Fastener Coatings and Platings: Designation, Thickness and Durability Baselines

What a Coating Designation Fixes

A coating on a threaded fastener is specified as a designation in its own right, appended to the fastener designation. GB/T 5267.1-2023 §9.2 states that the electroplating designation is appended to the fastener designation under the general fastener designation method, that the data fields are separated by a solidus, and that the field order follows Table 11 of the same document.

Table 11 of GB/T 5267.1-2023 (PDF p.22-23) splits the designation into five elements: the basis metal (steel carries no code), the metal layer, the conversion coating or passivation, the sealer or surface coating, and an additional lubricant. Table 12 of the same document (PDF p.23) fixes the order in which those fields appear in an order designation: metal layer, thickness, conversion coating, sealer or surface coating or lubricant, neutral salt spray duration to red rust, and a torque-clamping force requirement expressed as a total coefficient of friction or as a K value where one applies. A footnote to Table 12 permits the thickness field to be replaced by the minimum neutral salt spray duration, in which case the thickness is left out of the designation.

The conversion coating carries a two-part code. GB/T 5267.1-2023 Table 13 (PDF p.24) lists An, Cn, Fn and Gn for bright, iridescent, black and yellow films, plus U for no conversion coating, and a footnote to that table states the naming rule: the first letter is the conversion coating type, and the second letter, n, records that the film contains no hexavalent chromium. Table 14 of the same document lists the parallel set for films that do contain hexavalent chromium, coded A for bright, C for iridescent, D for opaque olive green and F for black. Table 15 (PDF p.24-25) covers the outer layer: T0 for no sealer and no surface coating, T2 for a sealer, T7 for a surface coating, T4 for added lubricant, and nL for the absence of lubricant.

Worked examples in GB/T 5267.1-2023 §9.3 (PDF p.25) show the assembled form: a designation ending -GB/T5267.1/Zn8/An; one ending -GB/T5267.1/Zn12/Cn/T2; one ending -GB/T5267.1/ZnNi/An/720h, where the thickness field is replaced by a 720 h neutral salt spray requirement; and one ending -GB/T5267.1/Zn12/Fn/T7/T4(μtot 0.10-0.20), where the trailing bracket carries the torque-clamping force requirement.

GB/T 5267.1-2023 §9.2 states that stress relief and baking are not entered in the designation and are specified separately.

Annex F of GB/T 5267.1-2023 is informative, and it exists to decode older drawings rather than to specify new work: the annex opens by stating that the earlier code system is not to be used for new applications.

In the older form recorded by GB/T 5267.1-2023 Annex F, table F.2 codes the metal or alloy as a single letter (A for zinc) and table F.3 codes the minimum thickness as a single digit (2 for 5 µm), so the code A2L reads as a zinc coating of 5 µm minimum thickness plus a surface-treatment letter.

§1 of GB/T 5267.1-2023 sets the metal layer at one of four — zinc, zinc-nickel, zinc-iron and cadmium — and Table 1 of the same document (PDF p.10-11) gives the use codes P for corrosion protection, F for functional properties and D for decorative properties; a footnote records that cadmium is restricted or prohibited in many applications and that the cadmium coatings retained are used mainly in military and aerospace work. The scope covers steel fasteners, with the dimensional requirements also applying to copper and copper-alloy fasteners, over bolts, screws, studs and nuts with standard threads as well as fasteners with non-standard threads and non-threaded fasteners such as washers, pins, clamps and rivets; a note records that electroplating of stainless steel fasteners serves lubrication and the avoidance of thread galling; and the section carries a precedence statement that the requirements of GB/T 5267.1-2023 for electroplated fasteners take precedence over other documents that deal with electroplating.

Electroplated Zinc Systems and Salt Spray Baselines

Table 8 of GB/T 5267.1-2023 (PDF p.17) is the designation-to-duration matrix for zinc-based electroplated systems.

Within GB/T 5267.1-2023 Table 8 the two groups are read differently: the column for corrosion of the coating metal itself, which appears as white corrosion, carries one figure per system, while the column for corrosion of the basis metal, which appears as red rust, is split into three figures by coating thickness — 5 µm, 8 µm and 12 µm.

Coating system as listed	Symbol as printed	White corrosion, min	Red rust min at 5 µm	at 8 µm	at 12 µm	Stated in
Zinc, clear passivation	Zn//An/T0	8 h	48 h	72 h	96 h	GB/T 5267.1-2023 Table 8
Zinc, iridescent passivation	Zn//Cn/T0	72 h	120 h	192 h	240 h	GB/T 5267.1-2023 Table 8
Zinc, iridescent passivation, sealed	Zn//Cn/T2	120 h	168 h	240 h	288 h	GB/T 5267.1-2023 Table 8
Zinc, black passivation, sealed	Zn//Fn/T2	24 h (white haze allowed)	72 h	144 h	192 h	GB/T 5267.1-2023 Table 8
Zinc-iron, iridescent passivation	ZnFe//Cn/T0	96 h	144 h	216 h	264 h	GB/T 5267.1-2023 Table 8
Zinc-iron, iridescent passivation, sealed	ZnFe//Cn/T2	120 h	216 h	288 h	360 h	GB/T 5267.1-2023 Table 8
Zinc-iron, black passivation, sealed	ZnFe//Fn/T2	96 h (white haze allowed)	192 h	240 h	312 h	GB/T 5267.1-2023 Table 8
Zinc-nickel, silver-grey passivation	ZnNi//Cn/T0	120 h	480 h	720 h	720 h	GB/T 5267.1-2023 Table 8
Zinc-nickel, silver-grey passivation, sealed	ZnNi//Cn/T2	168 h	600 h	720 h	720 h	GB/T 5267.1-2023 Table 8
Zinc-nickel, black passivation	ZnNi//Fn/T0	48 h (white haze allowed)	360 h	600 h	720 h	GB/T 5267.1-2023 Table 8
Zinc-nickel, black passivation, sealed	ZnNi//Fn/T2	120 h (white haze allowed)	480 h	720 h	720 h	GB/T 5267.1-2023 Table 8

The figures sit inside stated conditions. In GB/T 5267.1-2023 §3 the values apply to systems with a chromium(VI)-free passivation film, in the coated condition, and the test is run 24 h after plating; the coated condition is defined there as the state after every plating operation, including sealer, surface coating and lubricant, and before the handling, conveying, sorting, storage and transport steps that reduce protection.

A footnote to GB/T 5267.1-2023 Table 8 records that the figures are for barrel-plated coatings and that rack plating reduces coating damage and raises corrosion resistance, and the same page states that if the test is run in the as-received condition the table figures do not apply and serve only as a starting point.

Four footnotes to Table 8 carry boundaries that travel with the numbers. Footnote a states that the minimum neutral salt spray duration is not to be treated as a direct basis for the corrosion resistance of electroplated fasteners in every environment it may meet, and points to GB/T 10125-2021.

Footnote c to Table 8 of GB/T 5267.1-2023 states that black spots do not impair corrosion resistance and are not a cause for rejection. Footnote d states that white haze is not counted as white corrosion, which is what the cells marked in the table allow.

Footnote e to Table 8 of GB/T 5267.1-2023 states that higher corrosion resistance is usually obtainable and that the corrosion test duration set by the document is 720 h, so 720 h is the ceiling printed in the table.

GB/T 5267.1-2023 §4.1 lists the thickness grades of the document as 3, 5, 8, 10, 12, 15, 20, 25 and 30 µm in Table 10, each carrying the nominal figure as its minimum local thickness, and only three of those grades appear in Table 8.

Table C.1 of GB/T 5267.1-2023 is informative and covers zinc coatings whose chromate conversion film contains hexavalent chromium. The same table is read in the two-group form used for Table 8: at a coating thickness of 12 µm with film code D, the table lists 96 h to white corrosion and 168 h to red rust.

GB/T 5267.1-2023 §3.2 records two boundaries for that table — chromate conversion treatment is not intended for new application areas, and a footnote states that thin coatings of 5 µm or less weaken the performance of the chromate film.

Coating thickness is measured by one of five methods. GB/T 5267.1-2023 §6.4 lists X-ray spectrometry, coulometry (not applicable when an additional non-conductive layer is present), metallography at any point on the fastener, the magnetic method applied before the sealer and lubricant are in place, and phase-sensitive eddy current, and states that in case of dispute the metallographic method is the one to use.

Two numeric limits sit on the plated thread itself. GB/T 5267.1-2023 §6.2 states that the coating thickness allowance applies to standard threads in external positions g, f and e and internal positions G or H, that the coating on an external thread is not to go beyond the zero line taken at basic size h, and that the coating on an internal thread is not to fall below the zero line taken at basic size H.

GB/T 5267.1-2023 §6.2 then sets a GO-gage limit: over the distance from the thread end to 1d, a GO ring gage on a bolt, screw or stud may need a maximum of 0.001d³ N·m, and a GO plug gage on a nut a maximum of 0.001D³ N·m.

GB/T 5267.1-2023 Table 9 tabulates that limit by diameter — 1.0 N·m at M10, 1.7 N·m at M12 and 4.1 N·m at M16.

Annex D of GB/T 5267.1-2023 is informative and relates coating thickness to thread allowance. A theoretical coating thickness t raises the pitch diameter of an external thread by 4t, so table D.1 gives 48 µm for t = 12 µm; table D.2 gives the maximum coating thickness each pitch position can hold, at 6 µm for position g, 10 µm for f and 15 µm for e at P = 1 (M6 and M7), and at 20 µm, 23 µm and 29 µm respectively at P = 6 (M64).

The same annex quantifies the skin effect: at a nominal length above 5d, the local thickness at mid-length of a bolt is one third to one half less than at the reference area.

Hydrogen embrittlement is governed by hardness and by property class. GB/T 5267.1-2023 §4.4 states that three conditions must be present together — high tensile strength or hardness, tensile stress including residual stress, and atomic hydrogen — and Table 2 of the same document bands the response by hardness: at 360 HV and below, class A, with no supplementary process verification, no product test and no baking; above 360 HV up to 390 HV, class B; above 390 HV, class C, with baking required and its temperature and time stated.

Table 3 keys those bands to the property classes of GB/T 3098.1-2010: below 10.9 is class A, 10.9 is class B at the manufacturer’s choice, and 12.9 is class C with baking.

GB/T 5267.1-2023 Annex B.4 states that zinc-electroplated fasteners above 390 HV are normally baked at 190 °C to 220 °C for at least 8 h to 10 h, that the period may extend to 24 h depending on type, size, hardness and coating system, that zinc-plated parts are normally kept at or below 220 °C and cadmium-plated parts at or below 200 °C, and that baking zinc-plated parts at 185 °C to 195 °C for 4 h is not enough to remove hydrogen.

Cathodic protection is stated as a condition on the coating metal. GB/T 5267.1-2023 §5.1 states that zinc, zinc-alloy and cadmium coatings have a lower potential than the basis metal, which is the precondition for cathodic protection, and that metals whose potential is higher than steel — nickel, copper and silver among them — cannot provide cathodic protection and accelerate corrosion instead once the coating is damaged or pitted.

The coefficient of friction can be made a requirement of the order. GB/T 5267.1-2023 §7.4 places torque-clamping force, where it is required, on an agreed test method and expresses it as μtot or as a K value, and Table 12 of the same document allows that requirement to be carried in the plating designation, as the (μtot 0.10-0.20) example in §9.3 shows. For torque values by size and property class, see the site’s bolt torque chart.

The Salt Spray Method Behind the Hours

GB/T 10125-2021 covers three methods — neutral salt spray (NSS), acetic acid salt spray (AASS) and copper-accelerated acetic acid salt spray (CASS) — and its §1 states the boundary that matters whenever a salt spray hour figure is read: the methods serve quality control and lot comparison, and they are not for putting different materials in corrosion-resistance order or for predicting long-term corrosion resistance. The same section notes that specimen dimensions, product test period and interpretation are fixed by the product specification, not by this document.

The running conditions are fixed numerically. GB/T 10125-2021 §2 (PDF p.9) gives sodium chloride at 50 g/L ± 5 g/L dissolved in distilled or deionised water of conductivity at or below 20 µS/cm at 25 °C ± 2 °C, with a relative density of 1.029 to 1.036 and total impurities at or below 0.5 percent.

The collected solution at 25 °C ± 2 °C is held at pH 6.5 to 7.2 for NSS, 3.1 to 3.3 for AASS and 3.1 to 3.3 for CASS, and GB/T 10125-2021 §2 adds copper(II) chloride dihydrate at 0.26 g/L ± 0.02 g/L to the CASS solution.

GB/T 10125-2021 Table 3 (PDF p.13) sets chamber temperature at 35 °C ± 2 °C for NSS, 35 °C ± 2 °C for AASS and 50 °C ± 2 °C for CASS, with a mean sedimentation rate of 1.5 mL/h ± 0.5 mL/h over an 80 cm² horizontal area for all three.

GB/T 10125-2021 §6.4.2 holds spray pressure between 70 kPa and 170 kPa, usually 98 kPa ± 10 kPa, and Table 1 (PDF p.10) gives the saturator water temperature against that pressure — at 98 kPa, 48 °C for NSS and AASS and 64 °C for CASS.

§11.1 of GB/T 10125-2021 (PDF p.13) lists the test periods as 2 h, 6 h, 24 h, 48 h, 96 h, 168 h, 240 h, 480 h, 720 h and 1008 h, and §11.2 holds the total opening time of the chamber to no more than 1 h per day.

GB/T 10125-2021 §9.2 places the test surface at 15° to 25° from the vertical, 20° where the geometry allows, and §9.4 requires inert non-metallic supports and suspension that is not metallic.

Table 2 (PDF p.12) gives the permitted mass loss for steel reference specimens — 70 g/m² ± 20 over 48 h for NSS, 40 g/m² ± 10 over 24 h for AASS and 55 g/m² ± 15 over 24 h for CASS — with the check carried out at intervals of about three months under GB/T 10125-2021 §7.

GB/T 10125-2021 §6.6 states that a chamber previously used for AASS or CASS is not to be used for NSS without thorough cleaning and verification, and §12.2 sets the post-test treatment: indoor drying for 0.5 h to 1 h, rinsing in clean running water at or below 40 °C, then air drying at or below 200 kPa from about 300 mm.

Hot-Dip Galvanizing: Thickness Set by Basis-Metal Thickness

GB/T 13912-2020 covers hot-dip galvanized coatings on steel and iron articles, fasteners among them, and adopts coating thickness as the acceptance quantity in two forms. §3 defines local thickness as the arithmetic mean of the readings taken within one basic measurement surface, and mean thickness as the arithmetic mean of the local thicknesses of the sampled articles or batch, so an order is judged against a local minimum and a mean minimum together.

Table 3 of GB/T 13912-2020 (PDF p.12) gives the minimum figures for articles that are not centrifuged, keyed to the thickness of the steel:

Article and steel thickness	Local min	Mean min	Stated in
Steel thicker than 6 mm	70 µm	85 µm	GB/T 13912-2020 Table 3
Steel over 3 mm and up to 6 mm	55 µm	70 µm	GB/T 13912-2020 Table 3
Steel from 1.5 mm to 3 mm	45 µm	55 µm	GB/T 13912-2020 Table 3
Steel thinner than 1.5 mm	35 µm	45 µm	GB/T 13912-2020 Table 3
Cast iron 6 mm and thicker	70 µm	80 µm	GB/T 13912-2020 Table 3
Cast iron thinner than 6 mm	60 µm	70 µm	GB/T 13912-2020 Table 3

Table 4 of GB/T 13912-2020 (PDF p.12) gives the parallel set for articles that are centrifuged — the condition in which threaded parts are usually processed, to clear zinc from the threads.

Under GB/T 13912-2020 Table 4, threaded parts with a diameter above 6 mm are set at a local minimum of 40 µm and a mean minimum of 50 µm, and threaded parts with a diameter of 6 mm or less at 20 µm and 25 µm; other articles, cast iron included, at 45 µm and 55 µm where the wall is 3 mm or thicker, and at 35 µm and 45 µm below that.

The measurement rules are fixed in GB/T 13912-2020 §6 (PDF p.10-12). A basic measurement surface is not smaller than 10 cm², and at least five magnetic readings are taken within one basic measurement surface and averaged; an individual reading below the table figure is permitted as long as the average of that surface reaches it.

Measurement under GB/T 13912-2020 §6 is not taken on cut faces, within 10 mm of an edge, on flame-cut faces or at corners.

Where there is a dispute, the gravimetric result expressed as coating mass per unit area takes precedence over the other thickness methods, and the conversion between coating mass and thickness uses a nominal coating density of 7.2 g/cm³.

Adhesion is normally not tested, or tested by agreement between supplier and purchaser, and an article may show dark grey or light grey colour variation as long as the thickness figures are met.

Black Oxide Coatings: Five Classes

MIL-DTL-13924F is dated 10 May 2023 and supersedes the preceding edition of 12 September 2018. It covers inorganic black coatings on ferrous metals — carbon, low-alloy and corrosion-resistant steels and wrought iron — and §1.1 states the level of protection the coating gives: limited corrosion protection under mildly corrosive conditions.

MIL-DTL-13924F §1.2 divides the coatings into five classes by process and by the material each one is for. Class 1 is black oxide by an alkaline oxidizing process, for wrought iron, cast and malleable irons and plain carbon or low-alloy steels.

Under MIL-DTL-13924F §1.2, Class 2 is black oxide by an alkaline chromate oxidizing process, for corrosion-resistant steel alloys tempered below 900 °F (482 °C).

Class 3 is black oxide by a fused salt oxidizing process, for corrosion-resistant steel alloys tempered at 900 °F (482 °C) or higher (MIL-DTL-13924F §1.2).

Class 4 is black oxide by an alkaline oxidizing process, for other corrosion-resistant steel alloys, and Class 5 covers black nitriding processes.

Process conditions are numeric. MIL-DTL-13924F §3.4 states that Class 1 coatings are formed from a boiling chrome-free alkaline oxidizing solution and Class 2 coatings are oxidized with chromated oxidizing compounds.

MIL-DTL-13924F §3.5 limits the molten oxidizing salt of Class 3 to not more than 900 °F (482 °C), with the treated parts cooled for eight to ten minutes and rinsed in water at 190 °F (88 °C).

MIL-DTL-13924F §3.4.3 sets the chromic acid dip used for Classes 2 and 3 at a minimum of 30 seconds in a 0.06 percent solution held at 150 to 190 °F (66 to 88 °C) and pH 2 to 3.

MIL-DTL-13924F Table I gives the working windows for the two alkaline baths: 285 to 305 °F (141 to 152 °C) for 5 to 60 minutes for the bath used on carbon and low-alloy steels, and 250 °F ± 10 °F (121 °C ± 5 °C) for 30 to 45 minutes for the alkaline chromate bath.

Appearance and verification follow from MIL-DTL-13924F §3.7 and §3.8. The colour of a finished part is to be similar to charcoal or soot black; a slight amount of smut inherent in the process is not a cause for rejection, while spottiness or streaking is unsatisfactory and grounds for reprocessing.

Classes 1, 2, 3 and 5 must pass the oxalic acid spot test before any preservative is applied, and MIL-DTL-13924F §4.4.2 describes that test as three drops of a five percent oxalic acid solution observed for 30 seconds and up to eight minutes, judged against the three comparison figures in the specification.

One durability figure is stated for a black coating. MIL-DTL-13924F §3.9, read with §4.4.3, requires the black coating on AISI type 300 series corrosion-resistant steel — Class 4 only — to show no signs of corrosion after 96 hours of exposure in a 5 percent salt spray (fog) test, applied before any preservative is in place.

MIL-DTL-13924F §6.3.3 adds the boundary that cast and malleable irons and martensitic 400 series corrosion-resistant steels can be processed in Class 4 proprietary baths but will not meet that salt spray requirement.

Hydrogen embrittlement is handled at both ends of the process. MIL-DTL-13924F §3.2.1 requires objectionable residual stress in ferrous alloy parts at Rockwell C 39 or greater to be relieved by heat treatment after forming and hardening and before cleaning and coating, and §3.10 requires a hydrogen embrittlement relief treatment for all classes on steel parts that are surface or through hardened at 39 HRC and above, carried out after the conversion coating and before any supplementary preservative.

MIL-DTL-13924F §6.1 carries the caution that high-strength steel at 39 HRC or greater may be subject to caustic embrittlement that could lead to spontaneous cracking if under internal or applied stress during the blackening treatment.

MIL-DTL-13924F §6.3.1 states that the coating produces no appreciable dimensional change, so the dimensions on a drawing are the dimensions after coating, and §4.2.1 fixes a lot as no more than 8 hours of continuous production.

Phosphate Coatings: Type M and Type Z

MIL-DTL-16232H is dated 26 March 2021 and carries Amendment 1; it supersedes the version of 15 September 2020. It covers two types of heavy phosphate coating for ferrous metals, applied by immersion — Type M on a manganese phosphate base and Type Z on a zinc phosphate base — each divided into classes 1 to 4 by the supplementary treatment it carries (§1.2). §6.1 states the intent: the heavy phosphate coating is a base that holds and retains a supplementary coating, and that supplementary coating supplies the major part of the corrosion resistance.

Coating weight is the primary acceptance quantity. MIL-DTL-16232H §3.5 sets Type M at a minimum of 16 g/m², or 11 g/m² where the order specifies the lower figure, and Type Z at a minimum of 11 g/m², both measured before any supplementary treatment is applied.

MIL-DTL-16232H §4.3.2 fixes a lot as items treated under the approved process control ranges during one shift in the same baths, or, where continuous or conveyor equipment is used, a maximum of four consecutive hours of production.

The durability requirements are stated per type and class, all of them in the salt spray test on the coated item free of supplementary treatment (MIL-DTL-16232H §3.6).

MIL-DTL-16232H §3.7.2 requires a Type M class 2 coating, treated with the specified lubricating oil, to show no signs of corrosion for a minimum of 48 hours. §3.7.4 puts Type Z class 2 at a minimum of 72 hours.

MIL-DTL-16232H §3.7.3 and §3.7.5 put Type M class 4 and Type Z class 4 at a minimum of 72 hours and state that the duration applies only if a supplementary treatment is applied.

Relief treatments are set by hardness. MIL-DTL-16232H §3.3 requires thermal stress relief before coating, at 177 to 204 °C (350 to 400 °F) for a minimum of four hours, for parts at Rockwell C 39 or greater that were machined, ground, cold formed or cold straightened after heat treatment, and at 104 to 155 °C (225 to 275 °F) for a minimum of eight hours for carburized parts.

MIL-DTL-16232H §3.4 sets the hydrogen embrittlement relief for Type Z and Type M coatings on steels at Rockwell C 39 or greater at 98 to 107 °C (210 to 225 °F) for a minimum of eight hours, or alternatively a rest at room temperature for 120 hours.

Where that relief is tested, MIL-DTL-16232H §4.7.2 specifies a sustained tensile load of 75 percent of the ultimate notched tensile strength of the material, held for a minimum of 200 hours and then examined for cracks at 10x magnification, with 4340 steel at Rockwell C 51 to 54 accepted for the worse case.

The finish stages are fixed in MIL-DTL-16232H §3.2.5 and §3.2.6. A non-chrome sealer rinse is applied to the manufacturer’s instructions; a chromic acid rinse may stand in its place when the drawing or contract calls for it, at 63 to 93 °C (150 to 200 °F), at approximately 300 grams of chromic acid flake per 1000 litres of water, held at pH 2 to 4, with a minimum of 60 seconds in each rinse and the final rinse checked at least every 8 hours.

Service boundaries come from MIL-DTL-16232H §6.1.1 and §6.1.2. MIL-DTL-16232H §6.1.1 records that Type M coatings resist alkaline environments more than Type Z and are not to be exposed above 121 °C (250 °F).

Type Z coatings are not to be used where contact with alkaline materials or exposure above 93 °C (200 °F) is expected; and MIL-DTL-16232H §6.1.1.d states that a class 4 coating reduces the variation in torque versus tension values usually found with threaded fasteners.

A parallel statement on the same coatings stands in another document, quoted here separately because it sets a different kind of limit.

MIL-STD-889C Appendix A §A.3.5 (22 August 2016) lists heavy phosphate conversion coating, described there as post treated with a supplemental treatment, among the protective treatments for carbon and low-alloy steels, and states that it is not for steels of strength greater than 220 ksi; that zinc phosphate coatings are prohibited for steels of strength greater than 220 ksi; and that for steels of strengths between 150 and 220 ksi stress relief is required before phosphating and hydrogen embrittlement relief after phosphating.

The two documents are read side by side, not in place of one another: MIL-DTL-16232H sets coating weight and salt spray durations by type and class, while MIL-STD-889C Appendix A sets a strength ceiling on the use of phosphate on steel.

Passivation of Corrosion-Resistant Steel

QQ-P-35C is dated 28 October 1988 and supersedes the preceding edition of 5 April 1973. It covers passivation treatments as well as the cleaning and descaling that precede them, for austenitic, ferritic, martensitic and precipitation-hardening corrosion-resistant steels (§1.2). §1.2.1 lists four active treatment types, numbered II, VI, VII and VIII, and §6.9 records that types I, III, IV and V were withdrawn in this revision.

Each active type is a solution and a cycle. QQ-P-35C §3.3.2 sets Type II at 20 to 25 percent by volume nitric acid with 2.5 ± 0.5 percent by weight sodium dichromate, for 20 minutes at 120 to 130 °F (49 to 54 °C).

QQ-P-35C §3.3.6 sets Type VI at 25 to 45 percent by volume nitric acid, for 30 minutes at 70 to 90 °F (21 to 32 °C).

QQ-P-35C §3.3.7 sets Type VII at 20 to 25 percent by volume nitric acid, for 20 minutes at 120 to 140 °F (49 to 60 °C). §3.3.8 sets Type VIII at 45 to 55 percent by volume nitric acid, for 30 minutes at 120 to 130 °F (49 to 54 °C).

Table I of the same specification restates those figures in one place; the clause text of QQ-P-35C §3.3.7 and Table I differ in the upper temperature for Type VII, and the figure printed in the clause text is 140 °F.

The document places the active types by alloy rather than offering one solution for all. §3.3.2.1 states that Type II is the treatment for the 440 series, for straight-chromium martensitic grades with 12 to 14 percent chromium, and for corrosion-resistant steels containing relatively large amounts (0.15 percent) of sulfur or selenium, naming 303, 303Se, 347Se, 416, 416Se, 430F and 430FSe among them. §3.3.6.1 places austenitic 200 and 300 series chromium-nickel and chromium grades of 17 percent chromium or greater, with the exception of the 440 series, with Type VI. §3.3.8.1 places high-carbon high-chromium 400 series grades and precipitation-hardening stainless steels with Type VIII.

Two further stages and two definitions complete the treatment. QQ-P-35C §3.5 requires a chromate treatment for all ferritic and martensitic parts within one hour of the final water rinse, at 4 to 6 percent by weight sodium dichromate held at 140 to 160 °F (60 to 71 °C) for 30 minutes, followed by a clean water rinse and thorough drying.

QQ-P-35C §3.6 requires the passivated part to show a chemically clean surface, with no etching, pitting or frosting, and allows slight discoloration. §6.4 defines clean water as water with a total solids content of no more than 200 ppm.

QQ-P-35C §6.1.1 records the mechanism the treatment works on: iron particles smeared over or embedded in the surface during forming, machining, tumbling and lapping would appear as rust or stain spots, and the nitric acid and sodium dichromate solution dissolves them and allows a thin transparent passive film to form over the restored surface, so parts with newly formed passive films are to be handled as little as possible in the 24 hours after treatment.

Acceptance testing is stated in QQ-P-35C §3.7 and §4.4. §3.7.1 requires the passivated parts to show no staining attributable to embedded free iron in either a water immersion test or a high humidity test.

QQ-P-35C §3.7.2 permits a salt spray test or a copper sulfate test in addition to, or instead of, those two.

QQ-P-35C §3.7.2.1 permits the copper sulfate test as a substitute for the salt spray test on austenitic 300 series chromium-nickel steels, where the surface must show no copper deposits; and §4.4.2.1 conducts the salt spray test for a minimum of 2 hours with a 5 percent salt solution.

QQ-P-35C §4.4.1.2 sets the high humidity test at 95 to 100 percent humidity and 100 to 115 °F (38 to 46 °C) for 24 to 26 hours.

Three exclusions are recorded. §6.7 states that stainless steel parts with carburized surfaces cannot be passivated, because the carbon combines with the chromium to form chromium carbides at the surface. §6.8 states that nitrided stainless steel is not to be passivated, because the treatment severely corrodes the nitrided case, and that where passivation is required it must be performed before the nitriding treatment. §6.10 records that high-strength alloys such as 440C are subject to hydrogen embrittlement or intergranular attack when exposed to acids, and that cleaning by mechanical methods or other chemical methods is the route used for such parts; §6.1 adds that 440C grades selected for high hardness and wear resistance may be exempted from passivation at the discretion of the procuring activity.

Galvanic Couples Between Dissimilar Metals

MIL-STD-889C is dated 22 August 2016 and supersedes the preceding edition of 7 July 1976. It defines the terms the rest of this section uses. §3.1 defines metals as dissimilar when two metal specimens are in contact with each other, or otherwise electrically connected, in a conductive solution and capable of generating an electric current. §3.2 defines galvanic corrosion as the accelerated corrosion of the more active metal, the anode, of a dissimilar metal couple in an electrolyte solution or medium, together with decreased corrosive effect on the less active metal, the cathode, compared with the two metals unconnected in the same environment. §3.3 defines a galvanic series as a listing of metals and alloys by their order and tendency to corrode in a particular electrolyte, and records that the further apart two metals sit in the series the greater the difference in potential between them.

The design requirement is stated in §5.1: where dissimilar metals are used in intimate contact, protection against galvanic corrosion is to be applied, and the attention of the requirement is directed to the anodic member of the couple. §5.1.1 places Table II of the standard as the ordering tool for a sea water environment, from the more active (anodic) end down to the least active (cathodic) end, and records that a galvanic series belongs to one electrolyte, so a different service environment produces a different order. In Table II the list opens with magnesium, Mg alloy AZ-31B and zinc (hot-dip, die cast or plated) and closes with silver, gold and graphite.

§5.1.2 addresses metals that are widely separated in the series and lists the measures: a sacrificial metal coating applied to the cathodic member with a potential near that of the anodic member; sealing to make the faying surfaces watertight; painting or coating all surfaces to raise the resistance of the electrical circuit; or inserting an inert, non-absorbing barrier where the joint is not required to conduct. §5.1.3 turns to the area ratio and states that a small anodic area against a cathodic area is to be avoided, and that the same metal or a more noble metal is to be used for small fasteners and bolts. §5.2 gives the joining methods, and its first item carries the worked case: bolts or screws of stainless steel for fastening aluminium sheet, but not the reverse, so the cathodic area is the smaller one.

Two coating statements in the same standard belong with this section. MIL-STD-889C Appendix A §A.3.2 lists the protective treatments for zinc and zinc coatings, puts a chromate conversion coating followed by a paint or resin system at the top of that list, and carries the note that bare or plated zinc is not to be used in a marine environment.

MIL-STD-889C Appendix A §A.3.5, for carbon and low-alloy steels, records that metallic coatings on steels of strengths above 220 ksi are to be applied by non-electrolytic methods, and that steels up to 220 ksi may be coated electrolytically provided the steel is stress relieved before plating and hydrogen embrittlement relieved after plating.

The numeric table behind that ordering is a table of standard electrode potentials, and the standard itself states its limit.

Table B-I of MIL-STD-889C lists the standard electrode potential of each metal against its own ion: magnesium −2.37 V, aluminium −1.67 V, zinc −0.76 V, chromium −0.74 V, iron −0.44 V, cadmium −0.40 V, tin −0.14 V, lead −0.13 V, hydrogen 0.00 V, copper +0.34 V, silver +0.80 V, mercury +0.85 V, platinum +1.2 V and gold +1.69 V.

MIL-STD-889C Appendix B §B.3.4 states that those standard potentials are of little value in establishing galvanic corrosion relationships in actual environments, because a galvanic system is not at equilibrium and the reaction is controlled by the polarization of the anode, the cathode or both, and by the resulting current.

The same appendix gives the illustration: MIL-STD-889C Table B-I puts aluminium anodic to zinc, while the sea water series of Table II puts zinc anodic to aluminium.

Coating Families Side by Side

The two tables below place one figure from each coating family next to the document and clause that states it.

Coating family	Figure as stated	Stated in
Zinc electroplating, iridescent passivation, sealed	288 h to red rust at 12 µm; 120 h to white corrosion	GB/T 5267.1-2023 Table 8
Zinc-nickel electroplating, iridescent passivation, sealed	720 h to red rust at 8 µm and 12 µm; 168 h to white corrosion	GB/T 5267.1-2023 Table 8
Hot-dip galvanized, steel thicker than 6 mm, not centrifuged	local minimum 70 µm, mean minimum 85 µm	GB/T 13912-2020 Table 3
Hot-dip galvanized, threaded part above 6 mm diameter, centrifuged	local minimum 40 µm, mean minimum 50 µm	GB/T 13912-2020 Table 4
Black oxide, Class 4 on AISI type 300 series corrosion-resistant steel	no signs of corrosion after 96 h in a 5 percent salt spray test	MIL-DTL-13924F §3.9 with §4.4.3
Phosphate, Type M class 2	no signs of corrosion for a minimum of 48 h in the salt spray test	MIL-DTL-16232H §3.7.2
Phosphate, Type Z class 2	no signs of corrosion for a minimum of 72 h in the salt spray test	MIL-DTL-16232H §3.7.4
Passivated corrosion-resistant steel, where a salt spray test is specified	minimum of 2 h in a 5 percent salt solution	QQ-P-35C §4.4.2.1

Element of the classification	Documents it is stated in
Basis-metal hardness above which relief treatment is required	GB/T 5267.1-2023 §4.4 (360 HV and 390 HV bands); MIL-DTL-13924F §3.2.1 and §3.10 (Rockwell C 39); MIL-DTL-16232H §3.3 and §3.4 (Rockwell C 39)
Passivation film containing no hexavalent chromium	GB/T 5267.1-2023 Table 13, second letter n
Chromate conversion treatment restricted for new application areas	GB/T 5267.1-2023 §3.2, stated for the chromate systems of Table C.1; QQ-P-35C §3.5, which requires the chromate treatment for ferritic and martensitic parts as a stage of the treatment
Coating thickness grades tabulated	GB/T 5267.1-2023 Table 10 (3 to 30 µm); GB/T 13912-2020 Table 3 and Table 4 (by steel thickness and by threaded-part diameter)

The figures in these two tables are the figures printed by the document named in the same row. The documents do not cross-reference one another on these points, no figure here was converted from another, and no row asserts a rank between the rows above it.

What This Entry Does Not Cover

Coating selection is out of scope. Which of the systems above suits a given exposure, and any ordering between them, are not stated here: the documents consulted state requirements system by system rather than comparisons between systems, and GB/T 10125-2021 §1 states in its own words that salt spray methods are not for putting materials in corrosion-resistance order.

Two coating families are named but not covered. The introduction to GB/T 5267.1-2023 records that the same series of standards is planned in five parts, covering electroplating, non-electrolytic zinc-flake coatings, hot-dip galvanized coatings, passivation of corrosion-resistant stainless steels and thermal diffusion zinc coatings. Only the electroplating part was among the copies consulted here, so no figure for zinc-flake or thermal diffusion coatings appears; the hot-dip and passivation material in this entry comes from GB/T 13912-2020 and QQ-P-35C.

Organic finishing, application and acceptance are out of scope. Painting, powder coating and other resin systems are not treated beyond the places where MIL-DTL-16232H states that a supplementary treatment supplies the major part of the corrosion resistance and MIL-STD-889C Appendix A names one such system for a given steel. Nor are field application, touch-up, repair and workshop practice: the storage and handling conditions a source states are reported where the source states them and not extended beyond that. The figures here are the requirements the standards state, not measured results for any article, and they are not acceptance limits for an individual order — GB/T 5267.1-2023 §10 and MIL-DTL-16232H §6.2 both place the options that must be settled at the ordering stage.

Tables that belong to incorporated documents are out of scope. Limits of size and full thread tolerance zones are not reproduced here. GB/T 5267.1-2023 §6.2 states which external and internal tolerance positions the coating allowance applies to, but the limits behind those positions and the property-class tables behind its Table 3 sit in the incorporated documents, and no figure is taken from a document named by a clause but not among the copies consulted.

Every figure quoted in this entry is the figure printed in the edition named beside it: GB/T 5267.1-2023, GB/T 13912-2020, GB/T 10125-2021, MIL-DTL-13924F dated 10 May 2023, MIL-DTL-16232H dated 26 March 2021 with Amendment 1, QQ-P-35C dated 28 October 1988 and MIL-STD-889C dated 22 August 2016. A drawing that cites one of these designations under a different edition inherits whatever changed between the editions, and the distributed copies of the two federal specifications consulted carry a notice asking the reader to verify the current version at the source before use.

References

• GB/T 5267.1-2023 — Fasteners: Electroplated coatings, issued 23 May 2023, implemented 1 December 2023
• GB/T 13912-2020 — Metallic coatings: Hot dip galvanized coatings on fabricated iron and steel articles
• GB/T 10125-2021 — Corrosion tests in artificial atmospheres: Salt spray tests, issued 20 August 2021, implemented 1 March 2022
• MIL-DTL-13924F, 10 May 2023 — Coating, Inorganic, Black, for Ferrous Metals
• MIL-DTL-16232H with Amendment 1, 26 March 2021 — Phosphate Coating, Heavy, Manganese or Zinc Base
• QQ-P-35C, 28 October 1988 — Passivation Treatments for Corrosion-Resistant Steel
• MIL-STD-889C, 22 August 2016 — Dissimilar Metals