AD

K/ . / R-TR-75-034

IMPROVED MANGANESE PHOSPHATE COATINGS

HENRY CRAIN -

APRIL 1975

RESEARCH DIRECTORATE

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GENERAL THOMAS J. RODMAN LABORATORYROCK ISLAND ARSENAL

ROCK ISLAND, ILLINOIS 61201

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IMPROVED MANGANESE PHSHT OTNS'JTechnica

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2. Conversion coatings3. Phosphating bath

20 AGrjC onln odd*. ta It .. c..soMV midP 1J.,alft. by block noc.mb)Work was conducted to determine the mechanism by which superior manganese

phosphate coatings are producel. The phosphate coatings were applied at temp-eratures above 2124F and with -.anganese-organic compounds added to the phos-phating solution. Experimental baths and solubility studies have shown thatthe manganese concentration in the bath is critical and that the main functionof the manganese-organic compounds is to increase the manganese concentration.The necessity of a processing temperature higher than 212 F has beenattributed

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\20. Cantto an exponential increase in the reaction rate constant with temperaturefor the conversion of mlnganese dihydrogen phosphate [(P(HjPO')] tomanganese phosphate [Nns(PO4)J].

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1ABLE OF CONTENTS

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DD Form 1473 (Document Control Data - R&D) .. . . . ..... . I

TABLE OF CONTENTS ................ ........... II

TABULAR DATA .................. .......... ...... v

INTRODUCTION ................ ... ......... .... .. 1

PROCEDURE . ,........ ..........

RESULTS ..................................... 3

DISCUSSION .............................. ... . 6

CONCLUSIONS .. . . . . ......................... 8

RECOMMI4ENDATIONS ................................8

DISTRIBUTION............................... 9

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TABULAR DATA

TableI Analyses of Solution and Coating for Phosphating Baths

4of Di-ferlng Compositions

11 Atomic Absorption Analysis Results 5

III Percentage by Weight of Manganese and Iron Present with 7

Increasing Temperature in a Phosphate Bath Enriched withManganese Tartrate

iv

INTRODUCTION

Manganese phosphate coatings have been applied to ferrous articles formore than sixty years. The. coatings are simple to apply and offer an ex-cellent base for oil or p.int treatment. These coatings by themselves arerequired by specification MIL-P-16232 to withstand a 1-1/2 hour, S per centsalt spray test.

The conventional manganese phosphatubath contains manganese dihydrogenphosphate [1Mn(HIPO) 1], ferrous iron (Fe ), and phosphoric acid (H3PO4)The processing temperature range is usually Z0S*F - 210*F. The base metalis pickled Initially by the acid; h~ydrogen is liberated and consequentlythe pH of the solution is lowered at the Imtil-solution interface. Thefollowing hydrolytic reactions then occur to form the manganese and ironphosphate coating:

k *a. Mn(H 2PO4) 2 Nn-P0 4 + H3PO0

k2

k)b. 3MnHPO4 - Mn3 (P04) 2 + H3i'O4

k4

k5c. Fe(H 2PO4) 2 -01 FeHPO4 + H3P04

k6

k7d. 3FeHP04 - Fe 3 (PO4) 2 + H3 P04

ke

*k represents the reaction rate constant

Recently, a new and vastly improved manganese phosphate coating wasproduced in this laboratory. This new coating is formed by the additionof a metal-organic compound such as manganese tartrate, fianganese gluconate,or maganese citrate to the conventional manganese phosphating bath andprocessing at temperatures in excess of 212 0 F. This coating is capable of 4withstanding more than a thousand hours in a 5 per cent salt spray test andexhibits the same remarkable corrosion resistance even after being sub-jected to a 450*F heat treatment for one hour.

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Several problems, however, have arisen in attempts to control the con-tinuous reproduction of the superior coatings. To solve these problems re-quired initiation of this study to obtain a better understanding of theunderlying phenomena and reaction mechanisms of the modified manganese phos-phating process.

PROCEDURE

The several phosphating baths that were used were of the followingcompositions:

a. Manganese citrate bath - 10 grams/liter of manganese citrate wasadded to the conventional bath.

b. Manganese tartrate bath - 10 grams/liter of manganese tartrate wasadded to the conventional bath.

c. Iron-free bath - 20 grams/liter of manganese dihydrogen phosphateLMn(H 2PO)2] was added to deionized water. The free acid valuewas adjusted to 2.0 with phosphoric acid (H3PO4) and manganesecarbonate (MnC0 3 ).

d. Manganese dihydrogen phosphate bath - 10 grams/liter of Mn(H 2 PO,)2was added to the conventional bath.

All the above baths were operated at 2131F - 215'F. This temperature rangewas achieved by use of steam pressures less than one p.s.i.g. in an auto-clave type processing vessel.

The specimens were mild steel panels measuring 2 by 3 by 1/16 inch.The specimens were degredsed in trichloroethylene, abrasive-blasted witnsteel grit, and processed in the selected phosphcting bath. Total immer-sion time for the processing of steel panels was 35 minutes.

The phosphate coating was stripped by a five-minute immersion in asolution of chromic acid heated to 200*F. The difference between theweight of the panel after processing and the weight after stripping givesthe coating weight. The difference between the weight of the panel before jprocessing and the weight after stripping yields the amount of iron etched. J

Conicentrations of the manganese and iron present in the bath, coating,and sludge were obtained by atomic absorption analysis. Titrations withsodium hydroxide were used to determine the free and total acid values.

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Solubility studies were conducted in an attempt to measure the so-called"inverse solubility" of the phosphates. The filtered precipitate was driedand weighed from a known volume of phosphating solution that had been keptin an isothermal bath for a 30-minute period. The filtrate was then placedIn an Isothermal bath of higher temperature and the process outlined abovewas repeated. Additional solubility data were obtained with samples takenat regular temperature intervals frov, the processing autoclave, as the temp- ferature was increased,

A study of pHl at the metal-solution interface versus time of process-ing was attempted at various temperatures. In these experiments, a ballof degreased steel wool was wrapped around a hydrogen electrode, and the -resulting pH was measured continuously on a recorder.

RESULTS IVarious parameters of interest for the five types of phosphate baths i

used are summarized in Table 1. The free and total acid values are approxi- 3mately the same for the conventional manganese gluconate bath and for themanganese citrate bath. The Mn(H 2 PO4) and iron-free baths have a highertotal acid since an excess of Mn(H 2PO0) 2 was added to the solution. Thecoating weights of the panels processed in baths containing some form ofmanganese enrichment are much greater than those panels processed in the 1conventional bath. The salt spray tests results show a five-hundred foldincrease in corrosion resistance for these panels processed in a manganese- Ienriched bath as opposed to the conventional bath. The iron etched bypickling is also given in Table 1.

Atomic absorption analyses of coatings, baths, and sludge to determine Ipercentage manganese and iron are shown in Table !1. Analyses of manganesecitrate, tartrate, and gluconate sludges and coatings have been previouslyreported by Menke. 1 The manganese percentages for the coatings are veryclose to the calculated value of 46 percent for manganese in Mn3 (POQ)2.Percentages for the solutions are in weinht percent and show a decrease inthe manganese after each use of the processing solution. The insiqnificantamount of iron found in the coating of panels processed in the iron-freebath arises from the use of technical grade Mn(H 2 POQ)2 and from the ironetched from the panel- themselves.

1 Menke, J., "A Study of Manganese Phosphating Reactions", Rodman Laboratory,Rock Island Arsenal Technical Report RE-TR-71-60, 1971.

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TABLE II

Atomic Absorption Analysis Results

Description of Sample Percent Manganese b l Weight

A. Sludges:

Conventional 12,5%Manganese citrate 28.2%Manganese dihydrogen phosphate 18,6%

B. Coatings from:

Iron free bath (1st run) 39%Iron free bath (2nd run) 37%Iron free bath (3rd run 46%

C. Solutions: Before and after processing.Iron free before 0.65%Iron free after St run 0.60%,-S runIron free before 0.687' ,Iron free after> 2f d run 0 .54 >2nd run

Iron free before>rd 3rd run 0:.Iron free after " run 0.50 3 r

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Jata for the solulIIlty studies was (Cfficult to obtain. In the studyconducted by heat.ing pNoslohate solutions in isothermal baths, a larger amountof precipitate wts fornied as the temperaturp of the bath was increased. H1ow-ever, the solubilitv was not only teriperature-dependent but als;o time-depen-dent. Hence, even after the precipitate had been filtered from the solution,more formed because of the time dependency of precipitate formation. Atomicabsorption analysis results for a solubility study that had been performedby the withdrawing of samples from the processing vessel at regular temp-erature intervals show an increase in precipitation with temperature (TableI11). The largest increase in the amount of precipitate formed occurs betweenZ00F - 2200 F.

A study of pDi versus time of processing for various temperatures andadditives was cond,;ctPd to determine whether any relationship existed amongthese variables. hecause of difficulties with the pH electrodes at theincreased pressures, the results for temperatures above 210OF were impossibleto obtain. For temperatures below 2100F, the plots varied slightly; but nod.finite relationships or trends could be established.

DISCUSSION

In the first experiments performed to produce superior corrosion resis-tant manganese phosphate coatings, manganese organic compounds were used.At that time, the oeneficial effects of the manganese organic compounds wereattributed to tne or',anic part of the compound. The citrate, tartrate, andqlucontc ion.c 3- -:-. lnoThn for their zhelating action- thus, investiqaterspostulated that perlips the mnanganese ion wds neld i,, solution by the chelateor combined with the iron to prevent its incorporation into the coatinn.Possible catalytic activity or buffering effects were also offered i explana-tions of the increased corrosiut, resistance. These experiments, nowever,indicate that the common factor for iqcreased corrnsicn protection is manganeseenrichment of the bath. All tne ilaths that have yielded superior coatinas havecontained added amounts of manoanese. Of more importance is the success of the

conventional bath with 10 c1ra,,s/l'tor of excess Mn(H 2 PO) 2 ; thus, the completeabsence of any chelatinq ion indicates that the concentration of manganese isextremely critical in the production of corrosion resistant coatings. Forelimination of the prese,_e of iron as a factor in phosphatina, a bath contain-ing only Mn(H 2PO,) 2 , , and deionized water w..as used. This bath producedfour consecutive set.s of phosphated panels that sustained salt spray tests formore than 1000 hours. Tie only intermittent addition made to the Lath wasmanganese carbonate used to lower the free acid. The increase in manqanese con-centration permits more Mn3 (PO,,) 2 to be formed and, consequently, leads to athicker, more protective coatin,;. Because this coating is thicker, the amountof free pore area in the coatinn probably decreases so that less of the steelis exposed to corrosion.

6*

TABLE I II

Percentage by Weight of Manganese and Iron Present with

Enriched with Manganese Tartrate

Temperature Percent Manganese Percent Iron

W0OOF 0.24 0.0034 4

200OF 0.23 0.0030

220OF 0.17 0.0027

240OF 0.14 0.0034

2509F 0.13 0.0029

260F 0.12 0.0038

80*F* 0.18 0.0034 1

*stock bath unenriched

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The other important factor necessary to produce superior phosphatecoatings is that of a processing temperdture above 2120F. Some Investi-gators have used the term "inverse solubility" to explain the formationof more precipitate as the temperature is increased. However, the timedependency for precipitation, as shown in the solubility studies, indicatesthat the mechanism is a kinetic one rather than a solubility mechanism.The equation for a rate constant is given as:

k - AeE/RT

where A - ConstantEa= Energy of activationT a Absolute temperatureR - Constant

As temperature increases, the rate constant will increase exponentially.Thus, for this particular phosphating system, the increase in temperatureabove Z12 0F becomes very important for the conversion of the solubleMn(H 2 PO4) 2 to the insoluble Mn3 (P04 ) 2 . The increase in Mn3 (P04) 2 formationproduces thicker coatings and, consequently, less free pore area forcorrosion sites.

CONCLUSIONS

a. The manganese concentration in the phosphating solution is criticalfor producing coatings with superior corrosion resistance.

b. The main function of the manganese organic compounds is to providethe bath with an increased manganese concentration.

c. The exponential increase with temperature of the kinetic rate con-stant for the conversion of Mn(H 2 PO4) 2 to Mn3 (PO4) 2 is an important factorfor processing at temperatures higher than 212'F.

RECOMMENDAT IONS

Further work should be conducted to determine:

a. the critical manganese concentration needed to produce a superiorcoating,

b. the differences in free pore areas that exist between conventionaland Mn-enriched phosphate coatings, and

c. a method to convert the Mn3 (PO4) 2 sludge to the useful form ofMn(H 2PQ4) 2.

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