Designation:A967–01e1
Standard Specification for
Chemical Passivation Treatments for Stainless Steel Parts1 This standard is issued under thefixed designation A967;the number immediately following the designation indicates the year of original adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.A superscript epsilon(e)indicates an editorial change since the last revision or reapproval.
e1N OTE—Paragraph3.1.1.5was deleted editorially in April2002.Paragraph10.2was corrected editorially in April2002.
1.1This specification covers several different types of chemical passivation treatments for stainless steel parts.It includes recommendations and precautions for descaling, cleaning,and passivation of stainless steel parts.It includes several alternative tests,with acceptance criteria,for confirma-tion of effectiveness of such treatments for stainless steel parts.
1.2Practices for the mechanical and chemical treatments of stainless steel surfaces are discussed more thoroughly in Practice A380.
1.3Several alternative chemical treatments are defined for passivation of stainless steel parts.Appendix X1gives some nonmandatory information and provides some general guide-lines regarding the selection of passivation treatment appropri-ate to particular grades of stainless steel but makes no recommendations regarding the suitability of any grade,treat-ment,and acceptance criteria for any particular application or class of applications.
1.4The tests in this specification are intended to confirm the effectiveness of passivation,particularly with regard to the removal of free iron and other exogenous matter.These tests include the following practices:
1.4.1Practice A—Water Immersion Test,
1.4.2Practice B—High Humidity Test,
1.4.3Practice C—Salt Spray Test,
1.4.4Practice D—Copper Sulfate Test,
1.4.5Practice E—Potassium Ferricyanide–Nitric Acid Test, and
1.4.6Practice F—Free Iron Test.
1.5The values stated in inch-pound units are to be regarded as the standard.The SI units given in parentheses are for information only.
1.6The following precautionary caveat pertains only to the test method portions,Sections14through18of this specifica-tion:This standard does not purport to address all of the safety concerns,if any,associated with its use.It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2.Referenced Documents
2.1ASTM Standards:
A380Practice for Cleaning,Descaling,and Depassivation of Stainless Steel Parts,Equipment,and Systems2
B117Practice for Operating Salt Spray(Fog)Apparatus3 B254Practice for Preparation of and Electroplating on Stainless Steel4
2.2Federal Specification:
QQ-P-35C Passivation Treatments for Corrosion-Resistant Steels5
3.Terminology
3.1Definition of Term Specific to This Standard—It is necessary to define which of the several commonly used definitions of the term passivation will be used in this specification.(See Discussion.)
3.1.1Discussion—Stainless steels are autopassivating in the sense that the protective passivefilm is f
ormed spontaneously on exposure to air or moisture.The presence of exogenous surface contamination,including dirt,grease,free iron from contact with steel tooling,and so forth,may interfere with the formation of the passivefilm.The cleaning of these contami-nants from the stainless steel surface will facilitate the spon-taneous passivation by allowing the oxygen uniform access to the surface.The passivefilm may be augmented by chemical treatments that provide an oxidizing environment for the stainless steel surface.
3.1.1.1In this specification,passivation,unless otherwise specified,is defined as the chemical treatment of a stainless steel with a mild oxidant,such as a nitric acid solution,for the purpose of the removal of free iron or other foreign matter,but which is generally not effective in removal of heat tint or oxide scale on stainless steel.In the case of stainless steels with additions of sulfur for the purpose of improved machinability,
1This specification is under the jurisdiction of ASTM Committee A01on Steel, Stainless Steel,and Related Alloys and is the direct responsibility of Subcommittee A01.14on Methods of Corrosion Testing.
Current edition approved Oct.10,2001.Published December2001.Originally published as A967–96.Last previoius edition A967–99.
2Annual Book of ASTM Standards,V ol01.03.
3Annual Book of ASTM Standards,V ol03.02.
4Annual Book of ASTM Standards,V ol02.05.
5Available from Superintendent of Documents,U.S.Government Printing Office,Washington,DC20402.
1
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passivation may also include the removal of sulfides from the surface of the metal for the purpose of maximizing corrosion resistance.
3.1.1.2The formation of the protective passivefilm on a stainless steel,also called passivation in a more general context,will occur spontaneously in air or other oxygen-containing environment when the stainless steel surface is free of oxide scale and exogenous matter.
3.1.1.3Chemical treatments,such as sodium dichromate solutions,may facilitate the more rapid formation of the passivefilm on a stainless steel surface already free of scale or foreign matter.Such treatments,also sometimes called passi-vation in common usage,are designated as post-cleaning treatments in this specification in order to distinguish them from chemical treatments capable of removing free iron from stainless steels.
3.1.1.4The chemical treatments capable of removing heat tint or oxide scale from stainless steel and capable of dissolving the stainless steel itself,typically called pickling,are substan-tially more aggressive than treatments used for passivation,as defined in3.1.1.1.The surface of stainless steel that has been pickled is free of scale,free iron,and exogenous foreign matter,and does not require a separate treatment for passiva-tion as defined in3.1.1.1.The passivation process defined in 3.1.1.2will occur without further chemical treatment but may be augmented and improved by the post-cleaning treatments defined in3.1.1.3.
3.1.1.5Electrochemical treatments,including electropick-ling and electropolishing capable of removing heat tint or oxide scale from stainless steel and capable of dissolving the stainless steel itself,are substantially more aggressive than treatments used for passivation,as defined in3.1.1.1.The surface of stainless steel resulting from these treatments is free of scale, free iron,and exogenous for
eign matter,and does not require a separate treatment for passivation as defined in3.1.1.1.The passivation process defined in 3.1.1.2will occur without further chemical treatment,but may be augmented and im-proved by the post-cleaning treatments defined in 3.1.1.3. Statements regarding chemical treatments,unless otherwise specified,are taken to include electrochemical treatments.
4.Ordering Information
4.1It is the responsibility of the purchaser to specify a test practice appropriate to any particular material and application. This specification was written for the purpose of providing an alternative to United States Federal Specification QQ-P-35C. Determination of the suitability of this specification for that purpose is the responsibility of the purchaser.
4.2Unless specified by the purchaser,the chemical treat-ment applied to the stainless steel parts shall be selected by the seller from among the listed passivation treatments.
5.Materials and Preparation for Passivation Treatments 5.1The passivation treatments shall be of one or more of the following types.The effectiveness of a particular treatment for a particular grade of stainless steel in a particular application is demonstrated by meeting the specified testing requirements: 5.1.1Treatments in nitric acid,
5.1.2Treatments in citric acid,
5.1.3Other chemical treatments,including electrochemical treatments,
5.1.4Neutralization,and
5.1.5Post-cleaning treatments.
5.2Materials:
5.2.1The chemicals used for passivation treatments shall produce passivated surfaces that meet the requirements of one or more of the tests of this specification.Attention shall be given to maintaining adequate volume,concentration,purity, and temperature control appropriate to the size and amount of stainless steel to be treated.
5.2.2The processor shall maintain a record with regard to concentration and temperature of the passivation solution sufficient to demonstrate that the specified passivation condi-tions were maintained for each lot of stainless steel parts processed.Such records shall be available for inspection when specified in the purchase order.The processor is not required to reveal the precise composition of proprietary chemical mix-tures but shall maintain a unique identification of the mixture
that will ensure its accurate representation for subsequent use.
5.2.3The processor shall be responsible for the safe dis-posal of all material generated by this process.
5.3Preparation for Passivation Treatments:
5.3.1The pretreatment methods and procedures used prior to the passivation treatment,including mechanical and chemi-cal methods,singly or in combination,for descaling and pickling,shall be in accordance with Practice A380.When electrochemical cleaning is required,it shall be performed in accordance with Practice B254.
5.3.2The resulting pretreated surface shall be substantially free of oil,grease,rust,scale,and other foreign matter.
5.3.3When thefinal pretreatment of a part includes pickling of the entire surface of the part,no further passivation treatment is required prior to testing of the surface unless specified by the purchaser.
6.Treatments in Nitric Acid Solutions
6.1Passivation Treatment:
6.1.1Stainless steel parts shall be treated in one of the following aqueous solutions and maintained within the speci-fied temperature range for the specified time.
6.1.1.1Nitric1—The solution shall contain20to25volume percent of nitric acid and2.5+0.5weight percent of sodium dichromate.The parts shall be immersed for a minimum of20 min at a temperature in the range from120to130°F(49to 54°C).
6.1.1.2Nitric2—The solution shall contain20to45volume percent of nitric acid.The parts shall be immersed for a minimum of30min at a temperature in the range from70to 90°F(21to32°C).
6.1.1.3Nitric3—The solution shall contain20to25volume percent nitric acid.The parts shall be immersed for a minimum of20min at a temperature in the range from120to140°F(49 to60°C).
6.1.1.4Nitric4—The solution shall contain45to55volume percent of nitric acid.The parts shall be immersed for a minimum of30min at a temperature in the range from120to 130°F(49to
54°C).
6.1.1.5Nitric5—Other combinations of temperature,time, and concentration of nitric acid,with or without other chemi-cals,including accelerants,inhibitors,or proprietary solutions, capable of producing parts that pass the specified test require-ments.
6.2Water Rinse—Immediately after removal from the pas-sivating solution the parts shall be thoroughly rinsed,using stagnant,countercurrent,or spray washes singly or in combi-nation,with or without a separate chemical treatment for neutralization(see9.1)of the passivation media,with afinal rinse being carried out using water with a maximum total solids content of200ppm.
7.Treatments in Citric Acid
7.1Passivation Treatment:
7.1.1Stainless steel parts shall be treated in one of the following aqueous solutions and maintained within the speci-fied temperature range for the specified time.
7.1.1.1Citric1—The solution shall contain4to10weight percent of citric acid.The parts shall be immersed for a minimum of4min at a temperature in the range from140to 160°F(60to71°C).
7.1.1.2Citric2—The solution shall contain4to10weight percent of citric acid.The parts shall be immersed for a minimum of10min at a temperature in the range from120to 140°F(49to60°C).
7.1.1.3Citric3—The solution shall contain4to10weight percent of citric acid.The parts shall be immersed for a minimum of20min at a temperature in the range from70to 120°F(21to49°C).
7.1.1.4Citric4—Other combinations of temperature,time, and concentration of citric acid,with or without other chemi-cals to enhance cleaning,including accelerants,inhibitors,or proprietary solutions capable of producing parts that pass the specified test requirements.
7.1.1.5Citric5—Other combinations of temperature,time, and concentrations of citric acid,with or without other chemi-cals to enhance cleaning,including accelerants,inhibitors,or proprietary solutions capable of producing parts that pass the specified test requirements.Immersion bath to be controlled at
a pH of1.8–2.2.
7.2Water Rinse—Immediately after removal from the pas-sivating solution,the parts shall be thoroughly rinsed,using stagnant,countercurrent,or spray washes,singly or in combi-nation,with or wit
hout a separate chemical treatment for neutralization of the passivation media(see9.2),with afinal rinse being carried out using water with a maximum total solids content of200ppm.
8.Treatments in Other Chemical Solutions,Including
Electrochemical Treatments
8.1It is recognized that the purpose of removal of all exogenous matter from a stainless steel surface,including the removal of free iron,can be accomplished by different media, with potential for benefits to be gained from use of proprietary skills and art,including proprietary passivation media.Such treatments may include externally applying an electrical po-tential on the stainless steel parts,as in the case of electropol-ishing.The suitability of such passivation treatments for use in meeting the requirements of this specification shall be deter-mined by the capability of the processed parts meeting the specified test requirements.
8.2Stainless steel parts shall be treated in a specified aqueous solution,with or without externally applied electrical potential,and maintained within a specified temperature range for a time sufficient for the processed parts to meet the specified test requirement.
8.3Water Rinse—Immediately after removal from the pas-sivating solution,the parts shall be thoroughly rinsed,using stagnant,countercurrent,or spray washes,singly or in combi-nation,with or without a separate chemical treatment for neutralization of the passivation media(see9.2),with afinal rinse being carried out using water with a maximum total solids content of200ppm.
9.Neutralization
9.1The chemical reactions of the passivating media on the surface of the stainless steel shall be stopped by rinsing of the stainless steel part,with or without a separate neutralization treatment.
9.2The suitability of a neutralization procedure is deter-mined by the capability of the processed parts meeting the specified test requirements.(See Note1.)
N OTE1—The selection of medium and procedures for a neutralization depends of the chemistry of the passivation and on economic consider-ations.An example of a neutralizing treatment would be immersion of the part for a minimum of30min in a solution of5%NaOH at160to180°F (71to82°C),followed by a water rinse.
10.Post-Cleaning Treatments
10.1Although the passivefilm characteristic of stainless steel will form spontaneously in air or any other oxygen-containing environment,the processor shall,when specified, apply a chemical treatment that will accelerate the formation of the passivefilm on a chemically clean stainless steel surface. An example of a medium that serves to accelerate the forma-tion of the passivefilm but does not contribute to the removal of free iron from the stainless steel surface would be an aqueous solution of sodium dichromate.
10.2When specified,within one hour after thefinal water rinse as required in6.2,7.2,or8.3,all ferritic and martensitic steel parts shall be immersed in an aqueous solution containing 4to6weight percent of sodium dichromate at a temperature in the range from140to160°F(60to71°C)for a minimum of30 min,followed by a rinse in accordance with6.2,7.2,or8.3. The parts shall then be thoroughly dried.
10.3The purchaser may specify other post-cleaning treat-ments.
11.Finish
11.1The passivated parts shall exhibit a chemically clean surface and shall,on visual inspection,show no etching, pitting,or frosting resulting from the passivation procedures.
12.Testing Agency
12.1When required,the purchaser shall be permitted to perform such inspections as necessary to determine that the testing agency is capable of performing the specified
test.
13.Lot,Frequency of Testing,and Selection of Test
13.1Definition of Lot—A lot shall consist of one of the following,at the option of the processor:
13.1.1The passivated parts of similar alloy and manufac-turing methods that are pretreated and passivated in a single day or within a time frame that will ensure consistent passiva-tion results;
13.1.2The passivated parts of the same product of one size from one heat in one shipment;or
13.1.3When few parts are involved,the passivated parts from an entire production run.
13.2Unless a greater frequency of testing is specified on the purchase order,one test per lot shall be sufficient.
13.3One or more of the following tests,when specified on the purchase order,shall be performed on each lot of stainless steel parts.Not all of the following tests are suitable for all grades of stainless steel.(See Note2.)
13.3.1Practice A—Water Immersion Test,
13.3.2Practice B—High Humidity Test,
13.3.3Practice C—Salt Spray Test,
13.3.4Practice D—Copper Sulfate Test,and
13.3.5Practice E—Potassium Ferricyanide–Nitric Acid Test.
N OTE2—Some of the tests may produce positive indications not associated with the presence of free iron on the stainless steel surface.An example would be application of Practice C on some lesser-alloyed martensitic or ferritic stainless steels.
14.Practice A—Water Immersion Test
14.1This test is used for the detection of free iron or any other anodic surface contaminants on stainless steel.
14.2The sample representing the lot of passivated parts shall be alternately immersed in a non-rusting tank of distilled water for1h and allowed to dry in air for1h.This cycle shall be repeated a minimum of twelve times.
14.3The tested sample shall not exhibit rust or staining attributable to the presence of free iron particles embedded in the surface.
15.Practice B—High Humidity Test
15.1This test is used for the detection of free iron or any other anodic surface contaminants on stainless steel.
15.2The test shall be performed using a humidity cabinet capable of maintaining the specified test conditions.
15.3The sample representing the lot of passivated parts shall be cleaned by immersion in acetone or methyl alcohol or by swabbing with a clean gauze saturated with acetone or methyl alcohol,and dried in an inert atmosphere or desiccated container.The cleaned and dried part shall be subjected to97 63%humidity at10065°F(3863°C)for a minimum of 24h.
15.4The tested sample shall not exhibit rust or staining attributable to the presence of free iron particles imbedded in the surface.
16.Practice C—Salt Spray Test
16.1This test is used for the detection of free iron or any other anodic surface contaminants on stainless steel.
16.2The sample representing the lot of passivated parts shall be tested by the salt spray test conducted in accordance with Practice B117for a minimum of2h using a5%salt solution.
16.3The tested sample shall not exhibit rust or staining attributable to the presence of free iron particles imbedded in the surface.
17.Practice D—Copper Sulfate Test
17.1This test is recommended for the detection of free iron on the surface of austenitic stainless steels in the200and300 series,precipitation hardened stainless steels,and ferritic400 series stainless steels having a minimum of16%chromium. This test is not recommended for martensitic400series stainless steels or for ferritic400series stainless steels with less than16%chromium because these steels will give a positive indication irrespective of the presence or absence of anodic surface contaminants.This test shall not be applied to parts to be used in food processing.
17.2The test solution is prepared by dissolving4g of copper sulfate pentahydrate(CuSO4·5H2O)in250mL of distilled water to which1mL of sulfuric acid(H2SO4,sp gr 1.84)has been added.Aqueous copper sulfate solutions more than two weeks old shall not be used for this test.
17.3The test solution is swabbed on the surface of the sample representing the lot of passivated parts,applying additional solution as needed to keep the surface wet for a period of at least6min.At the end of this period,the surface shall be carefully rinsed and dried with care taken not to disturb copper deposits if present.
17.4The tested sample shall not exhibit copper deposits.
18.Practice E—Potassium Ferricyanide–Nitric Acid Test 18.1This test is recommended when detection of very small amounts of free iron is required.It is recommended for detection of free iron on austenitic200and300series stainless steels.This test is not recommended for detection of free iron on ferritic or martensitic400series stainless steels,because these steels will give a positive indication irrespective of the presence or absence of anodic surface contaminants.This test shall not be applied to parts to be used in food processing. 18.2The test solution is prepared by adding10g of chemically pure potassium ferricyanide to500mL of distilled water,adding30mL of70%nitric acid,agitating until all of the ferricyanide is dissolved,and diluting to1000mL with distilled water.The test solution shall be mixed fresh on the day of the test.
18.3The test solution is swabbed on the surface of the sample representing the lot of passivated parts.The formation of a dark blue color within30s denotes the presence of metallic iron.
18.4The tested sample shall not exhibit the dark blue color indicative of free iron on the surface.
18.5When the test is negative,the surface shall be thor-oughly washed with warm water to removal all traces of the test solution.When the test is positive,the dark blue stain shall be removed with a solution of10%acetic acid and8%oxalic acid,followed by a thorough hot water
rinse.
19.Practice F—Free Iron Test
19.1This test is used for the detection of free iron on the surface of stainless steel.It is especially useful for large parts that have been uniformly cleaned but that are inconvenient for reasons of size of equipment or ease of handling of the part to place in the environments defined in Practice A(Section14)or Practice B(Section15).Unless otherwise specified by the purchaser,the number of tests and the locations of the tests shall be at the option of the processor to assure a representative testing of the part.
19.2The test is performed by placing a clean cloth pad that has been thoroughly soaked with distilled or deminaralized water on the surface of the part at a part temperature of50°F (10°C)or greater for a period of not less than60minutes.The cloth shall be in contact with the steel for an area of at least20
square inches(130cm2).The pad shall be maintained wet through the test period,either by a method of retarding external evaporation,by the further addition of potable water,or by backing the pad with a sponge or similar water source.The cloth pad used shall be used for only one such test,being changed for each test so as to avoid risk of contamination. After removal of the cloth pad,the surface of the part shall be allowed to dry in air before inspection.
19.3The tested part shall not exhibit rust or staining attributable to the presence of free iron particles embedded in the surface.
20.Rejection and Retest
20.1Any lot failing to meet the specified test requirements of the purchase order shall be rejected.A rejected lot may,at the option of the processor,be re-passivated,with or without re-pretreatment,and then be retested.The number of samples tested from a lot subject to retest shall be twice the original specified test frequency,to the limit of the number of pieces in the lot.All samples must pass the specified acceptance criterion for the specified test for the retested lot to be accepted. 21.Precision and Bias
21.1No statement is made concerning either the precision or bias of Practices A,B,C,D,and E becaus
e the results state merely whether there is conformance to the criteria for success specified in the procedure.
22.Certification
22.1When specified in the purchase order,a report of the practice and tests used,including the record of process condi-tions when specified in accordance with 3.1.1.2,shall be supplied to the purchaser.
APPENDIX
(Nonmandatory Information)
X1.INFORMATION REGARDING PASSIV ATION TREATMENTS
N OTE X1.1—The following information is based on a section of Federal Specification QQ-P-35C(Oct.28,1988)identified as information of a general or explanatory nature that may be helpful,but is not mandatory. Minor changes have been made in the text to facilitate references to the main document and to correct technical inaccuracies.
X1.1Intended Use—The passivation treatments provided by this specification are intended to improve the corrosion resistance of parts made from stainless steels of all types.
X1.1.1During processing operations such as forming,ma-chining,tumbling,and lapping,iron particles or other foreign particles may become smeared over or imbedded into the surface of stainless steel parts.These particles must be re-moved or they will appear as rust or stain spots.This condition may be prevented by chemically treating the parts to remove the iron particles or other foreign particles,and then allowing the passivefilm to form on the cleaned surface,with or without chemical enhancement of the formation of this oxidefilm.
X1.1.2This specification is not intended for the black oxide coating of parts typically used for photographic or optical instruments.
X1.2Ordering Data—Purchasers should select the pre-ferred options permitted by this specification and include the following information in the purchase order:
X1.2.1Title,number,and date of this specification;
X1.2.2Identification of material by type and applicable product specification;
X1.2.3Test practices to be imposed(see1.4);
X1.2.4Definition of lot size,if other than described in this specification;and
X1.2.5Required documentation,if other than the minimum required by this specification.
X1.3Grades of Stainless Steel—Different types of stainless steel are selected on a basis of properties required,for example, corrosion resistance and design criteria,and fabrication re-quirements.Table X1.1is a compilation that serves as a guide for the selection of passivation treatment for different grades, but is far from complete either in grades or in passivation treatments.
X1.4Clean Water—Clean water is defined as water con-taining a maximum total solid content of200ppm.Rinsing can be accomplished by a combination of stagnant,countercurrent or spray rinses,or both,prior tofinal rinse.
X1.5Chemically Clean Surface—A chemically clean sur-face is defined as a surface upon which water,when applied momentarily to the surface,will remain on that surface in an even,continuousfilm,and in addition is free of any foreign material or residualfilm deposit which would be detrimental to the quality of the
part.
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