3.1.3 Method 2 – Passivation in Citric Acid
3.1.3.1 Bath Composition
Passivation shall be accomplished by immersion in an aqueous solution of 4 to 10 weight percent anhydrous citric acid (see 8.10). Additives such as wetting agents, inhibitors, and algicide or fungicide may be used as applicable. Other additives, such as ammonium hydroxide, may be used in order to stabilize or fix the pH of the bath as applicable.
3.1.3.2 Operating Conditions
Bath temperature shall be 70 to 160 °F (21 to 71 °C), with an immersion time of not less than 4 minutes for baths operating over 140 °F (60 °C), not less than 10 minutes for baths operating in the range of 120 to 140 °F (49 to 60 °C), not less than 20 minutes for baths operating in the range of 100 to 119 °F (38 to 48 °C), or not less than 30 minutes for baths operating below 100 °F (38 °C).
3.1.4 Rinsing
Immediately after removal from the passivating solution, the parts shall be thoroughly rinsed.
3.1.4.1 The final rinse shall be carried out in clean water (see 8.12), followed by drying.
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3.1.5 Post-Treatment
When post-treatment is specified, directly after rinsing, parts made from ferritic, martensitic, or precipitation-hardening steels shall be immersed in a solution containing 4 to 6% by weight of sodium dichromate dihydrate (Na2Cr2O7·2H2O) at 140 to 160 °F (60 to 71 °C) for 1 hour, followed by rinsing and drying.
3.1.5.1 When post-treatment is not specified, see 8.2 for guidance.
3.2 Properties
3.2.1 Corrosion Resistance
Parts shall meet one of the following conditions or, when specified, a test in AMS-STD-753. When a test is not specified, any of these tests may be used. Parts containing 0.85% carbon or more (such as SAE 440C) shall be exempt from these tests.
3.2.1.1 Humidity Test
Parts shall be free from visible red rust after exposure to 95% minimum relative humidity at 100 to 115 °F (38 to 46 °C) for not less than 24 hours.
3.2.1.2 Water Immersion Test
Parts shall be free from visible red rust after alternately immersing in deionized or distilled water having an initial conductivity of 5 μS/cm or less for at least 1 hour and allowing to dry in room temperature air for at least 1 hour, until 12 cycles of immersion and drying have elapsed (see 8.13).
3.2.1.3 Copper Sulfate Test
A test solution containing 8 g of copper sulfate (CuSO4·5H2O) and 2 to 3 mL of sulfuric acid (H2SO4, sp. gr. 1.84) in 500 mL of high-purity water having a conductivity of 5 μS/cm or less (see 8.14) shall be applied to the part, or the part shall be immersed in the test solution. The part’s surface shall be kept wet for 6 to 6.5 minutes. Rinse and dry the surface without disturbing any deposits. Parts shall be free of any copper-colored deposits, which indicate the presence of unacceptable free iron. The effectiveness of copper sulfate solutions shall be validated by one of the following methods:
- Periodic chemical analysis in accordance with 4.2.2.1.
- Verification before use. A test specimen of any convenient size, made from any carbon or low-alloy steel (e.g., 1018 or 4130) and properly cleaned, shall exhibit a copper-colored deposit when subjected to the test above. Verification does not need to be performed more than once in any production day.
3.2.1.3.1 The copper sulfate test is not recommended for use on martensitic 400 series alloys or for use on ferritic 400 series alloys containing less than 16% chromium because a copper deposit can result even though the passivation treatment was adequate.
3.2.1.4 Salt Spray Test
Parts shall withstand exposure to 2 hours minimum in a salt spray environment operated in accordance with ASTM B117. Parts shall be free from visible red rust following completion of the test.
3.2.1.5 Ferroxyl Test
Parts shall not show any traces of iron or iron oxide when the ferroxyl test is performed in accordance with ASTM A380. This is a hypersensitive test that may be used when traces of free iron or iron oxide might be objectionable.
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3.2.2 Surface Appearance
After completion of processing, there shall be no evidence of etching, pitting, smutting, frosting, dimensional changes, or other chemical attack on the parts when visually examined without aid of magnification. However, loss of temper color is acceptable.
3.3 Written Procedure
All processing and testing shall be done in accordance with a written procedure acceptable to the cognizant engineering organization (see 4.4.3).
- QUALITY ASSURANCE PROVISIONS
4.1 Responsibility for Inspection
The processor shall supply all test specimens for the processor’s tests and shall be responsible for the performance of all required tests. When parts are to be tested, such parts shall be supplied by the purchaser and, if acceptable after testing, may be included with the lot of processed parts. The cognizant engineering organization reserves the right to perform any confirmatory testing deemed necessary to ensure that processing conforms to specified requirements.
4.2 Classification of Tests
4.2.1 Acceptance Tests
4.2.1.1 Classes 1, 2, and 4
Corrosion resistance (see 3.2.1) and surface appearance (see 3.2.2) are acceptance tests and shall be performed on each lot.
4.2.1.2 Class 3
Surface appearance (see 3.2.2) is an acceptance test and shall be performed on each lot.
4.2.2 Periodic Tests
4.2.2.1 Compositions of passivating and post-treatment solutions are periodic tests and shall be performed at a frequency selected by the processor (see 8.7).
4.2.2.2 Class 3 Parts
Corrosion resistance (see 3.2.1) is a periodic test and shall be performed at a frequency selected by the processor.
4.2.3 Preproduction Tests
All technical requirements (see 3.2 and 3.3) of this specification are preproduction tests and shall be performed prior to production and when the cognizant engineering organization deems confirmatory testing is required.
4.3 Sampling for Testing
Sampling for testing shall not be less than the following: A lot shall be all parts of the same part number, processed in the same set of solutions within a 24-hour period, and presented for the processor’s inspection at the same time. Tested parts shall be selected randomly from all parts in each lot. Where parts are not available for testing, as in the case of large parts or parts that might be damaged by such testing, identically processed specimens fabricated from the same alloy as the parts represented may be used (see 8.8).
4.3.1 Class 1 Parts
The minimum number of parts or specimens selected for testing shall be as shown in Table 2.
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Table 2 – Number of parts to be tested
Number of Parts in Lot
Surface
Appearance
Corrosion-Resistance Test
1 to 6
All
2
7 to 15
7
2
16 to 40
10
3
41 to 50
15
3
51 to 110
15
5
111 to 150
25
8
151 to 500
35
8
501 to 700
50
13
701 to 1200
75
13
Over 1200
125
13
4.3.2 Class 2 parts or specimens shall be corrosion-resistance tested at a frequency of one part per lot and visually examined at the frequency given in Table 2.
4.3.3 For Class 3 parts or specimens, sampling shall be established by the processor.
4.3.4 Class 4 parts or specimens shall be selected for testing as shown in Table 3.
Table 3 – Number of parts to be tested for Class 4
Number of Parts in Lot(1)
Surface Appearance
Corrosion-Resistance Test(2)
1 to 13
All
All
14 to 1200
13
13
1201 to 35000
50
13
35001 to 500000
80
50
(1) For Class 4, a lot shall consist of one of the following:
a. Parts of similar alloy and manufacturing methods that are pretreated and passivated in one day’s production or within a time frame that will ensure consistent passivation results.
b. The same product of one size from one heat in one shipment.
c. When the quantity of passivated parts in one day’s production does not warrant daily testing, the lot size shall be as agreed upon by the cognizant engineering organization and the processor.
(2) Identically processed specimens, made from the same alloys used to fabricate the parts, may be used for test purposes. The test specimens shall be randomly distributed throughout the lot during processing. When multiple tests are to be performed, separate specimens are required for each test.
4.4 Approval
4.4.1 The process and control procedures, a preproduction processed part, or both, whichever is specified, shall be approved by the cognizant engineering organization before production parts are supplied.
4.4.2 If the processor makes a significant change to any material, process, or control factor that was used for process approval, all preproduction tests shall be performed and the results submitted to the cognizant engineering organization for process reapproval, unless the change is approved by the cognizant engineering organization. A significant change is one which, in the judgment of the cognizant engineering organization, could affect the properties or performance of the parts.
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4.4.3 Control factors shall include, but are not limited to, the following:
Method(s) for precleaning in preparation for passivating (see 3.1.1)
Composition and composition limits of each processing solution
Temperature and temperature limits of each processing solution
Immersion time and time limits of the process for each processing solution
Test(s) used for corrosion-resistance testing (see 3.2.1)
Periodic test plan (see 4.2.2 and 8.7)
4.5 Reports
The processor shall furnish with each shipment a report stating that parts have been processed and tested in accordance with the specified requirements and that they conform to acceptance test requirements. Where post-treatment is used, the report shall so indicate that it was completed. The report shall state the method and, if applicable, type of passivation used, as well as the class and corrosion-resistance test(s) used. This report shall also include AMS2700G, the purchase order number, lot number(s), part number(s), and quantity of parts processed.
4.6 Resampling and Retesting
4.6.1 If any part subjected to surface appearance testing fails to meet requirements, that part shall be subject to rejection, and all parts in the lot shall be visually examined for conformance to surface appearance requirements or be subject to rejection.
4.6.2 If any part subjected to corrosion-resistance testing fails to meet requirements, that part shall be subject to rejection. The balance of the lot may be reprocessed and retested using the sampling defined by Table 2 for the original number of parts in the lot, or all of the remaining parts in the lot shall be tested. - PREPARATION FOR DELIVERY
Packages of passivated parts shall be prepared for shipment in accordance with commercial practice and in compliance with applicable rules and regulations pertaining to the handling, packaging, and transportation of the processed parts. - ACKNOWLEDGMENT
A processor shall mention this specification number and its revision letter in all quotations and when acknowledging purchase orders. - REJECTIONS
Parts that are not processed in accordance with the requirements of this specification, or are processed with modifications not authorized by the cognizant engineering organization, will be subject to rejection. - NOTES
NOTICE
This specification may reference the use of substances, products, or processes that are restricted or banned by local (regional) chemical substance regulations. Users of this specification should consider the implications of local legislation on the products, substances, and processes referred to within the document.
8.1 Revision Indicator
A change bar (I) located in the left margin is for the convenience of the user in locating areas where technical revisions, not editorial changes, have been made to the previous issue of this document. An (R) symbol to the left of the document title indicates a complete revision of the document, including technical revisions. Change bars and (R) are not used in original publications, nor in documents that contain editorial changes only.
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8.2 When post-treatment is not specified, parts should be neutralized, preferably in a solution of 2 to 5% sodium hydroxide, followed by rinsing and drying.
8.3 Dimensions and properties in inch/pound units and the Fahrenheit temperatures are primary; dimensions and properties in SI units and the Celsius temperatures are shown as the approximate equivalents of the primary units and are presented only for information.
8.4 These processes have been used primarily to enhance the corrosion resistance of corrosion-resistant steel alloys, but the passivation process has also been successfully and historically applied to nickel-chromium high-temperature alloys and other nonferrous alloys for removal of free iron resulting from machining or other processing. Different types of smeared metal on the corrosion-resistant surfaces, or the presence of other surface treatments such as plating or braze filler metals, may dictate the use of either nitric acid or citric acid as applicable to the specific case.
8.5 It is recommended that this process be used prior to heating corrosion-resistant steel parts to temperatures exceeding 1200 °F (649 °C) to prevent diffusion of contaminant free iron from the surface into the near-surface layer of parts.
8.6 This document does not address methods for removal of scale or foreign materials from the surfaces of parts prior to passivation. Methods for accomplishing this may be found in such other documents as ISO 8074, ISO 8075, or ASTM A380.
8.7 ARP4992 is recommended to satisfy the requirements for control of processing solutions.
8.8 “Identically processed” as used in 4.3 refers to such operations as machining, grinding, heat treating, welding, media finishing, and similar processes.
8.9 Passivating should be accomplished after completion of all manufacturing and inspection operations that could affect the passivity of the surface of the material, including but not limited to: forming, turning, milling, heat treatment, shot peening, media finishing, penetrant testing, or magnetic testing.
8.9.1 Reworking operations that expose new surfaces or could affect the passivity of the surface should result in the subsequent re-passivation of the area (see 8.11.4).
8.9.2 Where other surface-altering operations are performed, such as electroplating or nitriding, the purchaser should specify at which point the passivation operation is accomplished within the manufacturing sequence.
8.9.3 Carburized and nitrided surfaces should not be passivated. Passivation solutions will severely corrode or pit nitrided surfaces. On carburized surfaces, the chromium combines with the carbon to form chromium carbides on the surface, which does not benefit from passivation. These surfaces should either be passivated prior to surface hardening or masked prior to passivation.
8.10 Method 1 solutions may be made up and maintained with nitric acid at other than the specified 42 degree Baumé if the concentration is adjusted to compensate. Method 2 solutions may be made up and maintained with citric acid monohydrate if the formulation is adjusted accordingly to the equivalent amount of 4.4 to 10.9 weight percent. Method 2 solutions may be made up and maintained with citric acid stock solutions of a higher concentration when they are diluted appropriately.
8.11 Guidelines for alternative passivation solutions that may be useful to the cognizant engineering organization are as follows:
8.11 Guidelines for alternative passivation solutions that may be useful to the cognizant engineering organization are as follows:
8.11.1 Bath Composition
Passivation has been accomplished by immersion in a bath of an aqueous solution of 20 to 55% by volume of 42 degree Baumé (sp. gr. 1.4) nitric acid (HNO3) (see 8.10).
8.11.1.1 It is recommended that the concentration of the nitric acid be above 40% for free machining steels.
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8.11.1.2 Where the acid concentration is less than 35% by volume and for ferritic and martensitic steels, it is recommended that additional oxidizers be added to the bath in the form of 2 to 6% by weight of sodium dichromate dihydrate (Na2Cr2O7·2H2O).
8.11.1.3 For the purpose of removing lead alloys from surfaces, molybdic acid (HMoO3) may be added to the bath at a concentration of up to 0.35 weight percent.
8.11.2 Operating Conditions
Bath temperature should be in the range of 70 to 155 °F (21 to 68 °C) with an immersion time of not less than 30 minutes for baths operating at temperatures below 100 °F (38 °C), not less than 20 minutes for baths operating at temperatures below 125 °F (52 °C), or not less than 10 minutes for baths operating at temperatures above 125 °F (52 °C).
8.11.3 For certain high-carbon corrosion-resistant steels, such as SAE 440C, it may be desirable to passivate with the parts anodic for 2 to 3 minutes at 2 to 3 V to prevent etching.
8.11.4 For local areas or where immersion is impractical, passivation has been accomplished by the use of thickened passivation solutions, such as pastes or gels, to maintain contact with stainless steel surfaces for the required time.
8.12 It has been found that water containing up to 200 ppm total dissolved solids may be considered to be clean, but this limit is not a requirement. Rinsing may be accomplished with stagnant, countercurrent, and/or spray rinses.
8.13 Distilled water may give more consistent results in the water immersion test compared to deionized water. Water used in the water immersion test will not maintain its initial conductivity during the test.
8.14 High-purity water may be prepared by distillation, ion exchange, continuous electrodeionization, reverse osmosis, electrodialysis, or a combination thereof.
8.15 It has been found that iron concentration in the passivating solution exceeding 2 weight percent may reduce the ability to remove iron contamination from parts.
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