DISTRIBUTED BY: Information Service U. S. · PDF fileTHE APPLICATION OF HEAT AND CORROSION ... solution and raised the pH. ... ino," Rock Island Arsenal Laboratory Report 47-250,

AD/A-O00 633

THE APPLICATION OF HEAT AND CORROSIONRESISTANT PHOSPHATE COATINGS UNDERSTEAM PRESSURE

Linden H. Wagner

Rock Island Arsenal,Rock Island, Illinois

March 1974

DISTRIBUTED BY:

National Technical Information ServiceU. S. DEPARTMENT OF COMMERCE

DISPOSITION INSTRUCTIONS:

Destroy'this report when it is no longer needed. Do notreturn itfto the originator.

DISCLAIMER:

The findings of this report are not to be construed asan official Department of the Army position unless sodesignated by other authorized documents.

. ....'-, AVII ',IJ ........

, sWtei C'Zt!

UnclassifiedJ

SLCURITY CLASSIFICATION OF THIS PAGE ("oen Data Entored_

REPORT DOCUMENTATION PAGE BEFORE COMPLETI FORML REPORT NUMBER 2. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

R-TR-74-01 2 ,_4ar ...I. TiTL" (and Subtleia) 5. APE OF REPORT I PERIOD COVERED

T;E APPLICATION OF HEAT AND CORROSION; SummaryRESISTANT PHOSPHATE COATINGS UNDERSTEAM PRESSURE 6, PERFOPMIMG ORG. REPORT NIUMBtR

7. AUTNOR(ax, S. CONTRACT OR GRANT NUMBER(&)

Linden H. Wagner

9. PERFORMIIIG ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT. TASK

CDR, Rock Island Arsenal AREA & WORK UNIT NUMBERS

General Thomas J. Rodman Laboratory PRON Al-2-23035-03-Mi-M2Rock Island, Illinois 61201 AMS Code 3297.16.6785It. CONTROLLING OFFICE I; ADDRESS 12, REPORT DATE

CDR, Rock Islan( .nal March 1974GEN Thomas J. Ro Laboratory, SARRI-LR 1,. NUMBERn PAGES

Rock Island, Ill, .s '0114. MONITORING AGENCY NAME ' 17-0. 4fetant from Controllng Office) 15. SECURITY CLASS. (of t s report)

Unclassified1Sa. CECLASSIFICATION/DOWNGRADING

SCHEDULE

16. DISTRIBUTION STATEMENT (of this -!aport)

Approved for public release, distribution unlimited.

17. DISTRI13UTION STATEMENT (of the abstract entered In Block 20, it different from Report)

\\0j

18. SUPPLEMENTARY NorEs

19. KEY 'WORDS (Continue on reverae aide if neceasary and Identify by block number)

1. Phosphate coatings 4. Corrosion resistance

2. Manganese phosphate 5. Pressure Vessel

3. Heat resistance

20. ABSTRACT (Continue on reveae aide it necesary and Identify by bloch number)

Processing methods to produce zinc and manganese phosphate coat-ings with greater heat and corrosion resistance on ferrous itemswere investigated. Conventional phosphaiing solutions enrichedwith salts of the alpha-reactive-carboxylic-acids, such asmanganese salts of citric, tartaric, and gluconic acids were pro-cessed under steam and atmospheric pressure. Manganese carbonateand manganese dihydrogen phosphate were added to control the free

DD I JAN 73 u473 EDITION Or INOV65 IS OBSOLETEi , , , , , i , l, I., U n c l a s s i f i e d

NATIONAL TECI INICAI SECURITY CLASSIFICATION OF THIS PAGE (When Data Entered)

INFOPMATION 1,FRVICFI) S, r)rrp itl,..nt ' n n,

UnclassifiedSECURITY CLASSIFICATION OF THIS PAGE(flh Data Entered)

20. Continued

and total acids. Evaluations were made of the coating weight,amount of iron etched from the surface, loss of coating weightdue to thermal exposure, and resistance of the coatings to heatand corrosion in the salt-spray tests. The results showed thatzinc coatings were not greatly improved by increases in steampressure; whereas, manganese coatings that had been producedunder steam pressure exhibited improved heat and corrosion re-sistance. Manganese tartrate enrichments produced coatings withthermal resistance to 4501F and corrosion resistance for 500hours in the salt-spray tests. Coatings processed with manganesegluconate additions provided thermal resistance to 350*F andsalt-spray resistance of 380 hours; coatings processed withmanganese citrate enrichment exhibited only a slight improvementcrier conventionally processed coatings.

Fax film replicas were made to examine the crystallinity,porosity and continuity of the phosphate coatings. Conductometrictitration curves indicated that the additives buffered thesolution and raised the pH.

fit UnclassifiedSECURITY CLASSIFICATION OF THIS PAGE(ften Data Entered)

......................................

FOREWORD

The project was carried out under the title "Applicationof Heat and Corrosion Resi.stant Phosphate Coatings." Thiswork was authorized as part of the Manufacturing Methods andTechnology Program of the U. S. Army Materiel Command and wasadministered by the U. S. Army Production Eqripment Agency.

iii The following page is blank

CONTENTS

Page

DD FORM 1473 Report Documentation Page i

FOREWORD iii

TABLE OF CONTENTS v

TABULAR DATA vii

LIST OF ILLUSTRATIONS ix

OBJECTIVE 1

BACKGROUND 3

EXPERIMENTAL PROCEDURE 7

RESULTS AND DISCUSSION 11

CONCLUSIONS 39

RECOMMENDATIONS 41

LITERATURE CITED 43

DISTRIBUTION 45

v The following page is blank

TABULAR DATA

Table Title Page

1 Zinc Phosphate Coatings Applied in a Con- 12ventional Bath with and without theaddition of Zinc Citrate

2 Manganese Phosphate Coatings Applied in a 15Conventional Bath with and without Mangan-ese Citrate

3 Me ganese Phosphate Coatings Applied in a 18Bath with and without Enrichment underSteam Pressure

4 Manganese Phosphate Coatings Applied in a 27Cunventional Bath with and without En-richment at Atmospheric Pressure

5 Manganese Phosphate Coatings Applied under 29Steam Pressure in a Bath Previously usedat Atmospheric Pressure

vii The following page is blank

LIST OF ILLUSTRATIONS

Figure Title Pa2e

1 Manganese Phosphate Coated Panels after Salt- 19Spray Exposure Processed under Steam Pressure

2 Manganese Phosphate Coated Panels after Salt- 19Spray Exposure Processed in a Bath Enrichedwith Manganese Citrate

3 Manganese Phosphate Coated Panels after Salt- 20Spray Exposure Processed in a Bath Enrichedwith Manganese Tartrate under Steam Pressure

4 Manganese Phosphate Coated Panels after Salt- 20Spray Exposure Processed in a Bath Enrichedwith Manganese Gluconate under Steam Pressure

Fax Film Replica of Manganese Phosphate Coat- 21ing Processed in a Bath at AtmosphericPressure

6 Fax Film Replica of Manganese Phosphate Coat- 22

ing Processed in a Bath under Pressure

7 Fax Film Replica of Manganese Phosphate Coat- 23ing Processed in a Bath Enriched withManganese Citrate under Pressure

8 Fax Film Replica of Manganese Phosphate Coat- 24ing Processed in a Bath Enriched withManganese Tartrate under Pressure

9 Fax Film Replica of Manganese Phosphate Coat- 25ing Processed in a Bath Enriched withManganese Gluconate under Pressure

10 Titration Curve of Conventional Manganese 31Phosphate Bath at Room Temperature andPreheated before Processing

ix

LIST OF ILLUSTRATIONS

Figure Title Page

11 Titration Curve of Conventional Manganese 32Phosphate Bath Preheated and Processed atPressures of 1 psig and 3 psig

12 Titration Curve of Manganese Phosphate Bath 33Preheated, Enriched with Manganese Citrateand Processed at Pressures of 1 psig and3 psig

13 Titration Curve of Manganese Phosphate Bath 34Preheated, Enriched with Manganese Tartrateand PrQcessed at Pressures of 1 psig and3 psig

14 Titration Curve of Manganese Phosphate Bath 35Preheated, Enriched with Manganese Gluconateand Processed at Pressu,-.s of 1 psig and3 psig

x

OBJECTIVE

The objective of this study was to evaluate a methodfor the application of high temperature, corrosion resis-tant phosphate coatings on ferrous alloys in a conventionalphosphating solution containing metal salts of the alpha-reactive-carboxylic acids under steam pressure in anautoclave.l ,2

Hache, A., "The Corrosion Protection of Steel by the

Process of Phosphating under Pressure," French Ironand Steel Research Inst., St. Germaine-in-Laye,France. Presented at N.A.C.E. 2nd InternationalCongress on Metallic Corrosion, New York City,March 1963.

Menke, Joseph, "A Study of Manganese Phosphating Re-actions," Research Directorate, Weapons Laboratory atRock Island, Research, Development and EngineeringDirectorate, U. S. Army Weapons Command, TechnicalReport RE-TR-71-60, September 1971.

1 The following page is blank

BACKGROUND

Modern rapid-fire weapons require a phosphate coatingthat will withstand high temperatures without thermal de-composition. Such a coating is of prime importance to themilitary services because the resistance of the presentlyused coating to heat 3 4,,, 6 and corrosion 7 ,8, 9 has not beentoo satisfactory.

3 Doss, Jodie and W. D. McHenry, "Study of the Water ofHydration Contained in Phosphate Coatings by Radio-metric Techniques," Rick Island Arsenal LaboratoryReport 54-900, March %u54.

4 Doss, Jodie, "Corrosion Resistance of Phosphated Steelafter Heating under Oil," Rock Island ArsenalLaboratory Report 55-3256, August 1955.

Bessey, R. E. and W. M. Kisner, "Heat Resistance of Phos-

phate Protective Coatings," Technical Report SA-MR18-1026, Springfield Armory, Massachusetts, January 1953

6 Wagner, L. H., "Heat Resistant Conversion Coatings -for

Steel," Rock Island Arsenal Laboratory Report63-3345, August 1963.

Doss, Jodie, "Comparative Corrosion Resistance Tests onPhosphate Coatings," Rock Island Arsenal LaboratoryReport 58-1842, July 1958.

Doss, Jodie, "Composition of Zinc Phosphate Coatings,"Rock Island Arsenal Laboratory Report 57-2612,January 1956.

Gilbert, L. 0., "The Effect of Phosphate SolutionAnalysis on the Decomposition of the Phosphate Cout-ino," Rock Island Arsenal Laboratory Report 47-250,May 1947.

3

The following information was obtained from literaturepertaining to thermal stability of phosphate coating.Conventional zinc and manganese phosphating baths havebeen operated at atmospheric pressure at a temperature of705 + 50 F10 1 1

1,1 2

21 3,1 to provide an insoluble protective

coating for small arms weapons. The minimum requirementsof the current military specification (MIL-P-16232, Phos-phate Coatings, Heavy, Manganese or Zinc Base) for ferrousmetals prior to the application of any supplementary treat-ment is that the metal "shall show no signs oi corrosionwhen subjected to the salt spray test for l- hours Formanganese and 2 hours for zinc phosphate coatings." How-ever, no thermal requirement is given for either of thesecoatings. Gilbert 9 found that the phosphate coatings pro-duced in baths with a ferrous iron content below 0.06percent showed little or no decomposition when heated to500'F. He also pointed out that coatings formed in a bathof 60 points total acidity and in excess of 0.65 percent

0 oKnanishu, J., "A Study of Innovations of Salt Spray10 (Fog) Testing Equipment," U. S. Army Weapons

Command, Rock Island Arsenal, Research and Engineer-ing Division, Report 65-1191, May 1965.

11 Tinsley, E. r., "Phosphating Treatments - PatentLiterature Survey," Rock Island Arsenal Laboratory,Report 57-1022, April 1957.

12 Gilbert, L. 0., "Phosphating Materials and Process,"Rock Island Arsenal Laboratory, Report 54-2906,May 1954.

'3 Gilbert, L. 0., "A Study of Phosphate Treatment ofMetals," Rock Island Arsenal Laboratory, Report56-2995, June 1956.

" Jenkins, II. A. H. and J. 0. Surrey, "Production ofPhosphate Coatings on Metals," U.S. Patent 3,338,755,Hooker Chemical Corp., 1967.

4

ferrous iron content decomposed when heated in air above350F. The generally accepted temperature at which man-ganese coatings were believed to decompose was 350°F .When Doss and McHenry 3 heated manganese phosphate coatingsin air, they found that the coatings did not lose water ofhydration until a temperature of 2501F had been reached.The coatings did lose water of hydration fairly linearlywith temperature up to 360°F where a retention of about 10percent occurred. No effort was made to relate loss ofwater of hydration with deterioration of the coating bythermal decomposition. Doss, 4 on the otherhand, heatedphosphate coated panels in the absence of air. Zinc andmanganese coatings were heated under oil at 25-degree in-tervals in the range of 175OF through 4500F. Subsequently,Doss subjected the coatings to the salt-spray test. Hefound that the zinc phosphate coatings heated in the absenceof air lose their corrosion resistance between 300OF and325°F. Manganese phosphate coatings heated in the absenceof air lose their corrosion resistance between 400°F and425'F. Bessey and Kisner' determined the weight loss fromzinc and manganese phosphate coated specimens at varioustemperatures. They also tested zinc and manganese phos-phate coatings i, the salt-spray tests. These coatings hadbeen heated in air at 212F, 300 0 F, and at 100-degree in-tervals up to 14000F. Bessey and Kisner reported that thecorrosion products initially appeared on phosphate coated9pecimens at 212OF for zinc and 400°F for manganese. Theinvestigations given above on the thermal stability of thephosphate coatings"5 showed that corrosion tests should beconducted on the coatings after these have been heated inair. The present work was intended to evaluate a methodwhereby manganese phosphate coatings processed in manganeseenriched solutions under low steam pressure' would provideimproved resistance to heat and salt spray corrosion.

's Wagner, L. H. and P. G. Chamberlain, "Method and Compo-sition for Phosphatizing Steel under Pressure,"U. S. Patent 3,767,476, October 1973.

5 !he following page is blank

1L

EXPERIMENTAL PROCEDURE

Construction of Pressure Vessel

A pressure vessel was fabricated from a 12-inch-diametersteam pipe and designed to operate in a vertical position.The bottom was welded shut, and a lid was bolted to the topso that it could be removed manually. Appurtenances in-cluded a steam gauge, a purging valve, a bypass steam valve,and a manually operated steam pressure regulating valve.

Preparation of Steel Panels

SAE 1020 steel panels, 2 inches by 3 inches by 1/8 inch,were vapor-degreased in trichloroethylene and abraded withNo. 80 steel grit. They were weighed and placed in a desic-cator before use.

Preparation of Metal Phosphates

In the preparation of the metal salts, an alpha-reactivecarboxylic acid was combined with manganese carbonate in anaqueous solution at room temperature. The insoluble saltwas decanted, wasned with water, and dried in an oven beforeuse. The i.ecal salts consisted of zinc citrate, manganesecitrate, manganese tartrate, and manganese gluconate. Con-ventional zinc and manganese phosphating stock solutionswere prepared. Six liter portions of the zinc stock solu-tion were enriched with zinc citrate. Six liter portionsof the manganese stock solution were enriched with manganesecitrate, manganese tartrate, and manganese gluconate, re-spectively.

Processing Procedure

The study was divided into two processing procedures.A new method of processing steel panels under steam pressurewith and without enrichment of the bath was investigated.Steel panels were also processed in a conventional bath atatmospheric press. re. Comparative evaluations were madebetween the two methods. Tne following processing procedure,*as used: A stainless steel beaker containing 6 liters ofthe stock solution at 160OF-170°F was placed in the pressure

7

I

vessel. The thermocouple was inserted, and the bath wasenriched with a metal organic salt. A potentiometer witha copper-constantan thermocouple was used to measure thetemperature of the solution under pressure. A sample ofthe bath was taken before processing for chemical analysis.Steel panels previously grit-blasted and weighed were put

into the solution. The time of immersion was noted, andthe lid was clamped in position. The steam was turned onand the air was purged from the vessel. Potentiometricreadings were taken about every two minutes. When the tem-perature of the bath reached 210°F-212°F, the purging andthe bypass valves were closed. The following example isgiven for the processing of panels at the maximum pressure.Similar procedures were used for the coating of panels atlower pressures. When the bath temperature reached 230 0 F,the initial processing time was noted and the pressure wasmaintained at 22 pounds per square inch gauge (PSIG) untilthe temperature reached 260'F. When the processing timewas completed, the steam was turned off, the purging andthe bypass valves were opened, the pressure was reduced,and the cover lid was removed. The work load was quicklywithdrawn and immersed in ti rinse water. The time ofimmersion was noted, and the total time of processing wasdetermined. A sample of the bath was withdrawn after pro-cessing for chemical analysis. The coated panels were re-moved, dried in air, and weighed.

Testing Procedure

The coated panels were evaluated for resistance tothermil dzcomposition and to salt-spray corrosion. Somecoat.d panels as processed were not heated and were usedas control specimens. Other coated panels were weighedand then heated in an air convection oven at test tempera-tures of 350 0F, 400 0 F, and 4501F for one-hour exposures.They were cooled at room temperature and reweighed to de-termine the loss in coating weight due to thermal exposure.The thermally treated coatings and the as-processed coatingswere exposed in the 5% salt-spray test described in ASTMMethod B117. The coated panels were examined visually atperiodic intervals. Initiation of rust was noted when threeor more dots appeared on the coatings. Failure was notedwhen about 5% of the surface area was rusted.

8

The coated panels from each processing bath weredivided and tested as follows:

No. of

Panels Tests Remarks

1 none for display purposes

2 removal of coating determine coating weightand iron etched from panel

3 350°F exposure + salt spray determine resistance tocorrosion and loss incoating weight

3 400°F exposure + salt spray determine resistance tocorrosinn and loss incoating weight

3 450'F exposure + salt spray determine resistance tocorrosion and loss incoating weight

3 salt spray only control specimens forcomparison purposes

9i9 The following page is blank

RESULTS AND DISCUSSION

Zinc Phosphate Coatings

Zinc phosphate coatings were evaluated as follows:

Steel panels were processed in a conventional zinc bathunder steam pressures of 1, 5, 16, and 22 PSIG for 10 minutes.In the next series of runs, the procedure given above wasrepeated except that the bath was enriched with 10 grams perliter of zinc citrate and processed under 1, 10, 16, and 22PSIG for 10 minutes. Steel panels were also processed in aconventional zinc bath at atmospheric pressure at 205°F-208°F for 30 minutes to serve as controls for comparativepurposes.

The chemical composition of the baths is shown inTable 1 under the headings as free acid (FA), total acid (TA),

TAand the ratio of (g), percentage of ferrous iron (Fe) and pH.The FA and TA values are given in points (a point is equiva-lent to one-milliliter of O.IN NaOH solution when titratinga 10-milliliter sample of the bath). Analysis of the con-ventional zinc bath processed at atmospheric pressure for30 minutes are shown in the first line of figures. Process-ing tests conducted in the conventional bath under 1, 5, 16,and 22 PSIG are also listed. Results of the processing testswith zinc citrate as the addition agent in the bath are shownunder 1, 10, 16, and 22 PSIG. Because of the poor resultsshown in the salt-spray tests, no analyses were made of thebaths. The total processing time is calculated from thetime the steel panels were placed in the phosphating bathuntil they were removed and immersed in the rinse water.The 10-minute processing period was initiated when the steampressure reached the desired PSIG reading at a given tempera-ture. The initifl temperature of the six-liter baths wasnot identical, so additional time was necessary to bringthe bath up to the processing temperature. This is reflec-ted in the total processing time.

Coating Weights

The coating weight of 2590 milligrams per square footwas obtained on the steel specimens processed in the

11

-. 0 .. F

0 x

1.- ~ 0< +0 0 . - --

4--4- LO.-

Lj-a.T , %C) .. %N N c

0: 0-0 z

t U- W. V_ _ r4 -N pn - - 4

o 4- U- c -0 0 0% ' N N0N 1n4 N 04 ~0 U

tA C. _ _ .- _ _ _ -, cr -,N ci %1- 0 x 0 '0i 'IT' 0 t

0- 4- 0 N- N Do No- +- A N n -- N N l"

C 0

-C 0

N em N

_ - c -n

in O al 1.C'I N N in

0-

co co 0 cL Nt '0 -<A~'

an 0 0 0-.LJ 0In fa ~ '0 0' N. U 0 t- N . L

w 4- m A w' .0 44 o'0.- L . L . 0 N )ICi e 00 0 4 0 0 ) Uc: 0 0 0

CL 0 . L 0 an Nll '0 L. w

o N-U-. 0 C' 0 0. 0 CL0CL'4c4-I -;I- l

a. r I Mt0f 0. (3 S'

.2 - n 0Oj a 0 0 0 00+- __._ __ _ __ _ - . -co -Z 2 (LA 1 2 01Cc ZD- 71-4-CC

o ~ ~ ~ ' an0 L7- n0V4) an N0 cl2 n L: C>-L. > .) E. 0 ,> 0

C- ____ c- -C- _ _ .- - - .0

00 4- 0 00c

uI m-' N~ .. U 14

N1

conventional bath at atmospheric pressure. Steel specimensprocessed in the pressurized conventional baths withoutenrichment showed a decrease from 2490 to 1500 mg-per-sq-ftas the pressure was increased from 1 PSIG to 22 PSIG. Inthe pressurize6 enriched baths, heavier coatings were ex-hibited in which the values varied from 2620 to 3260mg-per-sq-ft. A comparison of the coating weights obtainedat 22 PSIG with and without enrichment showed that the coat-ings i. the enriched baths with zinc citrate doubled inI weight during the total processing time of 30 minutes.

Iron Etched from the Surface

The conventional zinc bath after processing at atmo-spheric pressure for 30 minutes etched 235 mg-per-sq-ft ofiron from the surface of the panels. The amount of ironetched varied from 429 to 562 mg-per-sq-ft during pressurizedprocessing. Therefore, the least amount of iron was etched.~en the panels were processed at atmospheric pressure.When the conventional bath was enriched with zinc citrate,the amount of iron etched varied from 388 to 611 mg-per-sq-ftduring pressurized processing. These values were a littleless on the average, than those processed without enrich-ment under 1, 5, 16, and 22 PSIG.

Resistance of Coating to Heat and Corrosion

The coatings as-processed in the conventional bath atatmospheric pressure and under 1 PSIG provided corrosionprotection for 9 hours. However, when the coatings wereheated to 350 0 F, 400 0 F, or 450 0 F, the resistance to saltspray corrosion was reduced to only one hour. When thephosphating bath was enriched with 10 grams per liter ofzinc citrate, the as-processed coatings showed an increasein salt-spray resistance from 22 to 62 hours. Again, afterthermal exposure, the coatings did not provide salt-sprayresistance beyond one hour.

Loss in -oating Weight

The loss in coating weight due to heat was determinedafter one-hour exposure at 350'F, 400 0F, and 450"F. At350 0 F, the coatings applied in the conventional zinc bath

13

under pressure showed a decrease in loss of weight from154 to 89 mg-per-sq-ft. The coatings applied in the zincenriched baths under pressure showed a greater loss inweight than those applied under pressure in the conventionalbath. An explanation can be given for the increased lossfrom the coatings, Coatings processed under pressure with-out enrichment of the bath weigh less than those processedunder pressure with enrichment of the bath. Generally, theweight losF relates to the as-processed coatirg weight.Therefore, the weight loss probably is due to the water ofhydration in the original coating. Results of this studyshowed that the conventional zinc bath enriched with zinccitrate and processed under pressure did not improve thecorrosion resistance after thermal exposure of the appliedcoatings. However, the as-processed coatings without thermalexposure did show an improvement in the salt-spray corrosiontest. Consequently, this phase of the research work on zincphosphate coatings was discontinued. The present effort wasdirected to determine whether superior heat and corrosionresistant manganese phosphate coatings could be applied tosteel by processing at lower pressures and temperatures ina bath enriched with metal organic compounds of the alpha-reactive carboxylic acids by which the process is moreeconomical.

Manganese Phosphate Coatings

Steel panels were processed at atmospheric pressure ina conventional bath enriched with 5 and 10 grains per literof manganese citrate at 202°F to 204°F for periods of 15,30, and 45 minutes. Th, Fesults are shown in Table 2. Acomparison may be made between values obtained in the con-ventional bath versus t'Pose ut ained by enrichment of thebath. The addition of 5 ard 10 grams p'er liter of manganesecitrate to the phosphating bath, as thr. processing time was

TAextended, increased the FA and the TA. The ratio TA showedFA

only a slight change. The coating weight and the ironetched in the conventional bath increased with the processingtime. With the 5 gram/liter enrichment, the coating weightdecreased with the processing time, and the amount of ironetched decreased slightly. With the 10 gram/liter enrichment,the coating weight shoqed an increase with the processing

14

04- 1.CO

-- In0

* I IfO405-

L- t

-V -

CO NO cc C % c

La

-"I N- --- I. I

4- 4- %

- - .0 ' 'INCJd

i- II

11~A 0%0 r

0

eq

a-.

SW%

0 O.o 0O n, O 0 o'(, On

A O •

-,, 5- -5 st - s- )

C!

8 ................_.

CN0

-. 4-

US- 0 ) (fu w ) S 0 0S U

CZ) a 00f3~~0 (D L f . 0

'S CLcLC)a c-

4- 20l - L4C C) to0( 0f

4- 4- -+-~~ ~ ~ ~ c '.0-11 0L

to 05L -) 00

L 4- 904--

c C%4 C 4-

04-0 C 0CtC cm

15

time, and the amount of iron etched also increased. Theprocessing temperature of the bath operated at atmosphericpressure was too low to effect a coating that would provideprotection fro0i corrosion beyond 6 hours.

Phosphating Steel under Pressure of 1 PSIG

As shown in Table 2, steel panels were processed con-secutively in a conventional bath under 1 PSIG (212 0 F) forperiods of 15, 30, and 45 minutes. The ferrous iron of thebath was very low (0.06 percent). The free acid was 2.0points and the total acid was 15 points. When the totalacid is below 27 to 30 points, the bath is considered apoor phosphating bath. After a processing time of 45minutes, the coating weight showed an increase, the ironetched varied from 689 to 825 mg-per-sq-ft, and the cor-rosion resistance of the coatings remained at 9 hours eventhough the thickness of the coatings changed. The coatingsprocessed for a period of 15 minutes appeared to be assatisfactory in providing the same protection as those pro-cessed for 45 minutes.

Manganese Organic Compounds

As shown in Table 2, 15 minutes appeared to be a reason-able length of time for processing steel panels in the pres-sure vessel at 1 PSIG. Therefore, 15-minute pressure timeswere used in the following experiments for the steel panelsunder the following conditions:

1. In a conventional bath,

2. In a conventional bath enriched with 10 grams perliter of the following additives:

a. manganese citrate

b. manganese tartrate

c. manganese gluconate

The following changes were made in the processing pro-cedure: As each 6-liter bath was taken from the stock solu-tionnear the temperature of 160 0 F, enrichment was added.

16

The steel panels were then introduced into the vessel. Thepressure vessel was closed, the steam was turned on, andthe air was purged from the vessel. The initial time andtemperature were noted. The 15-minute processing periodwas initiated when the bath temperature reached 212°F.As shown in Table 3, the total processing time was about30 minutes. Enrichment additives of the bath appeared tobuffer the free acid and to increase the pH. Coatings ap-plied in the baths enriched with manganese tartrate andmanganese gluconate provided the best resistance to heatand corrosion. Coatings applied in the bath enriched withmanganese citrate at 1 PSIG provided less protection thanthose of the control coating. Photographs of the coatedpanels after exposure in the salt-spray test are shown inFigures 1, 2, 3, and 4. Coatings applied in the baths en-riched with manganese tartrate and manganese gluconateunder steam pressure showed a significant improvement inresistance to heat and corrosion.

Fax rilm Replica

A Fax film is an imprint made of the coated surfacearea and recorded on a photograph. Fax film replicas weremade of the phosphate coatings to examine the iature if +,hesurface layer. Examination of the coatings was made asfollows:

As shown in Figure 5, the coating as-processed in a

conventional bath at atmospheric pressure at 210°F for 45minutes exhibited a fine, dense, crystalline structure withdark spots of porusity.

The coatings ,hown in Figures 6, 7, 8, and 9 wereapplied under steim pressure of 1 PSIG (212^F) for 10minutes. The coating processed in the conventional bathwithout an additive had a mass deposition of coarse crys-tals with dark areas of porosity (Figure 6). The coatingprocessed in the bath with 10 g:ams per liter of manganesecitrate exhibited a conglomerate mass of fine crystalswiLh dark areas of porosity (Figure 7). The coating pro-cessed in the bath with 10 grams per liter of inanganesetartrate exhibited a fine, dense, crystalline structure,

17

0 0

0 0C E 0 0

m.fL- -0 -

U~L

OXE 0

E a -

0 4- LA-

0z a L 4- ) 0U) If l 0

0a

In C0 0I. 4- 0

0

0 4- cL. u I

L.. I-I %D at__

ma. 0)

3: - - -1 0 C%

4- 4 0

0

LA t-. LI A

<- 0W

4- u a0 - )(' '

-~ N.

0C14

U)L (7 to -

in U. r4_ 21-;

I- - C. r , 4 ICC +

o~0 to 0EC o0C f v QNu ) o C V ) 0 u0

4- 0 0 a 00 07 0n - 00 0 0 L~ 0 0

-0L 0 L C I- m

CY n C ( (A Q'

0n 3 . . 0 . L. .0 L 0 r. 0 7 0A )

C L- o'- C- 0

O~~C 10)O~ D0 - Q- I-

>_ _ >. . > 4-->0

4- C 0 0- go- 4

09 M IL U )t U 08 01 ~ 8i 3 G

.C ii 00 lf- 0 UC 0 18 0

Thermal Exposure:

No Heat 350OF 400F 450F

" -r .. .i L -

96 6 6 1

Corrosion Resistance (Hrs)

Figure 1 Manganese Phosphate Coated Panels after Salt-SprayExposure Processed under Steam Pressure

Thermal Exposure:

No Heat 350F 400F 450F

1 11

Corrosion Resistance (Hrs)

Figure 2 Manganese Phosphate Coated Panels after Salt-SprayExposure Processed in a Bath Enriched with ManganeseCitrate

19

Thermal Exposure:

No Heat 350"F 400°F 450 0 F

>720 >720 384 384

Corrosion Resistance (Hrs)

Figure 3 Manganese Phosphate Coated Panels after Salt-SrrayExposure Processed in a Bath Enriched with ManganeseTartrate under Steam Pressure

Thermal Exposure:No Heat 350°F 400F 450°F

>720 "'720 384 384

Corrosion Resistance (Ibs)

Figure 4 Manganese Phosphate Coated Panels after Salt-SprayExposure Processed in a Bath Enriched with ManganeseGluconate under Steam Pressure

20

-7 7. T

RI

iI 4

Figure 5 Fax Film Replica of Manganese Phosphate CoatingProcessed in a Bath at Atmospheric Pressure (100X)

21

Figure 6 Fax Film Replica of Manganese Phosphate CoatingProcessed in a Bath under Pressure (1OX)

22

4 f4

ik -1

4. r

Figure ~ ~ ~ ~ ~ ~ ~' 7 Fa imRelc f agnsePopat otn

Figureocss7 Fax Fil Reliat of ice wt Manganeseat Coin

Citrate under Pressure (10OX)

23

*~ I77

kl

.,let'

4 MA~

I- I~ ~V!,'~~'?4j"

N.6

~ \-4

Figure 8 Fax Film Replica of Manganese Phosphate CoatingProcessed in a Bath Enriched with ManganeseTartrate under Pressure (l0QX)

24

Figure 9 Fax Film Replica c( Manganese Phosphate CoatingProcessed in a Bath Enriched with ManganeseGluconate under Pressure (lOOX)

25

minimum porosity, and uniform continuity (Figure 8). Thecoating processed in the bath with 10 grams per liter ofmanganese gluconate exhibited a conglomerate mass of mediumsize crystals, minimum porosity, and continuity (Figure 9).The coating was without rust ifter 720 hours of salt-sprayexposure.

Phosphate Coatings Applied under Atmospheric Pressure

Manganese baths enriched with manganese tartrate andmanganese gluconate produced coatings under steam pressurewith superior resistance to heat and corrosion. For thisreason, data were needed to determine whether these samebaths would produce improved coatings when processed atatmospheric pressure after a period of 45 minutes at 203*Fand 206°F under the following conditions.

1. In a conventional manganese bath enriched withmanganese carbonate.

2. In a conventional manganese bath enriched with10 grams per liter of the following additives:

a. manganese citrate

b. manganese tartrate

c. manganese gluconate

3. After stabilization overnight, the above-listedbaths were processed under steam pressure of 3 PSIG (222 0 F)for a period of 30 minutes.

A phosphating bath during overnight stabilization, incontact with the sludge becomes changed in that a reversalof the equilibrium condition occurs. Phosphating bathsset aside to stabilize over the weekend have been noted byEisler and Chamberlain 1 6 to produce coatings possessing in-creased resistance to salt-spray corrosion. This resultwas attributed to increased ferric phosphatp redissolvedfrnm the sludge. The results are shown in !able 4.Initially the free acid of the stock solution was high, somanganese carbonate was added to reduce the acidity. The

16 Eisler, S. L. and P. G. Chamberlain, "Determination of theSolu'bility of Ferric Phosphate in Phosphating SolutionsUsing Radioiron," Rock Island Arsenal Laboratory,Report 53-638, June 1953.

26

L-

ttLu L0Q In In 0 U

cCxl C)

w (.tUU. I2

Z.) 0 0n- 4-. .- .L.

V)0 0 ODa 0 0 N I

E l OOL In

2- + L--

a0 l -

- "I 4 -t0t *-. I

w X to 7

w In N IV) 0

U.0 00%^ U 0d

I- - -4- :4 0

r- 0

., .. 0 - -, .-

* A 0 Pei 1

0J

0- 0 V1 pn pn N

4- OD in 70 %

4 N

ON I I i

1 OU *) I V v V N

-2- 0. -N

I . C0 I I

-~ ~ ~ : c -

-U)

< - In10--

0 0RO-L- . 4- - -

w 2

InJ 0' 0 0 0

4-u - '0 Uan

oD La oI0i L. 0In t 1.. .

-# 4~t0N- IA 'V4- - r 'j' * N N ('

4-) 0 Q-7 J Nf DCl 4 0 N:" 0 'V V 'V

cl 00. () =O

w 720O CW ) 6 o C

4- .2 4

X L -L. 40a C a-

0V 4- -C CC -C CC .c: it C. ' .- 1

W 47 1 CO IA A27

addition of manganese carbonate also enriched the bath inmanganese content. The heat and corrosion resistance ofthe coatings was best, however, when the bath was enrichedwith manganese tartrate. Manganese gluconate was secondbest. Manganese-citrate enriched baths produced coatingsinferior to those enriched with manganese carbonate. Theaddition of manganese carbonate improved the coating appliedin the conventional bath at atmospheric pressure.

After the previous tests, the baths were allowed tostabilize overnight in contact with the sludge. The controlbath consisted of a nep* portion taken from the stock solu-tion. Each of the erriched baths was placed in the pressurevessel and heated to 200°F. Steel panels were introducedand processed at 3 PSIG (222°F) for 30 minutes. The resultsare shown in Table 5. The control bath that had been setaside to stabilize overnight showed an improvement in thecoating. The coatings processed under steam pressure in theenriched baths did not show an improvement. In the previoustest, about 1.4 sq. ft. of steel had been processed in the6-liter bath at atmospheric pressure. After the bath hadstabilized overnight, a similar area of steel was processedin that same bath under steam pressure without replenishment.This, without doubt, accounted for the decrease in the heatand corrosion resistance of the coatings. In processingunder steam pressure as the temperature gradienc increasedthe rate of reaction, mainly dissociation of the primaryphosphates resulted in the formation of tertiary phosphatesof iron and manganese. A rapid coating-buildup took place,and the excess of iron and manganese phosphates was precip-itated as sludge. The sludge was low in ferric phosphatebecause the bath was initially low in ferrous phosphate.Manganese phosphate was in excess, so the sludge consistedlargely of manganese phospi te. Upon stabilization, over-night, the sludge was very slow to dissolve, so the contentof primary phosphates was greatly reduced. Processingunder steam pressure did not improve the phosphate coatingsbecause the baths, which had been previously used at atmo-spheric pressure, were low in primary phosphates.

Conductometric Titration Curves

Conductometric titration curves were made of the bathsto determine the free and total acids before and after each

28

______________________________________

0 - -C1

u w.LL - - -

L.0 on

0-0 -

w xLU 0, C l)2

in thCC

U) 0 0 9o

o LA

I--

IA.

4- CL%.

v I

I~A 10

'CL

4.-U '

Li in

Wi InL-.

lo W

1.))

U) .3 L C C

r_0___ 0

4) .2 4)

C. L -

ISA - I 1-4 4- -J.-. in <

I- Ii29

processing cycle. Analyses were made to determine the effectof additives and to follow the changes in the baths whenprocessing under two pressures. Initially, the free acidof the bath is kept at a low level to control the picklingaction of the free acid (H3PO4 ) on the work surface. Thetotal acid is a measure of the free acid and the combinedphosphates in the bath. From the analysis of the bath, theratio of total acid to free acid is calculated. The bathis most effective when the ratio is kept at 6 to 1 or greater.A conductometric titration curve was made of the stock bathat room temperature and after the bath was preheated to200 0 F. A work load of 1.4 sq. ft. of mild steel was intro-duced and processed for 30 minutes at 1 PSIG. The procedurewas repeated on a portion of th stock bath for the samelength of time at 3 PSIG. Samples of the bath were analyzedafter the preheat at 200 0 F, and before and after the 30-minute-processing periods at 1 and 3 PSIG, respectively.The same procedure was repeated as outlined except that 10grams per liter of manganese citrate, manganese tartrate,and manganese gluconate were added, respectively, to a por-tion of the stock bath and processed under 1 PSIG (2120F)and 3 PSIG (222 0F). Analyses were made on a recordingtitrimeter. A 10-milliliter sample was titrated with 0.3NNaOH solution. The analysis of the bath made at a tempera-ture of 70°F and after the bath had been heated to 200*Fwithout processing any work is shown in Figure 10. Changesin the FA, TA, and pH were noted. The analysis of the bathafter it had been preheated to 200OF is shown in Figure 11.A second curve shows the change in the bath after 1.4 sq.ft. of steel had been processed at 212°F for 30 minutes.A third curve shows the change in the bath after a similaramount of steel had been processed at 222 0 F for 30 minutes.The curves show that the free acid increased and the totalacid decreased as the bath temperature was increased. Theinitial free acid and the final total acid range when man-ganese citrate was added to the bath are shown in Figure 12.The relative closeness of the curves after the processingperiods with manganese tartrate is shown in Figure 13. Thecurves crossed each other as the temperature was increased.The interval between the curves at the initial pH and thefinal pH is shown in Figure 14. The curves on the tartrateanalysis of Figure 13 crossed each other at lower valuesthan those shown on Figure 14 of the gluconate analysis.The buffering action of the free acid is shown on Figure 13by the closeness of the curves at the initial pH readings.

30

0

to 4- ;4

3'1

0 00

Ccv

49,

0Z))(

~ ~.1~ (6m

') ... 0

%0i cr IYt

#-

e *e

cn

.0 cc(~ '

11(wN~ ~ ~ K' NIrr'T

/ 4J 32

IVI

, u

C, V

.i.o 0.

VSj K4CV+//

:i: 444

L)

C- a.% CWWI WOIUM1T

~ r33

N, 0

I M

-1 CV C1 t

0I 0

6;4 ~1'A

'- 0 N- h

t.. .34

00

r%

611 ..

CI

C.) 4.)

/ -I

00 10/osII1T

/ 35

Processing under Atmospheric Pressure

This process does not lend itself to the production ofquickly formed phosphate coatings for heat and corrosionresistance. However, the method has many advantages. Theoperation can be conducted in u tank open to the atmosphere.The size of the tank can be changed with relative ease tomeet the workload. The contents can be heated internallyor externally, and the temperature of the bath can be main-tained between 205°F and 210 0 F. Usually 35 to 45 minutesare required under these conditions to effect a coatingthat will successfully pass a two hour salt spray exposure.

Processing under Steam Pressure

The method of processing under steam pressure has itsadvantages. The processing is done in an autoclave. Thetemperature of the bath can be raised under pressure froma minimum of 210°F at atmosphere pressure to 260OF at 22PSIG. A relatively thick coating with a minimum porositycan be quickly formed in 10 to 20 minutes of processingtime. The coating is enriched with manganese from solution,and possesses excellent resistance to heat at 450OF withsubsequent resistance to salt spray corrosion. The oper-ational cost is dependent upon the size of the autoclave,the working pressure, and replenishment of the bath withacid salts. The most logical method is to conduct the pro-cessing at the lowest pressure that will effect a highquality phosphate coating in the shortest processing time.

Metal Organic Additives

Additives, such as manganese dihydrogen phosphate andphosphoric acid were used to maintain the total acid level,and manganese carbonate to control the free acid. Theferrous iron content of the bath should be less than 0.1percent. The conventional bath can be enriched with metalsalts of manganese citrate, manganese tartrate, and mangan-ese gluconate, respectively. The processing proceduresconducted at a minimum pressure of I PSIG (212 0F) has shownthat manganese tartrate and manganese gluconate aid in theformation of the insoluble coatings. These coatings affordsuperior resistance to heat and corrosion in the salt spraytests.

36

Improvement in the Rate of Phospiating by PressurizedProcessing

In phosphating under steam pressure, line steam is cir-culated over the bath, and air is removed from the vessel.The temperature of the bath follows a time-temperaturegradient as the pressure is increased. The following tem-peratures and pressures have been approximated:

Temperature (°F) Pressure (PSIG)

212 1

222 3

232 7

242 11

252 16

262 22

As the temperature of the solution is increased, therate of dissociation of the phosphating solution increases,the free acid increases, the total acid decreases, the ironin solution decreases, and the pH decreases. These changesaffect the solution and the cnating as follows: As the rateof reaction increases, some of the phosphate materials are"thrown out of solution" as a flocculent precipitate. Underthermal movement of the solution, the particles grow in sizeand some of them are codeposited in the coating. The totalacid is reduced through the loss of these materials. However,the free acid in solution increases and lcwers the pH. Thisincrease in free acid accelerates the etching action on themetal surface and reduces the rate of deposition of the in-soluble phosphate coating. When the ratio of total acidto free acid decreases below 6, the coating becomes thinnerand porous and may eventually result in only a picklingaction with discoloration of the surface. The corrosionprotection afforded by the porous coating is unsatisfactory.The addition of manganese citrate, manganese tartrate, andmanganese gluconate to the solution buffers the free acidand stabilizes the solution. This is shown in Figures 10through 14. Phosphating at two temperatures is shown by

37

a variation in the pH and at the point at which the curvescross each other.

Fax film replicas of the applied coatings show that theuse of metal salts in the baths aid in the rapid formationof conglomerate crystals in the insoluble coating. The heatand corrosion resistance of the coatings is directly relatedto the thickness of the insoluble coating applied as a con-glmerate mass of crystals in the shortest processing time.Coatings applied in the shortest time results in continuousand uniform distribution, minimal porosity, limited ironloss at the metal-solution interface, and enrichment of theinsoluble coating with metal from solution.

38

CONCLUSIONS

The following conclusions are made:

1. Steel specimens processed in a manganese solutionenriched with manganese tartrate and manganese gluconate,respectively, at I PSIG for 30 minutes, did provide a phos-phate coating that afforded over 720 hours of protectionin the salt spray tests.

2. In comparative tests, the manganese phosphate coat-ings applied in an autoclave bath with manganese compounds ofthe alpha-reactive-carboxylic acids afforded significantlysuperior heat resistance to 450*F and corrosion resistancethan conventional coatings processed under atmospheric pressure.

3. Steel specimens processed in a conventional zincbath enriched with zinc citrate under steam pressure producedcoatings which exhibited only slight improvement in saltspray corrosion resistance over conventional zinc phosphatecoatings,

39 The following page is blank

RECOMMENDATIONS

1. The process of phosphating under steam pressure atlow temperatures (212 0 F) with the use of manganese baths en-riched with manganese tartrate and manganese gluconate, re-spectively, should be adopted for use on ferrous metal itemsrequiring a high-temperature, corrosion-resistant, phosphatecoating.

2. The use of metallic chelating agents in phosphatingsolutions should be investigated to determine how they affectthe heat and corrosion resistance of the phosphate coatings.

3. Analytical methods should be developed to determinethe am.1unt of metal organic compounds in the phosphatingsolution so that p 'ncess control can be more effective.

4. The sludge evolved in the bath should be controlledso that it will not be codeposited in the coating.

5. A continuous process should be developed for phos-phating steel items under steam pressure.

41 The following page is blank

LITERATURE CITED

1. Hache, A., "The Corrosion Protection of Steel by theProcess of Phosphating under Pressure," French Ironand Steel Research Inst. , St. Germaine-in-Laye,France. Presented at N.A.C.E. 2nd InternationalCongress on Metallic Corrosion, New York City,March 1963.

2. Menke, Joseph, "A Study nf Manganese Phosphating Re-actions," Research Directorate, Weapons Laboratory atRock Island, Research, Development and EngineeringDirectorate, U. S. Army Weapons Command, TechnicalReport RE-TR-71-60, September 1971.

3. Doss, Jodie and W. D. McHenry, "Study of the Water ofHydration Contained in Phosphate Coatings by Radio-metric Techniques," Rock Island Arsenal LaboratoryReport 54-900, March 1954.

4. Doss, Jodie, "Corrosion Resistance of Phosphated Steelafter Heating under Oil," Rock Island ArsenalLaboratory Report 55-3256, August 1955.

5. Bessey, R. E. and W. M. Kisner, "Heat Resistance ofPhosphate Protective Coatings," Technical ReportSA-MR18-1026, Springfield Armory, Massachusetts,January 1953.

6. Wagner, L. H., "Heat Resistant Conversion Coatings forSteel," Rock Island Arsenal Laboratory Report63-3345, August 1963.

7. Doss, Jodie, "Comparative Corrosion Resistance Tests onPhosphate Coatings," Rock Island Arsenal LaboratoryReport 58-1842, July 1958.

8. Doss, Jodie, "Composition of Zinc Phosphate Coatings,"Rock Island Arsenal Laboratory Report 57-2612,January 1956.

43

LITERATURE CITED

9. Gilbert, L. 0., "The Effect of Phosphate SolutionAnalysis on the Decomposition of the PhosphateCoating," Rock Island Arsenal Laboratory Report47-250, May 1947.

10. Knanishu, J., "A Study of Innovations of Salt Spray(Fog) Testing Equipment," U. S. Army WeaponsCommand, Rock Island Arsenal, Research and Engin-eering Division Report 65-1191, May 1965.

11. Tinsley, E. C., "Phosphating Treatments - PatentLiterature Survey," Rock Island Arsenal LaboratoryReport 57-1022, April 1957.

12. Gilbert, L. 0., "Phosphating Materials and Process,"Rock Island Arsenal Laboratory Report 54-2906,May 1954.

13. Gilbert, L. 0., "A Study of Phosphate Treatment ofMetals," Rock Island Arsenal Laboratory Report56-2995, June 1956

14. Jenkins, H. A. H. and J. 0. Surrey, "Production ofPhosphate Coatings on Metals," U. S. Patent3,338,755, Hooker Chemical Corp., 1967.

15. Wagner, L. H. and P. G. Chamberlain, "Method andComposition for Phosphatizing Steel under Pressure,"U. S. Patent 3,767,476, October 1973.

16. Eisler, S. L. and P. G. Chamberlain, "Determinationof the Solubility of Ferric Phosphate in Phos-phating Solutions using Radioiron," Rock IslandArsenal Laboratory Report 53-638, June 1953.

44