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1 V4~-YRoc Island Arsenal_q LaboratoryU -npo1A hCI'y/i"E $ .TECHNICAL REPORT LeACORROSION RESISTANCE OF BLACK OXIDECOATINGS ON MILD AND CORROSION RESISTANT STEELSByLinden H. Wagner T'LLA jjEngineering Support of Standardization Project 76502-0001 AMC Code No..Report ('oy Nop .I~I'ID No .. I)t ..21 December 1964 DISTRIBUTED) BY TilE TIs REI'ORT .MAY iE I)IESTROYI.'I) VIIELNOFFICE OF "rI*ClIINICAI, S ERVICIES NO I,)N(;'R 1(QUIII.:1) FOR Ri:FIRIN(:U. S. I)EI'AR 'r,% ,N T of (:o .% l*R(:I,-WAShIINGTON 25. 1). findings in this report are not to be construedas an official Department of the Army position."Copies Available at Office of Technical Services S$ 7 Report No.

2 64-3580 Copy No. _CORROSION RESISTANCE OF BLACK OXIDECOATINGS ON MILD CORROSION RESISTANT STEELSByLinden H. WagnerApproved by:I 0. INMINA cting Laboratory Director21 December 1964 Engineering Support of Standardization 76502-9001 AMC Code No. Island ArsenalRock Island, IllinoisDDC Availability Notice:Qualified requesters may obtaincopies of this report from the revision of Military Specification NIL-C-13924A, dated 29 December 1956, the various processeswere reviewed and evaluated for their effectiveness inblackening mild steel, 300 and 400 corrosion The corrosion resiatance of these coatingswere also investigated.

3 A low temperature Class 4proprietary alkaline-oxidizing process was proposed forblackening only the 300 series stainless steels in lieuof the fuzed salt oxidizing process which operates Class 2 alkaline-chromate process did notblacken the 300 series stainless steels. Class 3 andClass 4 processes did. The mild steel control panels,coated and uncoated, failed the silt spray test in onehour. The electrodeposited coating failed in three tofour hours. Freshly abraded c'ntrol stainless steelpanels 302, 310 and 321 were without rust after 500hours. Proprietary coatings A-1 and B applied to freshlyabraded 302 and 310 series stainless steel panels werewithout rust after 500 hours.

4 On the 321 series stain-less steel panels, initial rust was noted on Coating A-1after 165 hours and on Coating B after 141 300 series stainless steels specimens coatedwith the type 3 (molten dichromate process) failed inthe salt spray test in one 64-3580 RECOMMENDATIONIt is recommended that:1. Proprietary compounds A-1 and B be used in theClass 4. low temperature alkaline-oxidizing process forblackening type 300 stainless steels only, and not beused on mild steel, or type 400 corrosion resistant The Class 4 process be used in place of the Class3, fuzed salt oxidizing process, for blackening type 300stainless steels, because of superior corrosion resistanceand simplicity of iiCORROSION RESISTANCE OF BLACK OXIDECOATINGS ON MILD AND CORROSION RESISTANT STEESLCONTENTSPage 1 Introduction 1 Procedure and Results 2 Materials 2 Treatments 2 Discussion 13 Literature References 16 List of Prior Reports 17 Appendix A -RIA Lab.

5 Rpt.,"Alkali Black Finishes" 18 Distribution 21iii 64-3580 CORROSION RESISTNCE OF BLCK OXIDECOATINGS ON MI) AND CORROSION RESISTANT STEELSOBJECTTo determine the effectiveness of low temperaturealkaline proprietary processes for blackening determine the corrosion resistance of thealkaline proprietary coatings applied to 300 seriesstainless steels and Ml, gas revise MIL-C-13924A according to the findings ofthis Specification, "Coating, Oxide, Black, forFerrous Metals," MIL-C-13924A, dated 26 December 1956,covers black oxide coatings applied to ferrous metals(wrought Jr% n: yaon, low alloy, and corrosion resistantsteels).

6 ,, , 1 Black sulfide coatings are notincluded in this black oxide coating processes are classified asfollows:Class 1 -Alkaline oxidizing process (for wrought iron,plain carbon, and low alloy steels)Class 2 -Alkaline-chromate oxidizing process (forcorrosion resistant steels)Class 3 -Fuzed salt oxidizing process (for all gradescorrosion resistant alloy and carbon steels having a drawtemperature above 9000F.)This standardization project was concerned with thefollowing problem: The Class 3 process for corrosionresistant steels utilizes molten dichromate (Na2Cr2O7)at 7600F. in the blackening of these steels.

7 This is ahazardous operation, and it is difficult for the Departmentof the Army to locate contractors who are willing to useit. As a result, it was brought to the attention of theDepartment of the Army that a low temperature, commerciallyavailable blackening material had been used to producesatisfactory black coatings on corrosion resistant samples of these materials were obtained; bathswere prepared, and several types of steel specimens werep-ocessed. The coat4-gs were comparatively evaluated forresistance to corrosion in the salt spray test. A revision64-3580 1*Iof the specification was made to include the Class 4proprietary coatings in MIL-C-13924B, dated 10 July 1964.

8 (FSC6850, Project No. 6850-0103). Appendix A, RockIsland Arsenal Report on "Alkali Black Finishes"describes the blackening process and properties of thecoating. It is included in this report for AND RESULTSThe materials used in the tests, their treatment,and the results of the salt spray tests are listed belowas follows:MaterialsMild carbon steel panels, 2" X 3" X 1/16", SAE#1020. Austenitic stainless steel panels, 2" X 4"' X 1/81Y,'ISI #302: 2" X 3" X 1/16", #310, and #321 Martensiticstainless steel; Ml, gas cylinders, AISI #410. Thechemical composition of the steels is shown in TableThese items were sandblasted to renew the surface,and cleaned in an alkaline derust MIL-C-14460 Type 2solution with periodic reversal of current.

9 After a hotand cold water rinse, they were blown dry with #1 -Controls. Representative items of theabove were selected as #2 -(MkFsivation). The clean items weretreated as outlined in Table V-A, par. (b) to (b) of Spec. MIL-STD-171A(ORD), dated 11 October1960. The procedure is outlined as follows. Cleaningand passivating:(a) Alkaline derust method was used instead offinish (Spec. TT-C-490, Method II.)(b) Immerse for 30 minutes in a hot (1200 to 1300F.)aqueous solution containing 20 percent by volume of nitricacid (sp. gr., ) and 2 percent by weight of sodiumdichromate.(c) Rinse in clean hot water.

10 (d) Immerse for one (1) hour in a hot (1400 to1600f.) aqueous solution containing 5 percent by weightof sodium 64-35804&~U) r404 1aE4 C02 0 1 I I to0 0054 0 D 0 * C4-EM 4 e q r- r44og a2'844) 0200093 * ..rm-4 r- 4 .10 040~ 0 0 0 0IE-44a 0 0 00 IA fU .* ..A4 r4 C4CO34464-358 3l0 .(e) Rinse in clean hot water.(f) Rinse in hot (1600 to 2100F.) aqueous solutionmaintained at pH 3 to pH 5 by the addition of flakechromic acid or proprietary mixture of chromic andphosphoric #3 -(Code A). The clean items were treatedin accordance with "Instructions for the Black Passivationof Corrosion Resistant Steel," a Bureau of Naval Weaponspublication, Code Ident.