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GIIIII1 ID

GENERAL DYNAMIC5,•' 1CO€NVFAIR

Report No. 8926-167

SMaterial - Finishes and Coatings - Primer Pigments k

Salt Spray Corrosion Resistance

IL. A. Mappus, R. H. Whidden, W. M. Sutherland

L.U.

!-.--

27 December 1956

Published and Distributedunder

Contract AF33(657)-8926

Post Office Box 1950, San Diego 12, California 296-6611

Material Post Office Box 2071 273-8000 j Accounting Post office Box 510

GIIIIIIIIDGENERAL DYNAMICE I CONVAIRMODEL PAGE

DATE REPORT NO.

Report No. 8926-167

Material - Finishes and Coatings - Primer Pigments

Salt Spray Corrosion Resistance

Abstract:

Five pigments, potassium zinc chromate; calcium chromate; strontium chromate;barium chromate; and.zinc tetroxychromate, were incorporated with twodifferent paint vehicles, an alkyd resin and a coumarone - indene resinvehicle, suitable driers and solventsand applied to clad 7075-T6 aluminumalloy, AZ31, Condition H magnesium alloy, normalized 4130 steel, and annealedType 321 stainless steel. The several samples were exposed to 20 percentsalt spray exposure for 1500 hours to observe weight losses resulting fromthe corrosion exposure. The weight loss data showed the effectiveness ofthe five pigments when applied to steel and magnesium alloy ranked in theorder they are mentioned above. Those primers formulated with alkyd resinvehicle performed better when applied to alloy steel and magnesium alloythan those formulated with the cumarone - indene resin.

Reference: Mappus, L. A., Whidden, R. H., Sutherland, W. M.,"Chromate Pigments - Relative Efficiency in AircraftPrimers of," General Dynamics/Convair Report MP 56-264,San Diego, California, 27 December 1956. (Referenceattached).

FORM 12120 0FIEV.,2/t.1

ENGL\EJAI-I'LG 'i'hT L'(URATOURIES

C O N V A I R REPORT 56-264

SDIVISION OF GENERAL DYNAMICS CORPORATION DATE 12-27-56

SAN DIEGO MODEL ms 56-Is'5

TITLE

ijEPORT NO. 56-264CHROMATE PIGMELTS -

RELATIVE EFFICIENCY L%AIRCRAFT RIRia.RS OF

MOUEL MRS 56-185

PREPARED BY(•, 2- 2A.Q GR.OUPIATERQIAL & BR, CES$c)ES LAB.

L. A. Mappuf"'

REFERENCECHECKED BY R, 1i Whi e

CHECKED BY APPROVED BW. M. Sutherland, E. F. StrongGroup Engineer Chief, Enginee z"ng

NO. OF PAGES i5 Test LE.boratories

NO. OF DIAGRAMS i0

REVISIONS

NO. DATE By CHANGE PAGES AFFECTED

-

FORM 181A.4A-

ANALYWSI C O N V A I R PAGE3PREPARED BY Mappus/Whidden A DYIISION Of' SNUAL DYNAMICS COIMNAIWOM REPORT NO. 56-264CHECKED DY W. M. Sutlerland SAN Of£GO MORT No. 56-26 5

MODEL MRS 56-185PEVI3ED my DATE 12-27-56

REPORT NO. 56-264ChROMATE PIGMENTS -

RELATIVE EFFICIENCY fL\ AIRCRAF tRIMERS OF

INTRODUCTION:

Potassium zinc chromate has been used exclusively as a corrosion inhibitivepigment in the aircraft industry. A literature search revealed that otherchromate pigments might be more satisfactory, especially under conditionsof high heat or in acid environments. These conditions are being encounteredon todayts fighter type aircraft due to high speeds and missile exhaust gases.The most recent comprehensive comparison of the efficiencies of variouschromate pigments is that of H. G. Cole (Reference 1.).

This work was intended to be a partial replicate of the work done by Mr. Cole.Originally, it was planned to include an alkaline pigment, especially formagnesium and steel protection, and barium potassium chromate (Pigment E).However, a supplier could not be located for these two pigments. The followingfive chromate pigments were selected for evaluation: potassium zinc chromate;strontium chromate; calcium chromate; barium chromate; and zinc tetroxy-chromate. These pigments were formulated into priming paints using a coumarone-indene vehicle and also an alkyd vehicle similar to that used in specificationMIL-P-8585 primer. The primers were applied to a clad aluminum alloy, amagnesium alloy, a chromium-molybdenum alloy steel, and a stainless steel.Test conditions for evaluation were the standard 20% salt spray cabinet andan exterior exposure with two daily sprays of sea water.

This report covers the results obtained after 1500 hours salt spray ex-posure on the magnesium and 4hromium-molybdenum alloy steel specimens.Addendum reports will be issued on the other specimens as soon as theresults become significant.

OBJECT:

To determine the relative efficiency of five different chromate pigmentsas corrosion inhibitors in primer formulations, using a coumarone-indeneresin vehicle and also the iIL-P-8585 alkyd vehicle, on four typical air-craft construction metals.

CONCLUSIONS:

1. The relative overall performance of the five pigments tested after 1500hours salt spray exposure on magnesium and 4130 steel specimens was asfollows, in the order of decreasing effectiveness:

(a) Potassium zinc chromate(b) Calcium chromate(c) Strontium chromate(d) Barium chromate(e) Zinc tetroxychromate

POAlM IGIS-A

ANALYSIS C 0 N V A I R PAGE2PREPARED BY Mappus/Whidden A KMRm W GINU" L syhn, CflPOWAI REPORT NO. 56-264CHECKED BY W. M. Suth#rlard SAN 01960 MODELMRS 56-185REVISED BY DATE 12-27-56

CONCLUSIONS. (Cont'd.)

2. As a group, the primers formulated with the alkyd resin vehicle per-formed better than those formulated with the coumarone-indene resinvehicle on magnesium and steel specimens after 1500 hours salt sprayexposure.

MATERIALS:

Metals -

Clad 7075T6 aluminum alloy, QQ-A-287Magnesium alloY, QQ-M-44, AZ 31, Cond. H4130 steel, AN-QQ-$-685, normalized18-8 stainless steel, MIL-S-6721, Type 321, annealed

Pigments -

Potassium zinc chromate, Reichh-old Chemicals # 1425Strontium chromate, Mineral Pigments Corp. # 1365Calcium chromate, Mineral Pigments Corp. # 1376Barium chromate, Mineral Pigments Corp. # 1355Zinc tetroxychromate, Reichhold Chemicals # 1430

Vehicles -

Coumarone-Indene, Neville Chem. Co. # R-12-ALinseed oil, 900 poise, Archer-Daniels-MidlandMaleic anhydride, Barrett Chemical Co.Phenol aldehyde, Bakelite # BKS 3962Alkyd resin, Reichhold Chemicals # P-372

Driers -

Manganese drier, 6% solids, NuodexCobalt drier, 6% solids, NuodexLead drier, 24% solids, Nuodex

Solvents -

Naphtha (12-15% aliphatic), heville Chem. Co. # 250WXylol, McGuire Chemical Co.Butyl Alcohol, Union Carbide Corp.

TEST SPE CIMN PREPARATION:

Metal substrates- The composition of the alloys selected for test is shownin Table I. Specimens for each alloy were cut from a single sheet of materialto assure uniformity of composition. Representative samples of each sheetwere analyzed by the Chemical Laboratory. All specimens were .064 x 3 x 4inches. The edges and corners of the specimens were rounded by filing.

FORM 1812-A

ANALYSis C 0 N V A I R PAGE 3PREPARED BY Mappus/Whidden A I V1ISON @*V 6lIN4AL DYNAMICS C¢POOATION REPORT NO. 56-264CHECKED BY. W. M. Sutherland SAN D1E0O MODEL MRS 56-185REVISED mY DATE 12-27 -56

TEST SPECIMEN PRE-PARATIOI. (Cont'd.)

Metal substrates, (Cont'dJ) -

The aluminum alloy specimens were solvent wiped with imthyl ethyl ketone,vapor degraased with trichoroethylene, alkaline cleaned and chromic acidanodized per MIL-A-8625, Type IL

Magnesium alloy specimens were vapor degreased, alkaline cleaned and di-chromate treated in accordance with MIL-M-3171, Type III.

The 4.130 steel specimens were sandblasted and solvent wiped prior topainting.

Stainless steel specimens were vapor degreased and passivated with nitricacid.

Paint formUlation - All of the experimental primers were formulated forthis test by the U. S. Paint and Chemical Co., 1133 Mariposa Street, SanFrancisco, California. Details of the formulations are shown in Tables IIIand IV. Unlike the Cole report, no final adjustment was made for glossafter manufacture. Details on the pigments used for formulations areshown in Table II.

Faint application - All of the primers were thinned with toluene to aZahn # 1 viscosity of 40 at 77 0 F. prior to application. Specimens receivingthe same primers were sprayed in the same batch to assure a uniform coaton the various substrates. Primers were applied within 24 hours after themetals were surface treated. After priming with one 8ood cover coat, thespecimens were allowed to cure at least 24 hours -nd then all specimens weretop coated with MIL-L-T178 lacquer, color no. 514 black. The specimenswere allowed to cure for two weeks prior to exposure.

Controls - Unpainted surface treated specimens of each alloy were usedas controls.

TEST PROCEDURE:

Salt spray cabinet exposure - Specimens were exposed in quadruplic:ýte to20% salt spray in accordance with QQ-M-151A. All specimens were mountedat an angle of 20 degrees from the vertical in a single rack (see Figure 1.).Each of the quadrupýiicate specimens was exposed at a different level on therack. Specimens of each of the four alloys were sub-grouped together.Control specimens were placed between those painted with coumarone-indenevehicle primers and those painted with the alkyd vehicle primers.

Exterior exposlure - Quadruplicate specimens mounted at an angle of 45degrees were exposed on a rack facing souta (see Figure 2.). The rack was -located ap".roximately 1/2 mile inland from San Diego Bay. The specimenswere attached to the rack with wooden pegs. Twice every working day, thespecimens were sprayed with sea waterhich had been collected in the neigh-borhood of the whistle buoy at the entrance to San Diego harbor.

FORM ISI-A

ANALYSIS C 0 N V A I R IRAGE 4PREPARED BY ppus/Whidden A WONM OF M.AL DYNAC$ COPOAON REORT NO. 56-264CHECKED BY W. M. Sutherlarn SAN DIZOo MODEL -MRS 56-185REV3SED BY DATE 12-27-56

TEST PROCEDURE, (Cont'd,)

Evaluation procedure - Corrosion was evaluated by weight loss, expressedas grams per square decimeter per ounce of paint applied per square y~rdof surface. All specimens were weighed to the nearest tenth of a milli-gram at the following intervals:

(1) After surface treatment(2) After priming(3) After top coating(4) After exposure (paint and corrosion products removed)

A paint stripper conforming to MIL-R-8633 was used to remove paint fromthe specimens.

Corrosion products were removed from the magnesium alloy specimens byimmersing overnight in a cold 10% solution of ammonium chrom~te throughwhich a stream of air was bubbled.

Lorayne Rust Remover, a proprietary material manufactured by Kruse Prod-ucts Co. of LaMesa, California, was used to de-rust the 4130 steelspecimens. This product was selected because of its negligible etchingeffect on 4130 steel (Reference TN 8811).

In addition to the weight loss evaluation, the m,gnesium and 4130 steelspecimens were visually examined and rated after. 1500 hours salt sprayexposure prior to the removal of paint ard corrosion products.

RESULTS:

These results are based on 1500 hours exposure to salt spray on magnesium 4and 4130 steel specimens only. Results are tabulated in Tables V, VI,VII, and VIII.

Performance of the potassium zinc chromate pigment was relatively good inboth vehicles on magnesium and 4130 steel specimens.

There is no signific.•nt difference between the performance of the strontiumchromate and the potassium zinc chromrate in the alkyd vehicle on eithermagnesium or 4130 steel specimens.

The overall p-rformance of calcium chromate was almost as good as thepotassium zinc chromate.

Efficiency of the oarium chrom nte w-as poor comp.red to the above t'-reepigments but much better th=n the zinc t:troxyc-hro:q'Lte except in t'healkyd vehicle on 4130 steul s•ecimens.

Perform.nce of zinc tetroxychrom.te was significantly poor in both vehicleson magnesium alloy s.eciniens.

FORM IGIS-A

ANALYSIS C 0 N V A I R PAGE 5

PREPARED BY Ma±~Pus/Whidden A DVIO•UO• orGNEAL DYNAMICS CORPORATION REPORT NO. 56-264CHECKED BY W. N. Sutherlxnd SAN D"IGO MODEL MS 56-lb5PF.iSED BY DATE 12-27-56

DISGUSSIOh Gil RESL'.LS:

In drawing conclusions from this data, it must be remembered that onlytwo alloys under an artificial exposure condition, namely magnesiumn and4130 steel alloys after 1500 hours salt spray exposure, are being considered.Results will be more significant after the completion of the exteriorexposure test.

Performances of the strontium chromate and calcium chromate were notexceptional enough on the tests completed to date to warrant substitutionfor potassium zinc chromate in all primer formulations. Perhaps thesepigments may be used advantageously under conditions of high tempErature,acidic environment, or in cases where potassium zinc chromate is incom-patable with the vehicle, such as in amine catalyzed epoxy primers.

Corrosion on the magnesium specimens coated with zinc tetroxychromatepigmented primers was of the concentrated cell type, indicating that therelative insoluble nature of this pigment may have something to do with itspoor performance. Zinc tetroxychromate is currently used in MilitarySpecification wash primers. The Paint Research Department of the Sherwin-Williams Company has found that substitution of strontiumchromate andsilicon dioxide as the pigementation in XIL-C-15328A wash primer improved itsperformance on magnesium (Reference 2.),

RE COMiENDAT IONS:

It is recommended that this study be continued until the exterior exposuretest results become significant.

REFEREN•CES:

1. "Tests on the Relative Efficiency of Chromate Pigments in AnticorrosivePrimers", by H. G. Cole, J. Appl. Chem., 5 May l955.

2. "Protective Coatings for ihgnesium", The Sherwin=Williams Company,WADC Technical Report 54-373.

NOTE: The data from which this report was prepared may be found inEngineering Test Laboratories Data Book # 979.

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ANALYSIS C 0 N V A I R PAGE 14PREPARED ByVappus/Whidden A DIVISION OF (ADINAL DYNAMICS CONPORATION REPORT NO. 56-264CHECKED BY W. M. Sutherland SAN DIEGO MODEL MRS 50-185

REVISD BYDATE 12-27-50

FIGUR~E

SPECIMENS PLACED IN SALT SPRAY EXPOSURE RACK

IFORN 1611-A

ANALYSIS C 0 N V A I R PAGE 15PREPARED BY Mvappus/Whiddefl A DIVISION OF GENERAL DYNAMICS CORPORATION REPORT NO. 56-264

CHECKED BY W. M~'. Sutherland SAN DIEGO MODEL M-S 56-1855REVISED BY DATE 12-27-56

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