B633 specification covers material and process requirements for electrodeposited zinc coatings applied to iron or steel articles to protect them from corrosion. The coatings are provided in four standard thickness classes below table, in the as-plated condition or with one of five types of supplementary finishes below table. High strength metals, unless otherwise specified, including high strength steels having a tensile strength greater than 1700 MPa (247 ksi, 46HRC) shall not be electroplated.
| Classification Number and Conversion Coating Suffix | Service Condition | Thickness, min μm |
|---|---|---|
| Fe/Zn 25 | SC 4 (very severe) | 25 |
| Fe/Zn 12 | SC 3 (severe) | 12 |
| Fe/Zn 8 | SC 2 (moderate) | 8 |
| Fe/Zn 5 | SC 1 (mild) | 5 |
| Type | Description | Minimum Salt Spray h |
|---|---|---|
| I | As-plated without supplementary treatments | |
| II | With colored chromate coatings | 96 |
| III | With colorless chromate conversion coatings | 12 |
| IV | With phosphate conversion coatings | |
| V | With colorless passivate | 72 |
| VI | With colored passivate | 120 |
1. The coatings shall be non-alloyed zinc produced by electrodeposition.
2. Defects in the surface of the basis metal, such as scratches, porosity, pits, inclusions, cracks, roll marks, and die marks, may adversely affect the appearance and performance of coatings applied there to despite the observance of the best electroplating practices. Accordingly, the electroplater’s responsibility for defects in the coating resulting from such conditions shall be waived, except when he is the prime contractor supplying electroplated parts. In this event, the basis metal shall be subjected to such polishing or buffing operations as are necessary to yield deposits with the desired final luster and appearance. To minimize problems of this sort, the specifications covering the basis material on the item to be electroplated shall contain appropriate limitations to such basis metal conditions.
3. Cleaning of Basis Metal—Proper preparatory procedures and thorough cleaning of the basis metal are essential to ensure satisfactory adhesion and corrosion resistance performance of the coating. It is recommended that the following appropriate recommended practices and guides be used: B183, B242, B254, B320, and B322.
4. Pretreatment of Iron or Steel for the Purpose of Reducing the Risk of Hydrogen Embrittlement—All steel parts having an ultimate tensile strength greater than 1000 MPa (31 HRC) and that have been machined, ground, cold formed, or cold straightened, shall be heat treated for stress relief to reduce the risk of hydrogen embrittlement in the part before clean and electroplate processes. If these heat treatments are not required, the purchaser shall specify in the ordering information their exception. If the purchaser does not specify an exception to heat treatment, then the plater shall use Table 1 in B849 to determine the appropriate heat treatment for the steel based on its tensile strength.
5. Post Coating Treatments of Iron and Steel for the Purpose of Reducing the Risk of Hydrogen Embrittlement—All electroplated steel parts having a tensile strength greater than 1000 MPa (31 HRC) as well as surface hardened parts, shall be baked to reduce the risk of hydrogen embrittlement. If these heat treatments are not required, the purchaser shall specify in the ordering information their exception. If the purchaser does not specify an exception to heat treatment, then the plater shall use Table 1 in B850 to determine the appropriate heat treatment for the steel based on its tensile strength. The baking treatment shall be done before the application of the supplementary treatments and within 4 h of removal from the last process. Electroplated springs and other parts subject to flexure shall not be flexed before the hydrogen embrittlement relief treatment. Baked parts shall not crack or fail by fracture when tested in accordance with 10.4.
6. Reactivation Treatment—Electroplated surfaces passivated as a result of the baking operation shall
be reactivated before receiving a supplementary treatment.
NOTE 1—Surfaces should be activated as soon as possible following baking and handled carefully to avoid
contamination and maintain an active surface for post processing. Proprietary methods are available to
prepare the surface or a 2% v/v sulfuric acid in deionized water or a 7-10 g/L solution of sulfamic acid
in deionized water can be used.
7. Supplementary Treatments—The supplementary film treatment for Types II, III, V, and VI shall be in
accordance with Practice B201 (see Notes 2 and 3). The treatment required for conversion to Type IV
shall be in accordance with Guide D2092.
NOTE 2—The zinc surface is attacked by supplementary treatments, thereby diminishing the amount of
metallic zinc present. With Classes Fe/Zn25 and Fe/Zn12, this reduction is insignificant; but it is
significant with Fe/Zn8 and Fe/Zn5. Therefore, it is recommended that supplementary treatments not be
applied to zinc coatings having a nominal thickness less than 5 μm.
NOTE 3—Although Types V and VI are technically not “chromate” films and they do not contain leachable
hexavalent chromium ions, they are supplemental coatings that render the active zinc surface passive and
provide added protection to the steel part.
Thickness—The thickness shall be specified in accordance with 4.1 and 5.1 (see Note 2).
1. Significant Surfaces—Significant surfaces are areas where minimum thicknesses to be met shall be
designated on the applicable drawing or by the provision of a suitably marked sample. Significant
surfaces may be defined as those normally visible, directly or by reflection, which are essential to the
appearance or serviceability of the article when assembled in normal position or which are the source of
corrosion products that deface visible surfaces on the assembled article.
2. Surfaces on which the specified thickness of deposit cannot readily be controlled, such as threads,
holes, deep recesses, bases of angles, and similar areas, are normally exempt from minimum thickness
requirements, unless they are specially designated as not exempt. When such areas are designated, and
thus made subject to minimum thickness requirements, the purchaser and the manufacturer shall recognize
the necessity for either thicker deposits on other areas or for special racking.
NOTE 4—The dimensional tolerance of most threaded articles, such as nuts, bolts, screws, and similar
fasteners with complementary threads, normally does not permit the application of a coating thickness
much greater than 8.0 μm. If heavier coatings are required, allowance for the deposit buildup must be
made during the manufacture of the threaded articles.
Adhesion—The adhesion of the coating shall be such that when examined in accordance with 10.2, the coating shall not show separation from the basis metal at the interface.
Luster—Unless otherwise specified by the purchaser, a bright, semi-bright, or dull finish shall be acceptable.
Corrosion Resistance-Zinc coatings with Types II, III, V, and VI treatments shall show neither corrosion products of zinc nor basis metal corrosion products at the end of the test periods describe in Table 2 when tested by continuous exposure to salt spray in accordance with 10.3. The appearance of corrosion products when examined with 20/20 eyesight at normal reading distance shall be cause for rejection, except that white corrosion products 6 mm or less from the edges of specimens shall not constitute failure. For corrosion resistance requirements, see Table 2.
Workmanship—The surface of the electroplated article shall be uniform in appearance, free of visible coating defects, such as blisters, pits, roughness, nodules, burning, cracks, or unplated areas, and other defects that will affect the function of the coating. The coating shall not be stained or discolored. However, superficial staining that results from rinsing or slight discoloration resulting from any drying or baking operation to relieve hydrogen embrittlement, shall not be cause for rejection. On articles in which a visible contact mark is unavoidable, its position shall be that chosen by the purchaser. The electroplated article shall be clean and free of damage.
The purchaser and producer are urged to employ statistical process control in the coating process. Properly performed, statisitical process control will assure coated products of satisfactory quality and will assure the amount of acceptance inspection. The sampling plan used for the inspection of the quality coated article shall be agreed upon between the purchaser and producer.
1. When a collection of coated articles (inspection lot, see 8.2) is examined for compliance with the requirements placed on the articles, a relatively small number of the articles (sample) is selected at random and is inspected. The inspection lot is then classified as complying with the requirements based on the results of the inspection of the sample. The size of the sample and the criteria for compliance are determined by the application of statistics. The procedure is known as sampling inspection. Test Method B602, Guide B697, and Test Method B762 contain sampling plans that are designed for sampling inspection of coatings.
2. Test Method B602 contains four sampling plans, three for use with tests that are nondestructive and one when they are destructive. Test Method B602 provides a default plan if one is not specified.
3. Guide B697 provides a large number of plans and also gives guidance in the selection of a plan. Guide B697 provides a default plan if one is not specified.
4. Test Method B762 can be used only for coating requirements that have a numerical limit, such as coating thickness. The test must yield a numeric value and certain statistical requirements must be met. Test Method B762 contains several plans and also gives instructions for calculating plans to meet special needs. Test Method B762 provides a default plan if one is not specified.
5. Guide F1470 may be used for fasteners such as internally threaded, externally threaded, and nonthreaded fasteners and washers. This guide provides for two plans: one designated the “detection process” and one designated the “prevention process.” The purchaser and producer shall agree on the plan to be used.
An inspection lot shall be defined as a collection of coated articles that are the same kind, that have been produced to the same specification, that have been coated by a single supplier at one time or approximately the same time, under essentially identical conditions, and that are submitted for acceptance or rejection as a group.
Electroplated Parts or Separate Specimens—When the electroplated parts are of such form, shape, size, and value as to prohibit use thereof, or are not readily adaptable to a test specified herein, or when destructive tests of small lot sizes are required, the test shall be made by the use of separate specimens plated concurrently with the articles represented. The separate specimens shall be of a basis metal equivalent to that of the articles represented. “Equivalent” basis metal includes chemical composition, grade, condition, and finish of surface before electroplating. For example, a cold-rolled steel surface shall not be used to represent a hot-rolled steel surface. Due to the impracticality of forging or casting separate test specimens, hot-rolled steel specimens may be used to represent forged and cast steel articles. The separate specimens may also be cut from scrap castings when ferrous alloy castings are being electroplated. These separate specimens shall be introduced into a lot at regular intervals before the cleaning operations, preliminary to electroplating, and shall not be separated therefrom until after completion of electroplating. Conditions affecting the electroplating of specimens, including the spacing, plating media, bath agitation, temperature, etc., in respect to other objects being electroplated, shall correspond as nearly as possible to those affecting the significant surfaces of the articles represented. Unless a need can be demonstrated, separately prepared specimens shall not be used in place of production items for nondestructive and visual examinations.
Thickness and Adhesion Specimens—If separate specimens for thickness and adhesion tests are required, they shall be strips approximately 25 mm wide, 100 mm long, and 1 mm thick.
Corrosion Resistance Specimens—If separate specimens for corrosion resistance tests are required, they shall be panels not less than 150 mm long, 100 mm wide, and approximately 1 mm thick.
Hydrogen Embrittlement Specimens—If specimens are required, the configuration shall be that specified by the purchaser (see 9.1).
Thickness:
1. Unless otherwise specified, the thickness of the coating shall be determined by Test Methods B487,
B499, B504, B567, B568, or B748 as applicable.
NOTE 5—Thickness methods that rely on the purity of the zinc deposit
may not be sufficiently accurate when evaluating coatings produced from
non-cyanide zinc solutions. These methods would include B504, B567,
and B568.
2. MIL-STD-1312, Test 12, is not prohibited from being used for thickness measurements of electroplated
fastener hardware.
3. Other methods may be used if it can be demonstrated that the uncertainty of the measurement with
these methods is less than 10 %.
4. Make thickness measurements of zinc electroplatings, Types II, III, IV, V, and VI after application
of the supplementary treatments. When Test Methods B504, B567, or B568 are used, remove the
supplementary treatment prior to testing. The chromate, passivate, or supplemental film may be removed
from the underlying zinc coating by using a very mild abrasive (a paste of levigated alumina rubbed on
with a suitable applicator such as a swab). The phosphate coating may be removed from Type IV coating by
a concentrated (28 %) ammonia solution that quickly dissolves the phosphate coating but does not attack
the underlying zinc.
Adhesion—Determine adhesion by any suitable procedure in accordance with Practice B571.
Corrosion Resistance—When specified in the contract or purchase order, determine the corrosion
resistance in accordance with Practice B117. Subject the selected samples to the salt spray test; the
length of time to be applicable for the type of supplementary coating shall be in accordance with the
requirements of 7.4. To secure uniformity of results, age Types II, III, V, and VI supplementary
coatings at room temperature for 24 h before subjection to the salt spray. The salt spray test shall
commence within 72 h of the completion of the aging period.
NOTE 6—Salt spray testing is much more severe on chromated zinc deposits when these deposits are
subjected to greater than 15° from vertical in the chamber as specified in B117 and very severe on
horizontal surfaces. For these reasons a flat test panel should be used to evaluate the process
capability and the angle should be controlled to ensure consistent results.
Hydrogen Embrittlement Relief—When specified in the contract or purchase order, prepare and test the satisfactory behavior of parts to indicate freedom from hydrogen embrittlement.
Visual Examination—Examine material for compliance with the requirements of luster (7.3) and workmanship (7.5) after electroplating.
The producer or supplier shall be responsible for the performance of all inspection requirements as specified herein. Except as otherwise specified in the contract or order, the supplier may use his own or any other facilities suitable for the performance of the inspection requirements specified herein, unless disapproved by the purchaser. The purchaser retains the right to perform any of the inspections and tests set forth in this specification where such inspections and tests are deemed necessary to ensure that supplies and services conform to the prescribed requirements.
Coatings not conforming to this specification or to authorized modification shall be rejected. They may be reconsidered for inspection in accordance with Test Method B602.
The purchaser may require in the purchase order or contract that the producer or supplier provide certification that the finish was produced and tested in accordance with this specification and found to meet the requirements. The purchaser may similarly require that a report of the test results be furnished.
When specified by the purchaser, the test report shall include the following information:
1. A reference to this specification,
2. A reference to the test method(s) used (see Section 10),
3. The location(s) of the test area(s) on each specimen,
4. The quantity of specimens tested,
5. The name of the operator and the testing laboratory,
6. The date(s) on which the test(s) was (were) performed,
7. Any circumstances or conditions thought likely to affect the results or their validity,
8. Any deviation from the test method specified, and
9. The results of the test(s) (see Section 7).
Preservation, packaging, and packing methods for zinc-electroplated parts or articles used by a supplier shall be such as to preclude damaging during shipment and handling.
Electrodeposited coatings, zinc; electrodeposited coatings, passivates, chromates.
1. The service life of zinc coating is a function of its thickness and the type of environment to which it is exposed. While the conditions of exposure and uses of electroplated metal are so varied that it is not definitely possible to predict the exact life of articles protected by a coating of a given thickness, those using zinc coatings can draw on the wealth of practical experience at hand, supplemented by results of corrosion tests carried out over the years. As a result of large-scale and long-range tests conducted by ASTM and other organizations, there is excellent information on the corrosion behavior of zinc and zinc coatings. The following data, based on worldwide testing, can be used to compare the behavior of electrodeposited coatings of zinc in various atmospheres. The values are only relative, since individual studies in various parts of the world have resulted in figures which vary widely from these averages.
| Atmosphere | Mean Corrosion Rate |
|---|---|
| Industrial | 5.6 μm/year |
| Urban nonindustrial or marine | 1.5 μm/year |
| Suburban | 1.3 μm/year |
| Rural | 0.8 μm/year |
| Indoors | considerably less than 0.5 μm/year |
NOTE- The mean corrosion rate given pertains to zinc only and does not include a corrosion rate when zinc is passivated or in contact with other materials.
2. Example Of Appropriate Service Conditions And Description Of Service Conditions
SC 4—Very Severe—Exposure to harsh conditions, or subject to frequent exposure to moisture, cleaners, and saline solutions, plus likely damage by denting, scratching, or abrasive wear. Examples are: plumbing fixtures, pole line hardware.
SC 3—Severe—Exposure to condensation, perspiration, infrequent wetting by rain, and cleaners. Examples are: tubular furniture, insect screens, window fittings, builder’s hardware, military hardware, washing machine parts, bicycle parts.
SC 2—Moderate—Exposure mostly to dry indoor atmospheres but subject to occasional condensation, wear, or abrasion. Examples are: tools, zippers, pull shelves, machine parts.
SC 1—Mild—Exposure to indoor atmospheres with rare condensation and subject to minimum wear or abrasion. Examples are: buttons, wire goods, fasteners.
ASTM Standards:
A591/A591M Specification for Steel Sheet, Electrolytic.
Zinc-Coated, for Light Coating Weight [Mass] Applications
(Withdrawn 2005).
B117 Practice for Operating Salt Spray (Fog) Apparatus.
B183 Practice for Preparation of Low-Carbon Steel for Electroplating.
B201 Practice for Testing Chromate Coatings on Zinc and Cadmium
Surfaces.
B242 Guide for Preparation of High-Carbon Steel for Electroplating.
B254 Practice for Preparation of and Electroplating on Stainless Steel.
B320 Practice for Preparation of Iron Castings for Electroplating.
B322 Guide for Cleaning Metals Prior to Electroplating.
B374 Terminology Relating to Electroplating.
B487 Test Method for Measurement of Metal and Oxide Coating Thickness by
Microscopical Examination of Cross Section.
B499 Test Method for Measurement of Coating Thicknesses by the Magnetic
Method: Nonmagnetic Coatings on Magnetic Basis Metals.
B504 Test Method for Measurement of Thickness of Metallic Coatings by the
Coulometric Method.
B567 Test Method for Measurement of Coating Thickness by the Beta Backscatter
Method.
B568 Test Method for Measurement of Coating Thickness by X-Ray
Spectrometry.
B571 Practice for Qualitative Adhesion Testing of Metallic Coatings.