At first look, it seems that surface  condition  can have 

a little or no practical influence on the  performance of materials in service. In practice, however, its effect on common material properties those include mechanical, phys- ical, trinological  (friction,  wear,  hardness),  metall-urgical, corrosion, aesthetic, etc. is  frequently exper-ienced. Present article deals with  the  effect of surface defects, contaminants, products, surface cleaning methods, etc. on the corrosion properties  and the  performance of metals or alloys

Gaur, B

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EFFECT OF SURFACE PREPARATION ON CORROSION OFMETALS AND ALLOYSB. GaurCorrosion Protection Division,National Metallurgical Laboratory , Jamshedpur (Bihar)At first look, it seems that surface condition can have a littleor no practical influence on the performance of materials inservice. In practice, however, its effect on common materialproperties those include mechanical, physical, tribological(friction, wear, hardness), metallurgical, corrosion, aesthetic,etc. is frequently experienced. Present article deals with theeffect of surface defects, contaminants, products, surfacecleaning methods, etc. on the 'corrosion properties and theperformance of metals or alloys.CORROSION PROPERTIEScorrosion ProbabilityEvan described a 'drop method' to obtain the effect of surfaceroughness on corrosion probability. The proportion of dropsproducing rust were indicated to be a measure of corrosionprobability. However, concrete conclusions could not be drawn onits basis except to realize the significance of surface conditionfor the corrosion occurring probability. Later on, other workersobtained some information by ruling 'scratch' lines on thesmooth thin nickel sheet, and counting the number of perforationsper unit length. When sheet was subjected to chloride sulphatesolution, the main surface remained uncorroded, but scratch lineregion suffered pitting resulting in perforation; the points ofperforation were easily countable. However, this was alsomentioned that the obtained results should not be taken as anabsolute measure of corrosion probability.Corrosion DistributionIn cases where the total corrosion rate is governed by anexternal factor, an attempt to enhance resistance by improvingthe surface is likely to confine and intensify the attack to1small area. For example, corrosion of high grade (having nometallurgical heterogeneities) steel in a fairly concentratedsalt solution is mainly fixed by the replenishment of oxygen atcathodic zones at the surface. In an experiment, a well preparedsteel plate immersed in O.1N KC1 was found corroded along the cutedges, where the cutting stresses or the exposure of internalsegregate might have favoured the attack. In next experiment cutedges were covered with a preventive coating but corrosion thenstarted at numerous points on the face. It seems that the surface(large cathode) was protected in fist experiment by strong attackupon edges* (small anode). Thus the elimination of sensitivepoints on steel, whether they were due to cut edges, surfaceblemishes, sulphide inclusions, may not always improve theperformance; if the total deterigration of metal is fixed,intensity of attack will increase with the decrease in the numberof sensitive points . In the cases where the total attack is notfixed such as in atmospheric attack; a smooth surface finish andsurface discontinuities, such as inclusions , are unlikely toaffect corrosion. Often the tiny rust spots on iron are due tosettlement of certain kind of dust and not due to surfacecondition.There are some interesting cases where the surface polishingaffects the role of metallurgical impurities, nature of attack,control measures etc. during corrosion. In a study conducted onsteel in an inhibited salt solution (5% NaCl with Na2CO3)1 thepolished surface was preferentially attacked at sulphideinclusion sites . However when surface was roughened (by abrasion)or scratched, the effect of inclusions ceased considerably, sincethere were alternate active points. In general, smooth surface isless prone to corrosion than coarser one and number of corrosioncentres steadily increases with degree of coarsening. This factis supported by electrochemical studies also. Polarizationexperiments made on SS 316 in sodium chloride solution have shownthat the value of pitting and passivation potential increaseswith improvement in the surface smoothening as evident from the* In dilute solutions, where the low conductivity confines themutual protective effects to short distances, there is attack atmany points on the face, even when the cut edges are leftunprotected.2following Table:Surface Finish Rest Pot.(mV)Pitting Pot.(mV)Repassivation Pot.(mV)1 micron -337 375 -60p1200 -256 275 -205p220 -344 125 -210However, attack will be deep if occurs at smooth surfaces due toestablished reasons. Similarly, if inhibitors are added, therequired dosage will be more for coarse surfaces but corrosionattack will be destructive on smooth surfaces if the dosage arenot maintained above a certain 'level. Moreover, it is alsoestablished that a rough surface of steel when plated orelectrodeposited with nickel will exhibit far more pores per unitarea than smoother steel carrying the same amount of nickel.SURFACE CLEANING FOR COATING AND CORROSION CONTROLProtective barriers (coatings) have been successfully used innumber of corrosive environments. These may be of metallic,inorganic and organic nature. Out of different coatings, organicones including paints, varnishes, lacquers, etc. protect metalson large scale than any other method to combat steel corrosion.Further, steels are the most widely used materials in number ofindustrial, municipal, transport, marine, domestic, etc.applications due to outstanding strength and other engineeringproperties and low cost. However, these frequently requirecorrosion resistant coatings due to their high susceptibility torusting. In view of the above, surface preparation has beendiscussed dominantly in the light of paint applications on steelcomponents, however, some of them are employed in other importantapplications also, i.e., pre and post fabrication cleaning ofchemical process equipments in industry, metallic coatings, etc.In most of the cases, poor paint performance, e.g., peel off,occurs due to poor application and inadequate surface preparationor pretreatments as shown in Photograph-1. Thus the surfacepreparation work lays the foundation for coating performance atthe coating - metal interface.3Natural Surface Condition and Finishing Required for PaintingMetals are being formed or shaped on large scale by six specificprocesses, each producing a slightly different end condition andimpurities at the surface, however, sufficiently resemblingthemselves in actual appearance that may be classified as:i) Surface containing mill scale and oxides including hoti olled, f:oi:q d, cr1E3t-. met-al.nii) Surface free of mill scale and oxides including cold rolled,cold drawn, extended metalsIn field, many users treat the metals with scales as hotrolled' and those free of scales a^; 'cold rolled'.The surface to which coating has to be applied must besufficiently free of rusts, mill scales, dirt, oil, grease, olddeteriorated paints, other contaminants, etc. This is requiredsince, paint holds to the metal surface by two basic mechanisms:chemical attraction or 'adhesion' and mechanical anchoring or'bonding'. Consequently, the finishing is desired to result in aclean, uniform and slightly roughened surface. Rougheningprovides a tooth' or 'anchor' (Photograph 2) for mechanicalbonding and more surface area for adhesion.Methods of Surface PreparationThe best finish for paint application is obtained with abrasiveblast cleaning (also known as 'sand blasting conventionally')which includes sand blast, grit blast and shot blast the steelsurface. Other- conventional surface preparation or cleaningtechniques used prior to various coating or clean-up (of processequipments) operations are: pickling, solvent degreasing,detergent washing and other types chemical treatments; scrapingand wire brushing; chiseling, chipping and hammering; grinding,polishing and buffing; flame cleaning (heating with a torch andscrap off dust and scale), electrochemical cleaning, etc.Besides, there are various pretreatments given to the surface ofstructures, pipes, equipments to prolong their life beforecoating or subjecting them to service. These may include wettingoil, wash primer, alkali treatments, phosphatizing, chromatising,4cyaniding, nitrate passivation etc. A study on paint life showed10.3 years for sand blasting , 9.6 years for pickling, 2.3 yearsfor weathering and than hand cleaning . In practice , however, theselection of optimum surface preparation method depends uponeconomic considerations, effectiveness, required degree of finish(specific coating or medium) and other technical factors, i.e.,the type of metal and paint system, alloy's self protection,existing surface condition, shape and size of component, physicalaccessibly to structure, environmental or chemical conditions,etc. In fact, methods are not mutually exclusive and sometimesused in combination also, for example, good surface preparationstarts with preliminary degreasing to remove oil or grease since,these are not effectively removed by mechanical methods otherthan abrasive blasting. In the -forthcoming paragraphs, anintroduction of a few mechanical cleaning methods is being givendue to lack of space. However, detailed information. andguidelines are available in standards or literature published byASTM ( including A380-78, ASTM STP 538), NACE, structural steelpaint corporation (SSPC), AISI, specialty steel producers, etc.Hand tool cleaning may be used to remove loose mill scales,contaminants, nonadherent old paints but not for tight ones ordeposits in pits and crevices . This is a slow operation andnormally recommended for uncontaminated atmospheric exposures or,often used to remove heavy deposits before employing more throughsurface preparation methods such as power tool cleaning andabrasive blasting . The common tools are chipping and rusthammers, chisels , scrapers , wire and dust brushes, abrasivecloths or papers, etc. The SSPC has established a standard forhand cleaning (SP-2-63). Power tool cleaning is employed on steelstructures where blast cleaning is impractical or uneconomical,and the coating systems used are those which tolerate thecontaminants left behind after this cleaning . Mostly this is alsochosen for uncontaminated atmospheric conditions. Power driventools include pneumatic chippers , chisels, needle hammers, rotarybrushes, grinding and polishing wheels, etc . The obtained surfacemay be excellent for painting if properly done as shown inPhotograph-2 (a) . An arbitrary standard for cleanliness has beenestablished by SSPC (SP-3-63).Abrasive blast cleaning may be defined as cleaning through theimpact of abrasive particles propelled (normally by jet action ofcompressed air) at high velocity against the surface to becleaned. This method is convenient, fast, portable, suitable for5different shapes, and is effectively used to remove all traces ofoil or grease, adherent or hard scales, old paints, etc. from thesurface and produces a desired cleaning and uniform roughening ofthe surface anchor or tooth pattern' as shown in Photograph-2(b).The SSPC has developed four standards of surface preparationto get varying degree of surface cleanliness which have beenaccepted and recognized by professional organizations andindustry. Following iscleanliness and cost:the list in the descending order of1. SSPC-SP 5 - 63 White metal blast cleaning2. SSPC-SP 10 - 63T Near white blast cleaning3. SSPC-SP 6 - 63 Commercial blast cleaning4. SSPC-SP 7 - 63 Brush off blast cleaningThe first two are specified only for coatings in immersionservice, while remained two are suitable for most of theatmospheric services. Sand is the most commonly used abrasive forair blast cleaning in the field due to its low cost and localavailability but due to its high breakdown rate (10-40%) it maynot be reused. The other common abrasives are: silica send, othernatural abrasives (crushed flint, garnet sand), slags (byproductof Pb and Cu ore reduction), metals (cast iron and steel shotsand grit), nonmetallics (SiC, aluminium oxides), etc. The shapesof abrasives may be classified as:Semisharp: Common shape of sand and slagGrit : Angular; gouges the surface; high cuttingefficiencyShot : Spherical; peens the surface pounds off brittledeposit and mill scale; may pound impurities intothe surfaceGrit and shot shapes usually apply to the metallic abrasives.Photograph-3 illustrates the type of surface obtained by theiruse. Nowadays, a mesh size range of 20-50 (US sieve series) isoften selected to obtain an optimum profile height (1.5-2.5mills) of the anchor patterns. There is one more method ofblasting namely glass bead blasting', but is used only forspecific applications.6RESIDUAL EFFECTS OF CLEANING OPERATIONSMetal forming processes, shipment mishandling and undesired mediaexposure, heating operations, e.g., hot bending, welding, stressrelieving and other heat treatments, etc. produce scales (oxides)on the surface of respective steel or stainless steel component.For certain services, complete removal of scale is required and,therefore, methods like pickling, abrasive blasting, grindingetc. have to be essentially employed. When improperly done, eachof these methods can result in damage or failure of structure. Afew consequences of such operations are dealt in the comingdiscussion.PicklingAcids containing corrosion inhibitors are largely used aspickling agents. If the article is pickled with electrolyticmethod by making it cathode, the attack on the metal is greatlyreduced. Cathodic action may, however, result in hydrogenembrittlement or blistering (Photograph-4 -a' and 'b'),especially if acid contains a trace of arsenic, sulphides, etc.which hinders the union of hydrogen atoms (it can diffuse intometal) to form a molecule. If the article is made anode, hydrogenabsorption is prevented but corrosion or roughening is increased.On carbon and alloy steels, cleaning of whole surface is notachieved at the same time. An attempt to get a complete finish byprolong pickling results in severe pitting (Photograph-5),intergranular corrosion (IGC) attack (mainly on austeniticstainless steels, Photograph-6), and stress corrosion cracking(SCC) if trace of acids are retained in grooves, pits or pockets.An example of the SCC failure of a boiler tube is given inPhotograph-7.Grinding of Welds and Improper Cleaning for Repair WeldsGrinding:There are many cases where grinding operations have resulted inlocalized corrosion and SCC of stainless steel, for example, a316L vessel of a distillation unit (handling mixed industrialwater having some chlorides) failed because of such attack.Photograph-8 shows hairline cracking and crevice corrosion in and7around a vertical weld as a band, several inches wide, spanningboth sides of the weld. The attacks took place in the regionwhere grinding (for cleaning) was done after the welding work.SCC was attributed to residual tensile stresses and localsensitization (IGSCC) introduced into vessel plate by grindingwork before and after welding. A 50 mm side branch connection (ofthe above mentioned vessel) also suffered severe localizedcorrosion due improper grinding (Photograph 9) . Besides, it hasbeen reported that the overemphasis on grinding duringradiographic examinations of welds reduced the wall thickness ofa process component 20% below the required minimum. This resultedin failure of equipment through cracking in heat affected zone.Surface Cleaning for Repair Welds: .During the replacement of a section of the pressurereactors/piping or repair (in plant), removal of surface andintergranular contaminants, e.g., Soft solders, etc. should beensured. Photograph-l0 illustrates IGC attack caused bypenetration of copper into a boiler tube which had containedcopper deposits along the inside surface prior to welding. Thehigh heat of welding caused the copper to penetrate into the heataffected zone of the steel along the grain boundaries. However,intergranular penetration of steel by copper comparatively wellknown. Less well known is that soft solder is capable ofpenetrating steel in a similar manner,, and the failure of a smallpressure vessel used as propane container was attributed to thiscause (Photograph-11). The soft solder was used to attach a nameplate at the top of vessel near outlet valve and melted duringmild fire hazard. Soft solder can penetrate (intergranularly) anysteel surface which it wets at a temperature between its meltingpoint and 400 degree celsius.Abrasive BlastingIn one case, shot cleaning was done on inside of feedwaterpreheater tubing of a boiler. The increased tensile stresses onthe outside of the tubing caused by the impact of, the shotparticles has produced corresponding tensile stresses on theinside of the light wall tubing (about 1/8" wall) amounting to8,500 to 17,000 psi. The resulting corrosion occurring only intubing containing water is associated with high oxygenconcentration. The attack involved pitting corrosion and IGC withsignificant oxidation present in the cracks. The attack wasattributed to be a sort of oxygen SCC. However contrary to the8ahovice di scllsseci failure case, abrasive blastincl sometimes helpsill 1 t'duc i nil the l.it y cal cot 1 0:, i on udder stre:^.;ed (SCC) andfatigue (corrosion fatigue) condition. This benefit is achievedi nd i I c'c nt 1 y by • shot pnell i llcl elf cc.l ' dtn^ i.nq ;allot 111 a;;I: i ny wll:i c,lllll(lllc i,: compressive tc'Ilslle :,tresses on the wall of the pressureor pip(-lines. In brief, most of the coarsening andgrc^cl. notch, crevices making finishing operations arec c^1.I:r.s icon promoting (except when especially created for paint,1l l l i.cations) .OTHER SURFACE DEFECTSThis is essential to properly remove surface products,contalllinants , irregularities before subjecting to serviceenvironment . Such defects may include embedded iron, grease, rustand deposits , slags, welding flux , weld spatters , debris, dirtand -odiments , crayon and paint marks, adhesive tapes, etc. Ifnot. carefully avoided , these may result serious corrosionproblems resulting in catastrophic failures of the component.BIBLIOGRAPHY1. Properties of Metallic Surfaces, Monograph and Report SeriesNo.13, Institute of Metals, London, 1953.2. Corrosion Engineering (by M.G. Fontana), Mc-Graw Hill Inc.,Sinqapore, 1986.3. Preparation of Metals for Painting (by S. Spring), ReinholdPublishing Corpn., New York, 1965.4. Application Manual for Paint and Protective Coatings (byWilliam F. Gross), Mc-Graw Hill. Book Co., New York, 1970.5. Sui-face Protection against Wear and Corrosion, ASM Intl.,Ohio, 1954.6. 71SM Handbook, Vol.. 5, Surface Engineering, ASM Intl., Ohio,1994 .7. Surface Engineering, Vol. I, Fundamentals of Coatings (by P.K.Datta and S.S. Gray), Royal Soc. of Chemistry, London, 1973.9i'1 iU I UUMU't 1i (a)Fragmented and SmearMetal-Above the LineGround Surface30 Microinches.A OlAft-0,4^b VWV15 st, ^, -4,qw %0 - j V3.1. (h}PHOTOGRAPHIt11PHOTOGRAPH9 1.011.

Effect of Surface Preparation on Corrosion of Metals and Alloys

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Effect of Surface Preparation on Corrosion of Metals and Alloys

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