat fragile), and—for process—the dry-in-place chromate/sili- safety grounds. many coating specifications—ready for ca complex oxide conversion coating is • The applied solution is immediately coating. It will provide generally consis- the only type of chromate chemistry spread over the surface by means of tent, even slightly improved, coating that will work on steel surfaces. It is the any suitable prescribed wiping tech- pe r f ormance. Figure 3 on p. 28 is an simplest form of chemical conversion nique, such as rubber squeegee blades, average assessment of many CD test coating technology to use. to give an even, thin wet film on the sur- results (TRANSCO/British Gas CW6 Again, correct application and face within 1–2 revolutions of the pipe. 30 day, ambient test) over many years process management is needed to Application can be monitored and from different plants and test pro- release these benefits and avoid any assessed visually because the resulting grammes showing the typical perfor - possible disasters. The details of how to coating will be light to medium golden mance of FBE with and without acid employ the definitive chromate treat- bronze, and defects such as runs, sags, wa s h i n g . ment for pipecoating would also fill and poor coverage will be apparent. another article. Pipe coaters should take • This thin film will begin to dry immedi- Chromate Pretreatment— advantage of the extensive industry ately from the steel surface heat. Conversion Coating Process experience with chromate treatment, • To complete the chemical reaction of But, for the more demanding coating and seek help and advice from the expe- the chromate treatment with the steel standards and specifications, even this rienced pretreatment supplier. su r f ace, the treated surface needs to be level of coating performance and quality heated to 120 C (248 F) or more for at may not be enough. To further improve When, How, and Why Is Chrom a t e least 20 seconds. In FBE/3LP coating co n s i s t e n c y , quality, and perfor m a n c e , Treatment Used in a Pipecoating Plant? operations, this happens automatically another (final) step in surface prepara- The “when” is, again, determined mainly as part of the pre-heat cycle before coat- tion is needed—changing the steel sur- by function but also by method of appli- ing application. No other actions are face to a truly stable, inert, adhesion- cation: Chromate treatment is essential- ne c e s s a r y . (Coating systems that do not promoting and corrosion-resistant con- ly part of the final coating procedure, require a surface temperature of at least dition by using powerful conversion not the surface preparation procedure. 120 C [248 F] will not benefit from coating technology to “replace” the thin Its operation, therefore, needs to be sited chromate treatment.) iron oxide layer with an alternative, on the coating line as close as possible to The result of chomate treatment is to beneficial, complex oxide layer. This the pre-heat prior to (FBE/3LP) coating change the cleaned, somewhat unstable, technology is the dry-in-place chro- application. The reasons for this have to invisibly oxidised steel surface to a com- mate/silica conversion coating, com- do with the method of its use and the plex mixed oxide conversion coating of monly referred to in pipecoating as need to dry the applied coating in place chromium, silicon and iron, 1–2 chromate (pre)treatment (Figs. 4 and 5 with heat. microns thick, integral with the steel on pp. 29 and 30). “How” the chromate treatment is itself and visible as a golden-bronze For the last 20 years, the pipecoating used in a pipecoating plant can be con- co l o u r . This surface is now extremely industry has been selectively using the sidered a one-step process during which stable, highly resistant to degradation metal finishing industry’s probably best a number of actions take place in rapid and will provide the maximum adhesion kept secret—chromate surface treat- succession in a very short length of and corrosion resistance possible from ment chemistry. Widely used on alu- space on the coating line. the applied epoxy layer. minium, zinc-coated steels, many other The procedure can be broken down as Chromate treatment will improve non-ferrous metals, and iron and fol l o w s . coating performance, easily meeting or steels—even stainless steels—chromate • The unheated, diluted chromate treat- exceeding the most demanding of coat- pretreatment provides unrivalled coat- ment chemical is applied at the pre- ing standards or specifications. ing adhesion performance and long-term scribed concentration to the rotating Dramatically improved CD and hot bond strength, corrosion resistance, and pipe steel surface that is heated to the water (HW) performance test results long-term corrosion protection perfor - required and controllable temperature. may be the easily measurable be n e f i t . www.paintsquare.com JPCL / August 2004 / PCE 31 Figure 6 is an extension of Fig. 3, show- ing the typical performance of FBE with all these various forms of surface prepa- ra t i o n . But this largely misses the point of using such technology and highlights more information about chemical treat- ment that needs to be clarified. • Chemical pretreatment provides better, more consistent surface preparation, with its results measured by the perfor - mance testing of the finished compo- nent. Chemical pretreatment is not a means of overcoming deficiencies in the su r f ace preparation or a shortcut to fin- Fig. 6: Comparison of CD performance—All surface preparation variants ish the job. • Chromate pretreatment will not turn a so w ’ s ear into a silk purse. Using this su r f ace of the chromate-treated pipe will Chemical Pret re a t m e n t — technology to mask the poor perfor - absorb the heat fas t e r , reducing heating The Disadvantages mance of low-quality coating systems is co s t s . To benefit from chemical pretreatment, a recipe for disaster. • Reduction in overall pipecoating cost one must use strong, potentially haz- • With such a stable and consistent sur- A significant amount of evidence con- ardous chemicals. For the pipecoating face that is guaranteed, the level of con- firms that the consistently high perfor - in d u s t r y , the growing concern is the haz- sistent coating quality, performance, and mance results regularly achieved in pro- ard of using such chemicals and the dis- ease of obtaining it become the norm duction translate directly into greatly posal of waste. rather than the exception. extended durability and retained pipe No national or international occupa- • By concentrating on the achievable end coating performance in the ground. tional health or environmental regula- result—a certain surface finish that can But this also begs a further question: tions prohibit the use of these processes, be guaranteed—the emphasis is no how does chromate treatment actually even the chromates. Many countries do, longer on concerns over the condition of in f luence coating performance? The ho w e v e r , regulate these materials heavi- the received steel but on confidence in answer is the subject of several major ly , setting stringent limits on how they achieving the right result, almost regard- technical papers, articles, and discus- may be handled, used, and disposed of. less of variation in the received pipe’s sions presented many years ago. Their The key hazards related to the chro- condition and the mechanical surfac e theory of CD and the reasons chromate mates—inhalation of hexavalent chromi- preparation procedure. treatment so effectively combated it are um in vapour or atomised form and the The real benefits of chromate treat- still unchallenged. It is not possible to environmental impact of chromium—do ment are almost hidden. prevent CD but it is possible to interfere not effectively apply to chromate treat- • Elimination of rejects for surfac e with its mechanism and “slow it down” ment in pipecoating. As a dry-in-place preparation problems so significantly that it appears to be pre- process, proper use and management • Higher quality and consistency of the vented. mean that there is virtually no waste to coating operation The application of the coating that fol - affect the environment. It is prohibited to • Ea s i e r , fas t e r , less troublesome coating lows acid washing and chromate pre- spray chromate, and the temperatures op e r a t i o n treatment is a whole other story but involved are nowhere near enough to • Potential for lower temperatures of inextricably linked to this one. generate a chrome-containing vapour, so cure of the epoxy Ho w e v e r , there are still questions and the inhalation risk is virtually nonexis- • For preheat systems that work by actions outstanding to complete the full tent. Once the applied process has been infrared heat absorption by the steel, picture of modern surface preparation through the preheat cycle and fully react- e.g., direct gas-fired ovens, the darker of steel pipe. ed with the steel, the chromate is present

32 JPCL / August 2004 / PCE www.paintsquare.com as an inert, insoluble and non-hazardous water quality, use treated water. re i n f orced rubbers, neoprenes. oxide layer, integral with the steel surface. • Air—Any blown or compressed air Brushes, rollers, matting, etc., made of The experienced pretreatment process used on the surface of the cleaned pipe inert but flexible, non-shredding materi- supplier will provide all the infor m a t i o n must be dessicated and oil-free; if used als, non-reactive with the chemical and assistance to allow any coating con- for drying, the air should be heated by processes, are also considered suitable. tractor anywhere in the world to utilise any means that does not contaminate it. Regular housekeeping and plant both chromate and phosphate technolo- A dewpoint for compressed air of -40 C maintenance can eliminate many gies with confidence and safety, both for (-40 F) has been suggested in recent potential causes of cross- or re-contam- the operators and the environment. sp e c i f i c a t i o n s . ination of the prepared pipe surface. • Heating—Indirect (induction) or The Pipecoating Plant direct (hot air, indirect or direct gas Co s t Features and needs of the plant itself flame) heating methods must not conta- The installation and use of these sur- can actually make or break a job. The minate the pipe surface or leave com- face preparation methods and proce- impact of the services used on the bustion residues on the pipe surface. dures imply additional cost. One can pl a n t — w a t e r , air, heating, and materials Direct, residue-free gas flame (i.e., also argue that the short-term benefit in constant or regular contact with the “blue” flame), such as propane or LPG of a simpler operation and reduced pipe—need to be addressed after the gas burners, may be used to provide prices is quickly lost with the long- achievement of the profile. low levels (up to 90 C [194 F]) of heat term cost penalty of poor quality and • Water—All water used in contact with input into the pipe surface. performance—and loss of business. the cleaned pipe surface should be of • Materials used for wiping/spreading Investing the time, effort, and, if nec- the following quality. chemical solutions on the pipe surfac e es s a r y , the money in setting up such a • Maximum anion content—100 ppm should not shred or leave residues on su r f ace preparation plant as described total chloride and sulphate where each the pipe surface. Typ i c a l l y , the same herein and establishing proper proce- anion shall not exceed 60 ppm material used as circular flanges at the dures and management will soon be • Conductivity—MAX 100 µS entry and exit ends of the wheelblast- rewarded with a low unit cost of opera- Where there is any doubt concerning ing units is recommended, e.g., hard or tion. A cost study is perhaps in order. Some Tips Su r face preparation for pipecoating perhaps needs to be rec o n - additional cleaning. si d e r ed to obtain the results now expected. For many coating • Use acid washing to guarantee surface cleanliness and to rem o v e systems, especially the increasingly demanding coating stan- all damaging contaminants, especially chloride, reg a r dless of da r ds and specifications employing the latest high-perfo rm a n c e whether the received pipe surface condition predicates its use. coatings such as FBE and 3LP, the following points can be made. • Set the target for cleanliness and profile to be achieved, inspect • Know what condition the received pipe is in and what impact and measure it at the end of these proc e d u r es only, and adjust the that can have on the surface prep a r a t i o n . pro c e d u r es if necessary. • Know what can be done initially to reduce any such impact on • Use chromate treatment to provide total consistency of coating the subsequent proc e d u r es, or to predict what needs to be done. qu a l i t y , not simply to optimise the coating perfo r mance or to meet • Know the grade and hardness of the steel and choose the type ce r tain coating specification req u i r ements. Such perfo r mance needs and size of the shot/grit mixture accordi n g l y . will be met as a matter of course anyway. • Use and manage the shot/grit blasting operation effectively and • Maintain and look after the plant, its materials, services, and oper- pr e-emptively to minimise the impact of received steel condition ation, and be aware of their impact on the effectiveness of the sur- variation. Separate shot blast and grit blast units are pref e r red , face preparation, including atmospheric conditions in the facility. but single units, managed caref u l l y , can be as effe c t i v e . • Plan and implement a complete surface preparation proc e d u r e that • Control and manage the abrasives mix ratios and their cleanli- can be controlled and managed easily and is preventative rather ness prop e r l y . than curative in its approach to possible problems. Prove it with • Concentrate on achieving the correct profile amplitude and qualification trials (including qualification of the coating system density. The acid wash process can complete any residual or itself), and stick to it. www.paintsquare.com JPCL / August 2004 / PCE 33 Saving just one reject pipe a week would more than pay for the extra technology and procedures.

Co n c l u s i o n In the high risk world of pipelines and high cost coating con- tracts, the small matter of surface preparation may not seem that important, but its impact on the service life of a pipeline can be dramatic and costly if poorly done. Done well and coupled with the best available coatings, it can also be signifi- cant in reducing pipeline corrosion protection operating costs in service. Where does CP fit in? CP and protective coatings are effec- tively mutually incompatible because the CP system is continu- ously trying to disrupt the coating from the steel surface. This brings us back to the beginning: surface preparation and its effects on coating quality and performance. Utilising these best practices with these high-performance coatings will result in an in-service pipeline drawing the minimum protection current from the CP system. Its life cycle protection costs will be min- imised and this alone justifies the rationale behind this article.

Ac k n o w l e d g e m e n t s The writer would like to acknowledge the other members of the original team that defined and developed the effective use of chromate pretreatment for pipecoating—George Higgins and Alan Koch—and the many long discussions over the years with one of the industry’s leading experts, David Norman. Their interest, support, and enthusiasm have ensured this writer's abiding fascination with this industry for over 20 ye a r s .

For the past 27 years, Chris Bates has been involved in most aspects of the surface fin- ishing of metals for painting and corrosion protection by chemical pretreatment process- es. Originally with Pyrene Chemicals (part of Brent International and now owned by Chemetall), he was responsible for the key development work that resulted in the chro- mate pretreatment system for pipecoating. He joined the Chemetall Group in 1991 and is currently global technical coor- dinator for all the CM Group's pipecoating interests worldwide.

This article is an edited version of a presentation given at the 15th International Conference on Pipeline Protection, Aachen, Ge r m a n y , 29–31 Oct 2003. It is published here with the permis- sion of the organisers, BHR Group Ltd, Cranfield, UK. The com- plete article, with full bibliography, can be found on JP C L - P C E ' s website ww w. p a i n t s q u a r e . c o m .

34 JPCL / August 2004 / PCE www.paintsquare.com