The paper describes results from recently published research focused on production of non-conventional magnesium phosphate Mg3(PO4)2・4H2O – bobierrite, or MgHPO4・3H2O – newberyite) coating for both magnesium alloys and/or mild steel. This new kind of coating is categorized in the context of current state of phosphating technology and its potential advantages and crystal structure is discussed. At the same time, the suitable comparison techniques for magnesium phosphate coating and conventional zinc phosphate coating are discussed

P. Pokorny

P. Szelag

P. Tej

English

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507METALURGIJA 55 (2016) 3, 507-510P. POKORNY, P. TEJ, P. SZELAGDISCUSSION ABOUT MAGNESIUM PHOSPHATINGReceived – Primljeno: 2015-05-16Accepted – Prihvaćeno: 2015-12-10Review Paper – Pregledni radP. Pokorny, P. Tej – Czech Technical University – Klokner Institute, Prague, Czech RepublicP. Szelag - PRAGOCHEMA Prague, Prague, Czech Republic The paper describes results from recently published research focused on production of non-conventional magne-sium phosphate (Mg3(PO4)2·4H2O – bobierrite, or MgHPO4·3H2O – newberyite) coating for both magnesium alloys and/or mild steel. This new kind of coating is categorized in the context of current state of phosphating technology and its potential advantages and crystal structure is discussed. At the same time, the suitable comparison tech-niques for magnesium phosphate coating and conventional zinc phosphate coating are discussed.Key words: mild steel, magnesium alloy, magnesium phosphating, bobierrite, newberyite INTRODUCTIONPhosphating is a popular surface treatment for both steel and other metals (Zn, Al, Mg) and their alloys. Various porous coatings of nonsoluble phosphates en-hance adherence of paints and prevent corrosion attack between the layers in the case of damage to the organic coating compromising its integrity [1, 2]. Conversion coating is formed on suitably prepared surface of metallic samples by joint influence of cations and anions – cations are formed by the corrosion reac-tion between the base metal and the phosphating bath; the anions come from the phosphating base itself. De-pending on the dominant cation, the dihydrogen phos-phate baths are often categorized in several groups: zinc, zinc - calcium, magnesium and multi - cation [2]. However, the standard EN ISO 9717 specifies the phos-phate coatings by their mass per area and the applica-tion purpose [3]. Most common multi - cation phos-phate coating is the “tri – cation” which found its place in the same industrial applications as the other above mentioned phosphate coatings. The “tri – cation” phosphate bath is very similar to common zinc phosphating baths, however the concen-tration of Zn2+ is reduced and the Ni2+, Mn2+, Co2+ (and sometimes also Mg2+) cations are added [1, 3, 4].Such baths produce coatings comprising by majority of Zn2Fe(PO4)2·4H2O (phosphophyllite) instead of Zn3(PO4)2·4H2O, i.e. hopeite. Phosphophyllite is, due to the higher iron content, more resistant in alkaline envi-ronment and is therefore preferred over hopeite as sur-face treatment before E - coating. During this process, alkaline environment is formed in the diffusion layer of coated component [1, 4].Addition of magnesium cations to the baths (as well as other cations: Ni2+, Mn2+, Co2+) in the “tri – cation” ISSN 0543-5846METABK 55(3) 507-510 (2016)UDC – UDK 669.14:669.721:661.632=111phosphating conclusively facilitates deposition of phos-phate coating and with their contribution, more compact coating with higher corrosion resistance is formed [2, 5]. Phosphating baths for production of magnesium-dominant phosphates coatings have not yet been con-ceived. The reason is probably the water solubility of magnesium ions [5]. If the composition of a new bath capable of reliably forming magnesium phosphate on the surface of differ-ent metals is successfully defined, it could replace some of current phosphating technologies [6]. Magnesium phosphating of magnesium alloysIn 2009, the first paper about possible application of magnesium phosphate coating on magnesium alloy AZ31 was published [7]. The reason for specifying completely new phosphating bath is the effort to create completely unique coating with extraordinary corrosion resistance on magnesium alloys [7]. The discussed phosphate coating was not yet pre-pared by conventional phosphating technologies. After immersion to the work bath (mixture of diluted ortho - phosphoric acid (1:10) and solution of α - Ca3PO4 (8,5 g.l-1 / 300 ml) prepared by 1:1 mixing) the MgHPO4 coating precursor was formed within one minute. This is described by the reaction (1):  (1)The sample of AZ31 is then exposed for 72 hours at 50 °C in glass container filled with work bath.This procedure forms highly rough coating, consist-ing of cubic, sheet up to paneled crystals of MgHPO4·3H2O. This compound is called newberyite and crystallizes in orthorhombic lattice. Prepared crystalline coating has thickness up to 50 μm and provides the underlying metal with increased corrosion resistance and also enhances its wettability. 508P. POKORNY et al.: DISCUSSION ABOUT MAGNESIUM PHOSPHATING METALURGIJA 55 (2016) 3, 507-510Good adherence of the coating was verified by scratch test, i.e. measurement of scratch critical load; the average value was about 15 mN [7].All the mentioned properties suggest that the new type of phosphate coating can be theoretically used even as an anti - corrosion surface treatment of bio im-plant bone replacements made from magnesium alloy.In this application, the porosity and brittleness must be taken into account. At the same time, their wettabil-ity, adherence, tissue biocompatibility and degree of surface coverage must also be verified in the case of application on rugged surface replacement (screws etc.) [6, 8]. Magnesium phosphating of mild steelPhosphate coatings are often used as surface treat-ment for mild steel. It has been mentioned before that these coatings are most commonly used to enhance ad-herence and overall corrosion resistance. Recently (2013), uniform coating of magnesium phosphate (bo-bierrite) was successfully prepared.bobierrite coatingsIn the 2004, M.F. Morks successfully deposited uni-form coating comprising of tertiary magnesium phos-phate (bobierrite – Mg3(PO4)2·4H2O) [9]. The coating on the mild steel was prepared by dipping into work bath (55 °C, 3-5 min). The bath was prepared by dilut-ing MgCO3 paste by diluted ortho - phosphoric and ni-tric acid. The author defines the parameters of the bath by points (also used in industrial practice): total acid - 17 points; free acid - 1 point. The bath was adjusted by accelerator (NaNO2 - 0,5 g·l-1) and salts: Ni2+ (2,5 g·l-1), Mn2+ (1,2 g·l-1) and Ca2+ (1,5 g·l-1). Effect of pH (ad-justed by NaOH) pH = 2,5; 2,78 and 3,0) on the quality of produced coating was also studied. The XRD data suggest that ideal conditions are pH = 3 and the immer-sion time is 5 min. Moreover, the author verified that the amount of produced dregs increases exponentially at pH > 3,0. Produced tri-phosphate coating is uniform; its mass per area is 3 g.m2. Author described the forma-tion reaction (2) as: (2)On the steel surface, the leaf - like shapes in petal arrangement are formed. The total length of these shapes is 10 - 15 μm.In practice, the performance of produced coating is compared to the hopeite coating (Zn3(PO4)2·4H2O) formed by the conventional zinc phosphating (5 min, pH = 3). The comparison is done directly by neutral salt spray test (NSS).Organic coating is deposited on the phosphate-coat-ed samples and both coatings are scribed to the base metal marking “X” on its surface. The corrosion attack was evaluated visually after 150 hours of exposition. Based on this test, the bobierrite coating was given sim-ilar corrosion resistance rating as the common zinc-phosphate coating (hopeite) [9].Nevertheless, from the industrial application point of view, this comparison cannot be considered a defini-tive one. The zinc phosphating technology is capable of producing various coatings in terms of thickness, poros-ity, grain size and even composition. It is therefore nec-essary to continue in the compare studies – verify the porosity and corrosion resistance by electrochemical measurements. The thermal stability of bobierrite also needs to be verified.The paper describes the structure of magnesium phosphate in detail; however detailed description of structure of zinc phosphate, to which it is compared, is missing.The crystallographic data suggest that the bobierrite crystallizes in orthorhombic lattice of the Cmc21 sym-metry group. Grains of this mineral can bind variable number of water molecules and thus form 3 other hy-drates, i.e. Mg3(PO4)2·5H2O or Mg3(PO4)2·8H2O or eventually Mg3(PO4)2·22H2O.It is therefore not possible to rule out that these hy-drates will not be present in the coating produced on steel [10]. Newberyite coatingsIn 2013, an article was published about preparation of uniform MgHPO4.2H2O layers (newberyite) on steel, similarly to layers on magnesium alloys [11, 12]. The procedure of depositing secondary magnesium phos-phate (also called magnesium phosphate dibasic) on steel surface was described by M. Fouladi and A. Ama-deh. The composition of working bath was specified by the authors as: 23 ml H3PO4 (85 wt. %); 8,5 g·l-1 MgCO3; 0,4 g·l-1 NaNO2; 6,8 g·l-1 NaOH.The procedure of coating is described by system of usual chemical formulae defining formation of typical soluble primary phosphate component (Mg(H2PO4)2) (3) and subsequently the final formula of coating for-mation (4): (3) (4)The authors reported acceleration of phosphate coat-ing reaction using sodium nitrite  according to conven-tional formulae (5) [12]:  (5)The magnesium hydrogen phosphate coating has again sheet-like shape of petal arrangement; see Figure 1.Authors are trying to optimize the conditions of quality coating of newberyite production, altering the 509P. POKORNY et al.: DISCUSSION ABOUT MAGNESIUM PHOSPHATINGMETALURGIJA 55 (2016) 3, 507-510bath temperature (tested: 20 °C, 40 °C, 60 °C, 80 °C, 90 °C; exposure time 20 min) and exposure time (tested: 1 min, 3 min,  5 min, 10 min, 20 min, 30 min; at 80 °C). Surface quality is evaluated by the degree of surface coverage (SEM), electrochemical test to qualitatively measure barrier properties of the coating and its poros-ity. Other parameters, such as: time dependence of cor-rosion potential (Ecorr/SCE), polarization resistance (Rp) and corrosion current density (icorr) were also measured. Porosity was calculated from measured data. The amount of produced dregs was evaluated by gravimetric method [12].The temperature of the work bath naturally affects the coating formation kinetics. Coating does not form at the temperatures of 25 °C and   40 °C. Coating at 60 °C pro-duces unsuitable coating; however, bath of 80 °C evi-dently yields surface coverage of 100 %. The thickness of the phosphate coating (~ 30 μm) does not change for the samples coated in the bath of 80 or 90 °C. Conversely, coating in 90 °C bath produces twice the amount of dregs (amorphous). Data from electrochemical measurements suggest as well that the optimal temperature is indeed 80 °C – the coating formed at higher temperature has in-creased porosity. One theory suggests that at higher tem-perature, fewer phosphate seeds are formed and more porous coating is thusly formed [12].If the new phosphating technology is to be imple-mented, optimal bathing time must be determined. Non - optimal phosphating cannot guarantee production of coating on 100 % surface, and also causes energy loss and coarsening of the coating. It is sometimes also sug-gested that growing phosphate grains can, by their ex-pansion, push out other, less - adherent grains from the surface. Authors reported that in 10 minutes time, only non - uniform coating is formed. Uniform layer is formed after 20 minutes. After 30 minutes, the unifor-mity of coating again decreases – size of some grains increases dramatically. It has been demonstrated, that the amount of dregs increases significantly after ten minutes of exposure. The authors also shown that new phosphate grains form on the surface of mild steel only during the first 5 minutes (nucleation stage). After this time, new grains stop forming and existing one grow in size. It was established by electrochemical tests that coating formed after 20 minutes is less porous than coating formed after 30 minute bathing [12]. Further, the overall corrosion resistance provided by the coating is compared to the common zinc phosphate coating (Zn3(PO4)2·4H2O) using the neutral salt spray test (NSS). Though this is not a common practice in industry as the phosphate coatings are not covered by organic coatings prior to the test. Authors noted that magnesium phosphate performs better in an environment of 3,5 % NaCl and 35 °C than the zinc phosphate coating [11]. However this can be only logical consequence of in-creased thickness of the coating (30 μm) contrary to the zinc phosphate (10 μm). From this viewpoint, the re-sults are not comparable. It is necessary to determine their mass per area to obtain information about the coat-ing compactness. Again, it is necessary to stress out that the technology of zinc phosphating can produce coat-ings of various thicknesses, mass per area weight and chemical composition. Comparative study used low - temperature phosphating bath (65 °C) without accelerator to produce the zinc phosphate coating. CONCLUSIONThe paper provides detailed sequence of magnesium phosphate (Mg3(PO4)2·4H2O or MgHPO4·3H2O) on mild steel and magnesium alloys. Due to complex ap-plication of phosphating in machinery industry as basic surface treatment before organic coating application, the research to enhance the quality of the crystalline surface itself must be further supported.In recent years, the research has been focused more on improving the conventional technologies by addition of small amount of inorganic or organic compounds to produce more compact coating or reduction of amount of produced dregs etc.Completely new kind of phosphate coating has not yet been discussed. Magnesium phosphating is poten-tially more suitable alternative to current coating tech-nologies. However, the new baths should be able to pro-duce coating of similar parameters (porosity, thickness, area weight, provided corrosion resistance in given en-vironment etc.), without requiring operation at higher temperature or yielding high amounts of dregs.Existing comparative tests cannot be considered completely relevant and finished. It will be necessary to compare corrosion resistance provided by the coating of similar area weight.It is also important to verify the thermal stability of deposited magnesium phosphates, since easy dehydra-tion of coating could render the coating unsuitable for some applications. The obvious application of magnesium coatings would be the corrosion - resistant layer for bio implants Figure 1  Morphology of MgHPO4·3H2O crystals deposited on steel (reprinted from [12])510P. POKORNY et al.: DISCUSSION ABOUT MAGNESIUM PHOSPHATING METALURGIJA 55 (2016) 3, 507-510from magnesium alloys, however it also requires nu-merous number of tests before implementation.AcknowledgmentsThis research has been supported by Czech Science Foundation, project GA ČR No 104 / 15-22670S (Ex-perimental and numerical analysis of bond behavior be-tween steel reinforcement and Ultra High Performance Concrete (UHPC)).REFERENCES[1] S. Narayanan, Surface pretreatment by phosphate conver-sion coatings – A review, Advanced Material Science 9 (2005), 130-177.[2] P. Pokorný, P. Szelag, M. Novák, L. Mastný, V. Brožek, Thermal stability of phosphate coatings on steel, Metalur-gija 54 (2015) 3, 489-492.[3] EN ISO 9717:2013, Metallic and other inorganic coatings – Phosphate conversion coatings of metals[4]  P. Pokorný, L. Mastný, V. Sýkora, Z. Pala, V. Brožek, Bond strength of plasma sprayed ceramic coatings on the phosphated steels, Metalurgija 54 (2015) 2, 411-414.[5]  U. B. Nair, Special types of phosphating – A Review, Cor-rosion reviews 13 (1995), 17-28.[6] B.Liu, X. Zhang, G. Xiao, Y. Lu, Phosphate chemical con-version coatings on metallic substrates for biomedical ap-plication: A review, Materials Science and Engineering C 47 (2015), 97-104.[7] T. Ishizaki, I. Shigematsu, N. Saito, Anticorrosive magne-sium phosphate coating on AZ31 magnesium alloy, Sur-face & Coatings Technology 203 (2009) 2288-2291. [8] F. Tamini, D. Nihouannen, D. C. Bassett, S. Ibasco, U. Gbureck, J. Knowles, A. Wright, A. Flynn, S. V. Koma-rova, J. E. Barralet, Biocompatibility of magnesium phos-phate minerals and their stability under physiological con-ditions, Acta Biomaterialia 7 (2011), 2678-2685.[9] M. F. Morks, Magnesium phosphate treatment for steel, Materials Letters 58 (2004), 3316-3319.[10]  K. O. Kongshaug, H. Fjellvåg, K. P. Lillerud, The synthe-sis and crystal structure of a hydrated magnesium phos-phate Mg3(PO4)2·4H2O, Solid State Science 3 (2001), 353 -360.[11]  M. Fouladi, A. Amadeh, Comparative study between novel magnesium phosphate and traditional zinc phosphate coat-ings, Materials Letters 98 (2013), 1-4.[12]  M. Fouladi, A. Amadeh, Effect of phosphating time and temperature on microstructure and corrosion behavior of magnesium phosphate coatings, Electrochimica Acta 106 (2013), 1-12.Note:  The responsible translator for the English  language is K. Štětková, CTU – Klokner Institute, Prague, Czech Republic 

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