SURFACE PRETREATMENT BY PHOSPHATE CONVERSION …

1 Sankara NarayananCorresponding author: Sankara Narayanan, e-mail: 9 (2005) 130-177 2005 Advanced Study Center Co. PRETREATMENT BY PHOSPHATECONVERSION COATINGS A Sankara NarayananNational Metallurgical Laboratory, Madras Centre CSIR, Complex, Taramani, Chennai-600 113, IndiaReceived: April 22, 2005 Abstract. Phosphating is the most widely used metal PRETREATMENT process for the surfacetreatment and finishing of ferrous and non-ferrous metals. Due to its economy, speed of opera-tion and ability to afford excellent corrosion resistance, wear resistance, adhesion and lubricativeproperties, it plays a significant role in the automobile, process and appliance industries. Thoughthe process was initially developed as a simple method of preventing corrosion, the changingend uses of phosphated articles have forced the modification of the existing processes anddevelopment of innovative methods to substitute the conventional ones.

2 To keep pace with therapid changing need of the finishing systems, numerous modifications have been put forth intheir development - both in the processing sequence as well as in the phosphating review addresses the various aspects of phosphating in detail. In spite of the numerousmodifications put forth on the deposition technologies to achieve different types of coatings anddesirable properties such as improved corrosion resistance, wear resistance, etc., phosphateconversion coating still plays a vital part in the automobile, process and appliance Introduction2. Chemical CONVERSION coatings3. History and development of thephosphating Chemistry of Acceleration of the Chemical Mechanical Electrochemical Kinetics of the phosphating Process Rinsing after Chromic acid Coating Structure and Coating thickness and coating Coating Stability of the PHOSPHATE Influence of various factors on Nature of the Composition of the Structure of the metal SURFACE SURFACE activation131 SURFACE PRETREATMENT by PHOSPHATE CONVERSION coatings a Thermal treatments and Phosphating Processing problems and Defects in coatings and Characterization of Testing the quality of Evaluation of Evaluation of corrosion Environmental impact4.

3 SummaryReferences1. INTRODUCTIONM etals have been the backbone of civilization. Ef-forts have been spared to find alternatives and re-placements for metals but these still play a majorrole in the manufacture and construction and arelikely to do so for many more years. This is due tothe combination of several useful properties likestrength, workability, low-cost and ability to be re-cycled, that the metals possess. However, metalswhich are extracted from their ores by chemical orelectrochemical means show a strong tendency torevert to their oxide form at the first available oppor-tunity, , they tend to corrode [1-4] and as a re-sult they create a tremendous economic loss be-sides posing a serious threat to the national re-sources of a methods of corrosion prevention are manyand varied. These methods may be generally clas-sified [3] as: Modification of the metal by alloying and/or sur-face modification; Modification of the environment by the use of in-hibitors; and Change of metal/environment potential by ca-thodic or anodic most commonly used method of corrosionprotection involves bulk alloying or SURFACE modifi-cation.

4 SURFACE modification is however, far moreeconomical than bulk alloying and is more widelypracticed. The methods generally used for surfacemodification involve the formation of a physical bar-rier to protect the metal against its corrosive envi-ronment [5]. This can be achieved by relatively moremodern methods such as: (i) physical vapour depo-sition (PVD); (ii) chemical vapour deposition (CVD);(iii) ion implantation; (iv) laser treatment; (v) deposi-tion by thermal spray, plasma spray and arc meth-ods; (vi) nitriding; (vii) carbiding; etc., or through moreconventional techniques such as: (i) painting; (ii)anodizing; and (iii) chemical CONVERSION the former methods are usually less economicas they involve the use of sophisticated applicationtechniques and are meant for specialized applica-tions, the latter methods are more cost-effective andhave a wider spectrum of end CHEMICAL CONVERSIONCOATINGSC hemical CONVERSION coatings are adherent, in-soluble, inorganic crystalline or amorphous surfacefilms, formed as an integral part of the metal sur-face by means of a non-electrolytic chemical reac-tion between the metal SURFACE and the dipped insolution [6].

5 In such coatings, a portion of the basemetal is converted into one of the components ofthe resultant protective film, which is much less re-active to subsequent corrosion than the original metalsurface. This film imparts an equal potential to themetal SURFACE , neutralizing the potential of the localanodic and cathodic galvanic corrosion sites [7].They also serve as absorptive bases for improvingthe adhesion to paints and other organic CONVERSION coatings are preferred becauseof their adherent nature and high speed of coatingformation besides being economical. Further thesecan be formed using simple equipment and withoutthe application of any external potential. Chemicalconversion coating processes are classified as phos-phating, chromating and oxalating according to theiressential constituents viz., phosphates, chromates,and oxalates respectively [8]. The present reviewfocuses on PHOSPHATE CONVERSION coatings with aspecial emphasize on zinc PHOSPHATE coatings onmild PHOSPHATINGP hosphating process can be defined as the treat-ment of a metal SURFACE so as to give a reasonablyhard, electrically non-conducting SURFACE coating ofinsoluble PHOSPHATE which is contiguous and highlyadherent to the underlying metal and is consider-ably more absorptive than the metal [9].

6 The coat-ing is formed as a result of a topochemical reac-tion, which causes the SURFACE of the base metal tointegrate itself as a part of the corrosion Sankara History and development of thephosphating processThe use of PHOSPHATE coatings for protecting steelsurfaces has been known since the turn of the cen-tury and during this period the greater part of theWorld s production of cars, refrigerators and furni-ture were treated this way. The first reliable recordof PHOSPHATE coatings applied to prevent rusting ofiron and steel is a British patent of 1869 granted toRoss [10]. In the method used by him, red hot ironarticles were plunged into the phosphoric acid toprevent them from rusting. Since then numerousdevelopments have taken place, of which the majordevelopments are listed in Table 1. Historical development of the phosphating Advancement made/process developedReferencesNo. of iron and steel using phosphoric acid and iron of PHOSPHATE coatings with oxidizing agents to reduce12process of the bath and formulation of zinc PHOSPHATE baths13,14requiring high temperature process time of one Formulation of manganese PHOSPHATE bath requiring high temperature - 15process time of process with maintenance of total acid to free acid ratio16, of PHOSPHATE coating as paint base18, process with the addition of Copper accelerator-coating20time: 10 minutes to 1 of oxidizing agents like nitrate for acceleration coating time:215 of PHOSPHATE coating for cold working operations for phosphating phospating time.

7 60-90 of non-coating PHOSPHATE process based on sodium24or ammonium of cold phosphating of aluminium surfaces using zinc PHOSPHATE and fluorides of disodium PHOSPHATE containing titanium as pre-dip sLarge scale application of manganese PHOSPHATE coatings as an oil28retaining medium for use on bearing or sliding surfaces sUse of special additives to control coating sSpray process at operating temperature of 25-30 C18, sImprovement in coating quality, use of spray cleaners based on18,19surfactant technologyDuring the last 30 years, work has been con-centrated mainly on improvements in quality, par-ticularly to keep in pace with the recent changingneeds of the organic finishing systems. Prominentamong these are: (i) use of low temperature phos-phating baths to overcome the energy crisis [30-32]; (ii) use of low zinc technology [18,19]; (iii) useof special additives in the phosphating bath [33-42];(iv) use of more than one heavy metal ions in exist-ing composition-particularly tri-cation phosphating[43]; etc.

8 New types of PHOSPHATE coatings suchas tin, nickel and lead PHOSPHATE coatings have beenintroduced [44,45] besides the development of com-positions for simultaneous phosphating of multiplemetal substrates [46,47]. There has been a grow-133 SURFACE PRETREATMENT by PHOSPHATE CONVERSION coatings a reviewTable 2. Special additives used in phosphating -hydroxy carboxylic acidsTo reduce the coating Improve bath life through 49-54like tartaric and citric acids,weightlesser consumption oftripolyphosphate, sodium,chemicalspotassium tartrate,nitrobenzene such as EDTA, NTA, To increase the coatingImproved corrosion42, 43 DTPA gluconic acids andweightprotection, shorter55-58polycarboxy o-amino ammoniumGrain refinementBetter adhesion of59-63compounds, N and Psubsequent finishes;compound containingbetter corrosionamido or amino , Formic acid ester,chelate of an acidic (II)To improve surfaceBetter adhesion of64, 65texturesubsequent finishes.

9 Better compounds, tungstateTo accelerate theReduction in66-69ions, gaseous nitrogenphosphating process processing timeperoxide, hydroxylaminesulphate, andTo prevent concentrationBetter utilization of70permonosulphuric acidof ferro-nitroso complex nitrite and reductionin nitrite accelerated in the evolution ofzinc phosphating bath. toxic trimetaphosphateTo reduce the operating Thinner, smoother and 71temperatureimproved corrosionTo increase theresistant coatingtolerance to , Palmitic, andTo improve lubricantImproved workability 72 Stearic acids with fattypropertiesof the metalamines and , dialkyl-To decrease scalingImprove the service73-75triaminepentakislife of heating coilsmethylene phosphonicand provide uniformacid and its salts andheating of the bathfluoborate or fluosilicate10. Phosphonic acid ester,To prevent the build upImproved service life76, 77 Magnesium or zinc nitrateof sludge on tank walls of the , Tin (IV), ArsenicAs inhibitorsImproves corrosion78, 79compounds, zinc salt ofresistancean organic N-compound12.

10 Zinc carbonateTo reduce the acidity Consistent performance 80of the bath and toof the bathmaintain the Sankara Narayananing use of substitutes to conventional Cr(VI) post-rinses [48] to suit the regulations imposed by thepollution control authorities on the use of Cr(VI) com-pounds. The special additives used in phosphatingbaths is complied in Table 2 and the alternatives toCr(VI) post-rinse treatment is given in Table Chemistry of phosphatingAll conventional phosphating solutions are dilutephosphoric acid based solutions of one or more al-kali metal/heavy metal ions, which essentially con-tain free phosphoric acid and primary phosphatesof the metal ions contained in the bath[18,19,24,127,128]. When a steel panel is intro-duced into the phosphating solution a topochemicalreaction takes place in which the iron dissolution isinitiated at the microanodes present on the sub-strate by the free phosphoric acid present in thebath.