II I I I I *II I li I II II MUIII I II

A- W

RE-IR-71-60

A STUDY OF MANGANESE PHOSPHATING REACTIONS

TECHNICAL REPORT

Joseph Monk.

Septe~mber 1971

RESEARCH DIRECTORATE

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS- COMMAND -

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J REPORT TITLE

A STUDY OF MANGANESE PHOSPHATING REACTIONS (U)

4 DESCRIPTIVE NOTES 1T7'p* of ,OpaI and Inclusive defo*)

I. AUTHOROISI (Pirit name, middle I.tlal, leaf na.me)

Joseph Menke

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September 1971 21 214i. CONTRACT Of GRANT NO. 90. ORlGINATOR'l* REPOIRT NuU.JINE1lh

b. PROJ, C T No. RE-TR-71-60

DA IT062105A328.. 12. OTHEIIR REPORT 1.40( (Any otIher nuibore thatl may be aalrtod

AMS Code 502E.11.294d.

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13. AOSTRACT

Work was conducted by the Research Directorate, Weapons Laboratory atRock Island, Research, Development and Engineering Directorate, U. S.Army Weapons Command to determine the composition of and the reactionsassociated with the formation of manganese-phosphate coatings. Animproved phosphate coating was studied as produced from a stockmanganese phosphating bath to which an addition of manganese citrate,tartrate, or gluconate was made. Processing was accomplished in anautoclave at temperatures above 212'F with steam pressure. Thecoatings produced by conventional and pressure methods were examined bydifferential thermal analysis, X-ray diffraction, and atomic absorptionanalysis. The chemical structure of the conventional coating wasFe3 (P04) .2MnHPO4.4Hz0, a form of hureaulite, and that of the pressurephosphate coating was mainly manganese phosphate heptahydrate,Mn3 (P04)?'7H2 0. (U) (Menke, Joseph)

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R__OL% C? ROEC W C ROLE

I. Improved Manganese Phosphate

2. Manganese Phosphating

3. Treatment tor Steel

4. Coatings

5. Conversion Coatings

6. Phosphate Coating Analysis

UnclassifiedMcurity Claesification

A'-

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9 1

S BLANK PAGE.

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M..---.

AD D

RESEARCH DIRECTORATE

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS COMMAND

TECHNICAL REPORT

RE-TR-71-60

A STUDY OF MANGANESE PHOSPHATING REACTIONS

Joseph Menke

September 1971

DA 1T062105A328 AMS Code 502E.11.294

Approved for public release, distribution unlimited.

i

ABSTRACT

Work Was condu(.Le6 Y n--k nirartnratp. Weai)ons

Laboratory at Rock Island, Research, Development and Engi-

ncering Directorate, U. S. Army Weapons Command to determine

the composition of and the reactions associated with the

formation of manganese-phosphate coatings. An improved phos-

phate coating was studied as produced from a stock manganese

phosphating bath to which an addition of manganese citrate,

tartrate, or gluconate was made. Processing was accomplished

in an autoclave at temperatures above 212OF with steam pres-

sure. The coatings produced by conventional and pressure

methods were examined by differential thermal analysis, X-ray

diffraction, and atomic absorption analysis. The chemical

structure of the conventional coating was Fe3(PO4)2-2MnHP04'

4H20, a form of hureaullte, and that of the pressure phos-

phate coating was mainly manganese phosphate heptahydrate,

Mn3(PO,)2-7H20,

CONTENTS

Page

Title Page

Abstract ii

Table of Contents ii

Objective

Background

Approach 2

Results 6

Discussion 9

Conclusions 11

Recommendations 12

Literature Cited 13

Distribution 14

DD Form 1473 (Document Control Data - R&D) 20

iii

OBJECTIVE

The objective of this project was to study the reactionswhich occur when metal-organic compounds are added to con-vent i urid i it.ci.es.- phzphz ti÷ nn lit nn; durinq applicationof pressure phosphate-coatings.

BACKGROUND

Manganese-phosphate coatings have been applied toferrous materials for many years. The coating consists ofan insoluble metal phosphate which is formed in a solutionof phosphoric acid and other chemicals. Corrosion resistanceobtained with a bare manganese-phosphate coating is usuallylimited, (about two hours in 5 per cent salt spray). There-fore, the coating usually serves as a base for various sup-plementary finishes. When supplementary coatings such asoils or paints are applied over phosphate coatings, thecorrosion resistance is increased; however, improvement ofthe inherent value uf the phosphate coating itself is highlydesirable. Recent research is involved with the developmentof a manganese-phosphate coating that provides a significantincrease in salt-spray corrosion resistance (i.e., in excessof 500 hours) when compared with conventional manganese-.phosphate coatings. This improved manganese-phosphate coatingalso has excellent corrosion resistance after being heatedfor an hour at temperatures of 4501F. With conventionalmanganese-phosphate coatings, heated to 450'F, salt-spraycorrosion resistance is severely reduced.

The reactions involved in the formation of a conventionalmanganese-phosphate coating are complex. The process maybest be described as a combination of the following reactions:

Mn(H2PO4) 2 - MnHPOL+H3PO (1)

3MnHP04 - Mn3 (PO4) 2 + H-PO 4 (2)

Fe(H 2 PO4)2- >FeHPO÷ H3 P01. (3)

3FeHP0O - I Fe 3 (PO4)j +H 3 PO4 (4)

Manganese dihydrogen phosphate, Mn(H 2 PO.)2. is soluble inthe water-phosphoric acid solution, When a steel specimenis immersed into the acidic solution, iron is dissolved and

hydrogen gas is evolved. As the pH at the solution-specimeninterface increases because of hydrogen ion depletion, thegassing ceases and insoluble manganese hydrogen phosphate,MnHPO 4 , is formed, (1). Dissolved iron from the specimenand iron in solution can also form an insoluble iron hydrogenphosphate, FeHPO4, which may become a part ot the coating, (3>.A further increase in the interface pH results in the for-nmation of manganese phosphate, Mn, (PO.)z or irnn phosphate,

Fe 3 (PO,)2. (See equations (2) and (4).) Thus, dependent uponthe solution equilibrium, manganese hydrogen phosphaLe, ironhydrogen phosphate, manganese phosphate, and iron phosphatecan all be present in the final coating. The possible pres-ence of various compounds In the manganese-phosphate coatingis also Indicated by X-ray analysis. According to the analysis,the coating has the following structure: (Fe,Mn)sH2 (PO)+.4H2 0.This is the structure of hureaulite, a naturally occurringphosphate mineral. The structure of hureaulite can be repre-sented in the following arrangements: Fe 3 (PO4)2a?2InHP0O,4H 2 0or Mna(PO 4 ) 2 -2FeHPO4'4H 20O.

The reactions involved in the formation of the improvedmanganese-phosphate coating are much more complex. The coat-ing is formed in an autoclave at temperatures above 212°Fwith the use of a conventional manganese-phosphating solutionto which is added a metal-organic compound. This project wasundertaken to determine the mechanism or reactions involvedin the formation of the improved phosphate coating and todetermine the causes involved in the improvement of corrosionprotection.

APPROACH

Work concurrently being conducted and unreported at thetime of this writing has led to process variations of steampressure and metal-organic chemical additions that producemanganese-phosphate coatings of exceptional corrosion resis-tance. Coatings were produced under some of these processconditions for sLudy and analysis in this work.

Specimens of SAE 1020 steel, 2"x3"xl/16", l"xl"xl/16"and 2"x6"xl/8" were vapor-degreased and abrasive-blastedwith steel grit. One group of the specimens was processedin a conventional manganese-phosphating bath to which 10 gramsper liter of manganese citrate had been added. Processingwas accomplished in an autoclave at 22 psig (260'F) of steampressure for 15 minutes. A second group of the specimenswas processed in a conventional manganese-phosphating bath

2

to whinh 10 grams per liter of manganese tartrate had beenadded. Processing wds accomolished in an autoclave at <-psig (214'F) of steam pressure for 30 minutes. A thirdgroup of the :p) i.eL i wa .Pt Ut::d Iae racvanti.namanganese-phosphating bath to which 10 grams per liter ofmanganese qluconate had been added. Processing was accom-plished in an autoclave at ,l psig (213'F) of steaml pressure

for 30 mInutes.

To determine how the higher temperature attained at22 psiy would affect the phosphating solution when steelspecimens we"e not being processed, solutions were heatedin an autoclave at 22 psig (260'F) of steam pressure for15 minutes. One solution contained 10 grams of manganesecitrate per liter added to a conventional manganese--phos-phating bath; the other solution was a conventional mangan-ese-phosphating bath with no additives.

Specimens were also coated with conventional manganese-phosphate so that atomic absorption, differential thermalanalysis, and X-ray diffraction data could be correlatedwith the pressure coating data. Sludge, which formed duringsome of the processing or heating cycles, was filtered out,drieJ at 212'F, and subjected to various analytical pro-cedures. Analyses uf solutions are given in Table I, andanalyses of coatings and sludge are in Table II. Some ofthe pressure manganese-phosphated specimens (2"x3"xl/16")were heated in a muffle furnace at 400'F, 800'F, 1000'F,and 1200'F for- one hour and were then subjected to the 5 percent salt-spray test. The remaining pressure phosphate-coated specimens, not heated, were tested as-processed.Specimens (l"xl"xl/16") coated with conventional and pressuremanganese-phosphate were subjected to X-ray diffractionanalysis. A known, reagent grade manganese-phosphate powderwas also subjected to X-ray di-,'raction analysis is a standard.Conventional and pressure manganese-phosphated specimens'2"x6"xl/8") were scraped to obtain samples of the coatingfor differential thrrmal analysis and for atomic absorptionanalysis.

An intensive literature search was conducted for chem-ical reactions that could be associated with the formationof phosphate coatings under the influence of steam pressureaod metal-organic additives,

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RESULTS

Pressure manganese-phosphate coatitgs heated in a mufflefurnace at 400°F for one hour did not change in physicaldppedrdnce. HudLir y dL 8OO F iur uric huur causeu th coatingto turn green after going through a transitional brown colorat atmospheres between 500°F and 700 0 F. The heating ofpressure manganese-phosphate coating at 1000F resulted ina green finish. After the coating was heated at this tem-perature, tiny areas of it spalled when the specimens wereremoved from the furnace; this was probably caused by thermalshock during cooling. The relative amount of spalling thatwas observed could be summarized as follows:

citrate >> tartrate > gluconate

Immediatel.y after the specimens were removed from the furnaceafter heating to 1200'F for one hour, all the pressure coatingswere intact. Within five minutes, more than 90 per cent ofall the coatings had spalled. The corrosion resistance ofheated and unheated coatings is summarized in Table Ill.

Examination of the coatings spalled at 1200OF showedthat t:ýe pressure manganese-phosphate finish consisted oftwo layers. The layer closest to the basis metal was gray,and was relatively smooth and crystalline, much like a con-ventional manganese-phosphate coating. The outer coatingwas green with a coarse crystalline structure. The spalledcoating segments were returned to the furnace at 1200'F forone hour to determine whether all the coating would becomegreen if the initial conversion to the green color was justa surface phenomenon. Examination of the coating after thissecond heating showed that the two distinct layers were stillpresent. The pressure manganese-citrace bath coating alsohad small areas that were yellow. These yellow areas, likethe green coating, were not present in +he basic gray coating.

Differential thermal analysis of coating material scrapedfrom the specimen surface showed that conventional manganese-phosphate coatings undergo some type of exothermic deterior-ation before an endothermic reaction. Interpretation of thedifferential thermal analysis data is difficult because thereactions associated with the various differential thermalpeaks are unavailable at the present time,

A previous examination of X-ray diffraction data byCrain' indicated that the conventional and pressure mangan-ese-phosphate coatings were essentially alike, However,two diffraction peaks in the pressure manganese-citrate X-ray

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Idiffraction pattern are more intense when compared with thediffraction pattern for conventional manganese-phosphate.These two peaks correspond to the two major peaks presentin the X-ray diffraction pattern of manganese pyrophosphate.!T date, the pr~otnre of ovroohosphate in the pressurephosphate-coating has not been confirmed.

X-ray diffraction analysis conducted during this inves-1 tigation showed that the diffraction pattern of conventionalS~manganese-phosphate is similar to the diffraction pattern

of hureaulite. The X-ray diffraction pattern of pressuremanganese-phosphate coatings more closely resemble the dif-fraction pattern of manganese-phosphate heptahydrate,Mn3 (PO4 )2a7H20. Some peaks in the diffraction pattern ofthe pressure phosphate-coatings are more intense than thosein the diffraction pattern of Mn3 (PO4) 2 "7H 2 0 These peaks,two major and four minor, correspond to the six most intensepeaks of manganese pyrophosphate. The intensity increasedue to the possible presence of manganese pyrophosphate isthe most pronounced in the pressure manganese-citrate coating.

Atomic absorption analysis included a determination ofthe manganese and iron content of various phosphate coatings,processing sludges, and otiver iron-manganese compounds. Theresults of the atomic absorption analyses are shown in TableII. The iron content of the conventional manganese-phosphatecoating was 26.6 per cent and the manganese content was 12.5per cent. These percentages correspond with the respectiveiron and manganese content of Fe 3 (POJ)2.2MnHPOk.4H 2 0, Sincethe pH at which iron phosphate begins to form is lower thanthe pH at which manganese phosphate begins to form, 2 theformation of iron phosphate would predominate and be presentin greater concentration. Composition of the conventionalmanganese-phosphate coat4ng is thus influenced largely bythe initial pH at which the coating begins to form.

Atomic absorption analysis of pressure manganese-phos-ptiate coatings shows that the major constituent is manganesewith a small amount of iron being present. Since manganeseion is present to a much greater extent than iron in theprocessing solution, the composition of a pressure manganese-phosphate coating is influenced by the precipitation ofmanganese-phosphate because of inverse solubility. Mostcompounds become more soluble when the solution temperatureincreases. Compounds which become less soluble with in-creasing temperature exhibit inverse solubility. Manganese,zinc, and iron phosphates are all inversely soluble, andprecipitation increases as the temperature rises

8

DISCUSSION

The reactions associated with the formation of a pres-sure , -p tc c'~tin" must t4 intn considerationtwo major factors: (1) the addition of manganese citrate,tartrate, or gluconate and (2) the inclusion of processingparameters above 212'F by steam pressure. The reactionswhich may occur when manganese citrate, tartrate, or glu-conate are added to a phosphating solution can be summarizedby consideration of the fact that organic radicals can:

1. Serve as metal ion sequestrants.

2. Buffer the solution, i.e., change free acid and pH.

3. Affect the specimen interface pH, i.e., lower thepH at which the coating forms.

4. Affect the solubility of the metal hydrogen-phos-phate formed.

5. Serve as a cathodic depolarizer.

6. Increase the available manganese-ion content ofthe phosphating solution.

When the phosphating solution is heated above 212°Funder steam pressure during processing, the reactions may:

1. Promote inverse solubility formation of metalphosphates.

2. Affect the possible formation of pyrophosphate andwater from orthophosphate.

3. Increase the solubility of manganese citrate,tartrate, and gluconate.

4. Increase the free acid as a result of the formationof the phosphate coating and precipitation of iron-manganesephosphate sludge.

Of the above-mentioned reactions, the following equationsare considered of greatest importance in the formation of

9

Ithe pressure phosphate coating:

H2 0,' ,UH 2

Gluconate + Fe+' H2 0 ) HO HQ" HO OHH + H6 O

C -C - -

I I0 H H H H

The organic radicals, citrate, tartrate, and gluconate, canserve as iron sequestrants and buffers. Their presence ina phosphating bath would reduce the iron present in the in-itial pressure phosphate layer and would increase the speci-men-solution interface pH; thus manganese-phosphate coatingis formed more quickly.

•0 /0

R - C + H2 - R - C + OH- (6a)

0 H

R C + 2H 2 - >R - C - OH + OH (6b)

""0 ~H

The organic radical could be used as a cathodic polarizer,thus the formation of the phosphate coating would be speededup by reaction with gaseous hydrogen liberated during pickling.Reaction with hydrogen may render the organic radical opticallyinactive and account for its apparent concentration decreasein the phosphating solution. Since no optically active or-ganic constituent is present in the sludge and no apparentreason is advanced for any organic radicals in the phosphatecoating, the only place in which the organic constituent canremain is in solution. Analysis of the processing solution

10

with the use of optical rotation techniques, reveals thatthe organic constituent is decreased from 1.0 to approxi-mately 0.5 per cent.

3 Mn+ + 2 P043 -A > Mn3 (PO4) 2 (7)

The manganese-ion content is increased by the addition ofmanganese citrate, tartrate, or gluconate to the phosphatingsolution. This Increase and the inverse solubility charac-teristic of manganese phosphate (as the temperature rises)drives the reaction (7) to the right; this action indicatesthat the greater portion of the pressure coating should bemanganese phosphate. This condition results in a coating

that contains very little iron phosphate. Atomic, absorptionanalysis and X-ray diffraction data support the conclusionthat the major constituent in the pressure coating is man-ganese phosphate.

2HPOý 2 >212 F > P2 O;, + H2 0 (8)

Processing in a phosphating bath above 212OF with steampressure could result in the formation of manganese pyrophos-phate in addition to the manganese phosphate formed by in-verse solubility (8). X-ray diffraction data indicate thatthe presence of manganese pyrophosphate is possible, Phos-phating solution at higher temperatures provide greaterpossibility of the formation of pyrophosphate.

CONCLUSIONS

1. The formation of a conventional manganese-phosphatecoating is dependent on pH changes which occur at the specimen-solution interface.

2. The composition of a conventional manganese-phosphatecoating is Fe3(PO4) 2 .2MnHPO 4 .4H 2 0.

3. The formation of the pressure phosphate-coating islargely dependent on the inverse solubility precipitationof manganese phosphate.

4. The pressure manganese-phosphate coating consistsof two distinct layers, a basis manganese-phosphate coatingcovered by a coating of manganese phosphate containing alimited amount of manganese pyrophosphate.

11

II5. The chemical reactions in phosphating solutions

modified as indicated in this work are capable of producingcoatings with exceptional resistance to heat and corrosion.

kLCUMMENDATiONS I

Further work should be conducted to determine:

1. the inverse solubility of manganese phosphate atvarious temperatures;

2. the pH at which the phosphate coating begins toform in conventional manganese-phosphating solutions andin solutions to which manganese citrate, tartrate, andgluconate have been added;

3. the effects of increasing te perature on the pHat which conventional and pressure phosphate-coatings form;

4. the effects of iron on the corrosion resistance ofconventional phosphate coatings.

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LITERATURE CITED

I. Crain, H., "Analysis of Manganese-Phosphate Coatings,"

Rock Island Arsenal Laboratory Report 68-650, 1'768.

2. Gilbert, L. 0., "A Study of Phosphate Treatment of

Metals," Rock Island Arsenal Laboratory Report

56-2995, 1956.

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