Coatings on metal fasteners

Transcription

1 BRITISH STANDARD Incorporating corrigendum no. 1 Coatings on metal fasteners Part 3: Specifications for electroplated zinc and cadmium coatings

2 This British Standard, having been prepared under the direction of the General Mechanical Engineering Standards Policy Committee, was published under the authority of the Standards Board and comes into effect on 15 September 1993 BSI 2007 The following BSI references relate to the work on this standard. Committee reference GME/9 Draft for comment 93/ DC ISBN Committees responsible for this British Standard The preparation of this British Standard was entrusted by the General Mechanical Engineering Standards Policy Committee (GME/-) to Technical Committee GME/9, upon which the following bodies were represented: BEAMA Ltd. British Constructional Steelwork Association Ltd. British Industrial Fasteners Federation British Railways Board British Steel Industry British Steel Industry (Wire Section) Gauge and Tool Makers Association Society of Motor Manufacturers and Traders Ltd. Washer Manufacturers Association of Great Britain The following bodies were also represented in the drafting of the standard, through subcommittees and panels: British Turned-parts Manufacturers Association Institute of Metal Finishing Metal Finishing Association Stainless Steel Fabricators Association of Great Britain National Centre of Tribology Amendments issued since publication Amd. No. Date Text affected 8181 Corrigendum no. 1 March 1994 Correction to Table C.1

3 Contents Committees responsible Foreword Page Inside front cover ii Specification 0 Introduction 1 1 Scope 2 2 References 2 3 Definitions 2 4 Application and type of coating 2 5 Coating 3 6 Chromate treatment 4 7 Suitability of coatings for components with parallel threads 4 8 Information to be documented and supplied to the coater 4 9 Identification 4 Annexes Annex A (normative) Coating thickness for threaded parts 7 Annex B (normative) Measurement of coating thickness 10 Annex C (informative) Guidance on procedures that may be adopted to accommodate thick coatings 11 Tables Table 1 Nominal coating thicknesses 3 Table 2 Coating designation and corrosion resistance 5 Table 3 Designation of chromate treatments 6 Table A.1 Upper limits of nominal coating thickness 8 Table C.1 Minimum fundamental deviation required to accommodate coatings too thick to be applied to standard threads. Metric threads 12 Figure Figure B.1 Measuring area for local coating thickness measurement on fasteners 11 List of references 13 i

4 ii Foreword This Part of BS 7371 has been prepared under the authority of the General Mechanical Engineering Standards Policy Committee to provide part of a series of standards on coatings on metal fasteners. The attention of user of this Part of BS 7371 is drawn to the restrictions placed on the marketing and use of cadmium and its compounds contained in the Environmental Protection (Controls on Injurious Substances (No. 4) Regulations 1992 [1]. WARNINGS. This British Standard calls for the use of substances and/or procedures that may be injurious to health if adequate precautions are not taken. It refers only to technical suitability and in no way absolves wither the designer, the producer, the supplier or the user from statutory obligations relating to health and safety at any stage of manufacture or use. Compliance with a British Standard cannot confer immunity from legal obligations.

5 0 Introduction BS 7371 deals with the performance and selection of coatings on metal fasteners and accessories. This Part of BS 7371 specifies the requirements for zinc and cadmium electroplated coatings. Other published Parts of BS 7371 are as follows: BS :1991 Specification for general requirements and selection guidelines 1) BS :1993 Specification for torque/clamping force relationship BS :1993 Specification for zinc flake non-electrolytically applied cured coatings Further Parts of the series to incorporate the following are in preparation: Specification for electroplated nickel, nickel/chromium and copper/nickel/chromium coatings Specification for electroplated tin and tin/lead coatings Specification for electroplated coatings for special purposes Specification for mechanically applied zinc coatings Specification for mechanically applied coatings for special purposes Specification for phosphate and oil coatings Specification for organic coatings Zinc and cadmium electroplated coatings covered by this Part of BS 7371 are for use on fasteners and accessories where the following apply: a) a coating given galvanic protection is required for ferrous fasteners; b) a substrate is required to improve the corrosion resistance of subsequent coatings on ferrous fasteners, e.g. organic; c) the risk of hydrogen embrittlement of high tensile steel fasteners is acceptable; d) the use of cadmium is acceptable. The coating has a silver appearance which can be modified to yellow, olive green or black by a chromate conversion (passivation) treatment applied to improve corrosion resistance. Additional coatings are available to further modify colour but these may influence coating performance, e.g. lubricity or electrical conductivity. The shape of certain parts may create difficulties in the application of a coating of uniform thickness and reference to the coating of uniform thickness and reference to the coater is recommended to enquire if a difficulty exists or if it can be overcome by the use of specialized processing methods. Typical difficulties as outlined in BS :1990 and BS :1990 arise when bulk processing: 1) long thin parts; 2) parts with flat surfaces that can stick together during processing, e.g. flat washers; 3) parts with recesses which do not receive the full coating; 4) parts with shapes that induce nesting and interlocking; 5) the internal surfaces of hollow parts. The coatings are suitable for application to metal components such as the following: i) parallel threaded fasteners and accessories; ii) screws that cut or form their own threads, e.g. self tapping screws (see for example BS 4174:1972); iii) woodscrews (see for example BS 1210:1963); iv) springs and spring components; v) pipe nuts and connectors with parallel or tapered threads. 1) This is supplementary to all other Parts of BS

6 When these coatings are applied to non-ferrous fasteners, the only part of the specification that will apply will be the coating thickness requirement. These coatings should not be specified for application to copper or copper alloys unless an undercoat is also specified, e.g. nickel. NOTE The coating processes covered by this standard may induce hydrogen embrittlement in certain ferrous fasteners. Attention is drawn to the appropriate requirements in BS : Scope This part of BS 7371 specifies requirements for coatings applied by the electrolysis of solutions of zinc or cadmium. The coatings consist principally of zinc or cadmium and can be lubricated to give controlled torque/clamping force properties when required (see for example BS :1993). In addition to the requirements in this Part of BS 7371, the requirements in Part 1 of the standard which are applicable to zinc and cadmium electroplated coatings apply. This Part of BS 7371 also specifies requirements for the following: a) the coating thickness; b) the type of chromate conversion treatment to be applied; c) quality assessment of the corrosion resistance of unused coated parts when subjected to neutral salt spray testing (see for example BS 7479:1991); d) adhesion test. Annexes include tables indicating suitable coating grades for standard screw threads and methods of determination of coating thickness. 2 References 2.1 Normative references This part of BS 7371 incorporates, by reference, provisions from specific editions of other publications. These normative references are cited at the appropriate points in the text and the publications are listed on the inside back cover. Subsequent amendments to, or revisions of, any of these publications apply to this Part of BS 7371 only when incorporated in it by updating or revision. 2.2 Informative references This Part of BS 7371 refers to other publications that provide information or guidance. Editions of these publications current at the time of issue of this standard are listed on the inside back cover, but reference should be made to the latest editions. 3 Definitions For the purposes of this Part of BS 7371, the definitions given in BS :1991 apply. 4 Application and type of coating 4.1 Cleaning and pretreatment Parts, fasteners and accessories shall be cleaned and pretreated in accordance with Clauses 4 and 5 of BS :1991. NOTE Restrictions applicable to the avoidance of embrittlement of high tensile parts should be observed. 4.2 Type of coating The coating shall be either silver in appearance or modified by passivation types C, D or Bk (see Clause 6). NOTE 1 The colour may be further modified by the addition of an organic top coat. The coating shall be electrically conductive and galvanically protective to steel under most climatic conditions. NOTE 2 The presence of passivates, particularly the coloured ones, decreases electrical conductivity at the surface. 2

7 Where specified by the purchaser (see item e) of Clause 8) the lubricity of the coating shall be modified by the addition of a lubricant and the torque/clamping force performance of a coated fastener shall conform to BS :1993. NOTE 3 The addition of an organic coating may affect dimensions, lubricity and electrical conductivity. 4.3 Application The coating of zinc or cadmium shall be applied by the electrolysis of an aqueous solution of a salt or salts of the appropriate metal. NOTE Stabilizers and brighteners may be added to the solution is necessary. 5 Coating 5.1 Surface condition The coating shall be clean, dry and of uniform appearance. All surfaces of a component shall be coated unless otherwise agreed. The coating shall be free from blisters or loosely adhering material. 5.2 Coating thickness The specified coating thickness (see item b) of Clause 8) shall be applied using the relationships between nominal, local and batch average thicknesses shown in Table 1. Coating thicknesses for threaded parts shall conform to Annex A. The coating thicknesses shall be measured by the methods described in Annex B. NOTE If a coating thickness other than those given in Table 1 is required, this should be agreed between the purchaser and the supplier. Nominal coating thickness 1) Table 1 Nominal coating thicknesses Effective coating thickness Minimum local thickness Thicknesses in 4m Batch average thickness Local thickness shall be measured on a significant surface (see Figure B.1). Exceeding the maximum batch average thickness shall not be a cause for rejection if the coated thread is still accepted by an appropriate go gauge, 4h for external threads and 4H for internal threads. min. max ) See Annex A. NOTE BS :1991 covers a range of thicknesses up to and including 12 4m. Greater thicknesses are given here as these may be ordered by the purchaser on agreement with the supplier 3

8 5.3 Corrosion resistance Unless otherwise agreed, representative samples from a batch shall be selected in accordance with Clause 7 of BS :1991 and shall be tested for resistance to neutral salt spray in accordance with BS 7479:1991. Coatings shall be graded by their corrosion resistance as given in Table 2. Surfaces which are not significant shall resist neutral salt spray for 50 % of the time given in Table 2. NOTE For routine quality control purposes, after the ability of the coating to conform to the corrosion test is established, evaluation of coating mass or thickness may be used, but for referee purposes the corrosion resistance requirements still apply. 5.4 Adhesion When tested in accordance with 2.1 of BS :1981 the metallic coating shall continue to adhere to basic metal. 6 Chromate treatment The chromate treatment (passivation) shall be designated by the symbols specified in Table 3. 7 Suitability of coatings for components with parallel threads NOTE 1 The suitability of the coatings specified in this Part of BS 7371 for parts with screw threads is limited by the clearance between external and internal threads. Maximum electroplating thickness to be applied to parts incorporating standard thread forms shall be as specified in Annex A, in order to minimize the risk of interference when both threaded elements of an assembly are similarly coated. NOTE 2 Annex C gives guidance for modifying thread dimensions to accommodate thicker coatings. NOTE 3 Modification of male thread dimensions from tolerance class 6g may influence the mechanical properties. 8 Information to be documented and supplied to the coater The following information shall be fully documented and supplied to the coater: 4 a) the number and date of this British Standard i.e. ; b) the nominal coating thickness (see Table 1) and the coating designation (see Table 2); c) the type of chromate conversion treatment required (see Clause 6); d) the hardness and property class of the component to be plated; e) if lubrication of the coating is required in accordance with BS :1993; f) the significant surfaces if not as defined in BS :1991; g) any additional protection required for delivery (see Clause 8 and Appendix D of BS :1991); h) the requirements of any subsequent operations (e.g. the application of an adhesive); i) any special requirements (e.g. selective plating: see Annex C). 9 Identification The coating shall be identified by the number and date of this British Standard, i.e. 2) together with the coating designation in accordance with Table 2. NOTE The chromate conversion treatment is included in the designation. A lubricated condition shall be designated by the letter L after the designation (see 4.2). Example A zinc electroplated coating of 8 4m thickness with yellow chromate conversion coating and added lubricant would be identified as Zn8CL. 2) Marking on or in relation to a product represents a manufacturer s declaration of conformity, i.e. a claim by or on behalf of the manufacturer that the product meets the requirements of the standard. The accuracy of the claim is solely the claimant s responsibility. Such a declaration is not to be confused with third party certification of conformity, which may also be desirable.

9 Designation Table 2 Coating designation and corrosion resistance Nominal coating treatment 4m Chromate treatment designation 1) First appearance of white corrosion product h First appearance of rust Cadmium h Zn or Cd 3A 3 2) A Zn or Cd 3B B Zn or Cd 3C C Zn or Cd 3D D Zn or Cd 5A 5 A Zn or Cd 5B B Zn or Cd 5C C Zn or Cd 5D D Zn or Cd 5Bk Bk 12 Zn or Cd 8A 8 A Zn or Cd 8B B Zn or Cd 8C C Zn or Cd 8D D Zn or Cd 8Bk Bk Zn or Cd 12A 12 A Zn or Cd 12B B Zn or Cd 12C C Zn or Cd 12D D Zn or Cd 12Bk Bk ) See Clause 6. 2) Low coating thicknesses impair chromate conversion performance. Zinc h 5

10 6 Table 3 Designation of chromate treatments Designation Type Typical appearance Corrosion protection A Clear Transparent clear, sometimes with a bluish tinge B Bleached Transparent with slight iridescence Bk Black Black with slight iridescence Slight, for example against staining during handling or against high humidity in mildly corrosive conditions C Iridescent Yellow iridescent Considerable, including protection D Opaque Olive green, shading to brown or bronze against certain organic vapours NOTE This table is modified from Table 1 of BS 6338:1982.

11 Annex A (normative) Coating thickness for threaded parts In order to reduce the risk of interference on assembly of threads with electroplated coatings, the nominal coating thickness shall not exceed one-quarter of the fundamental deviation of the thread as specified in Table A.1. The effective coating thickness measured according to one of the methods specified in Annex B shall conform to the values specified in Table 1. If batch average thickness measurement is used for threaded parts having a nominal thread length > 5d then a smaller nominal thickness shall be applied (see Table A.1). 7

12 8 Pitch P Nominal thread diameter 1) d Table A.1 Upper limits of nominal coating thickness Internal thread External thread Tolerance position G 4) Tolerance position g 4) Fundamenta l deviation Nominal coating thickness max. Fundamental deviation Normal coating thickness max. 2) 3) All nominal lengths Nominal length l l < 5d 5d < l < 10d 10d < l < 15d mm 4m 4m 4m 4m 4m 4m 4m p ; p p (1.8) p p (2.2) p p p p p p (7) p p p p (14) p (18; 22) p (27) p (33) p (39) p (45) p (52) p (60) p p ) Information for coarse pitch threads is given for convenience only. The determining characteristic is the thread pitch. 2) Maximum values of nominal coating thickness if local thickness measurement is agreed. 3) Maximum values of nominal coating thickness if batch average thickness measurement is agreed. 4) See BS and BS NOTE The additional deviations that can be applied to threads specially manufactured to accommodate thick coatings are given in Table C.1.

13 Tolerance position f 4) Tolerance position e 4) Fundamental deviation Normal coating thickness max. Fundamental deviation 2) 3) 2) 3) All nominal lengths Normal coating thickness max. Nominal length l All nominal Nominal length l l < 5d 5d < l < 10d 10d < l < 15d lengths l < 5d 5d < l < 10d 10d < l < 15d 4m 4m 4m 4m 4m 4m 4m 4m 4m 4m p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p

14 3) Bought by Mr Wayne Grealey, Bapp Group Ltd. Latest version as of 01/05/ :33. Not to be distributed/networked. For multi-user access go to Annex B (normative) Measurement of coating thickness B.1 Local thickness The local thickness shall be measured by one of the methods contained in the following British Standards. 10 BS :1987 BS :1986 BS :1984 BS :1991 The thickness shall only be measured on the measuring areas as shown in Figure B.1. B.2 Determination of group and batch average coating thickness B.2.1 Reagents A stripping solution consisting of: hydrochloric acid (density 1.16 g/ml to 1.18 g/ml) 800 ml distilled water 200 ml antimony trioxide 20 g B.2.2 Procedure Degrease the group of components in an organic solvent, dry thoroughly and weigh to an accuracy of 0.01 % of the total mass of the group of components; then totally immerse the components in the stripping solution and turn them over to allow free access to all surfaces. After the effervescence has ceased, remove the samples, wash immediately in running water, and wipe with a soft cloth to remove any loose deposits. Immerse in clean acetone, remove, dry thoroughly and reweigh. B.2.3 Calculations B Calculate the group average thickness of coating (in 4m) using the following formula: Thickness Where Km ( 0 m 1 ) = A K is a factor dependent on the density of the coating (see B.2.3.2); m 0 is the original mass of the group of components (in g); m 1 is the final mass of the group of components (in g); A is the total surface area of the group of components (in cm 2 ) 3). 3) Surface areas of common metric components are given in Appendix C of BS :1991.

15 B Values of K are as follows: for cadmium, K = 1160, assuming a density of 8.6 g/cm 3 for cadmium; for zinc, K = 1410, assuming a density of 7.1 g/cm 3 for zinc. B Calculate the batch average coating thickness from the following formula: Batch average coating thickness = Annex C (informative) Guidance on procedures that may be adopted to accommodate thick coatings C.1 Modification of thread dimensions Measuring area Figure B.1 Measuring area for local coating thickness measurement on fasteners Sum of all group average coating thicknesses Number of groups When, in order to provide improved corrosion resistance, it is required to deposit coatings thicker than those that appear in Table A.1 or alternatively, when it is required to apply coatings to components of pitches smaller than appear in Table A.1, it is necessary to manufacture threads to special limits and tolerances. The minimum pitch limits of applicability in Table A.1 can be lowered if for any particular thread the normal tolerance is restricted at the maximum metal limits. This provides a greater fundamental deviation or, in the case of tolerance position H, it provides a deviation which does not otherwise exist. Alternatively, the whole tolerance zone can be displaced to provide a greater fundamental deviation. The minimum fundamental deviation required for particular pitches and deposit thicknesses are given in Table C.1. C.2 Selective electroplating Where a thick deposit is required on a portion of the fastener, for example the heads of bolts or blind nuts, it is often possible to use the procedure of selective electroplating. In such cases the thickness of the deposits applied to the different areas of the fastener will need to be specified by the purchaser. 11

16 Table C.1 Minimum fundamental deviation required to accommodate coatings too thick to be applied to standard threads. Metric threads Normal coating thickness 12 Minimum fundamental deviation If local thickness measurement is agreed If batch average thickness measurement is agreed All nominal lengths 1 < 5d 5d < l < 10d 10d < l < 15d Dimensions in micrometres NOTE 1 If the tolerance class already has a basic deviation (for example, G, g, f or e), the deviation is deducted from the relevant deviation in Table C.1 to give the minimum additional basic deviation. NOTE 2 Since the modifications for the thicker deposits may significantly reduce the thread engagement, their application should be agreed between manufacturer and purchaser.

17 List of references (see Clause 2) Normative references BSI standards publications BRITISH STANDARDS INSTITUTION, London BS 3643 BS :1981 BS :1981 BS 5411 BS :1987 BS :1986 BS :1984 BS :1991 BS :1981 BS 7371 BS :1991 BS :1993 BS 7479:1991 Informative references BSI standards publications BRITISH STANDARDS INSTITUTION, London BS 1210:1963 4) BS 4174:1972 4) BS 4479 BS :1990 4) BS :1990 4) BS 6338:1982 Other references ISO metric screw threads Principles and basic data Specification for selected limits of size Methods of test for metallic and related coatings Review of methods of measurement of thickness Coulometric method for the measurement of coating thickness Measurement of local thickness of metal and oxide coatings by the microscopical examination of cross-sections Measurement of coating thickness of metallic coatings: X-ray spectrometric methods Review of methods and available for testing adhesion of electrodeposited and chemically deposited metallic coatings on metallic substrates Coatings on metal fasteners Specification for general requirements and selection guidelines Specification for torque/clamping force relationship Method for salt spray corrosion tests in artificial atmospheres Specification for wood screws Specification for self-tapping screws and metallic drive screws Design of articles that are to be coated General recommendations Recommendations for electroplated and autocatalytic coatings Specification for chromate conversion coatings on electroplated zinc and cadmium coatings [1] GREAT BRITAIN. Environmental Protection (Controls on Injurious Substances) (No. 4) Regulations London: HMSO 5) 4) Referred to in Clause 0 Introduction only. 5) Referred to in the foreword only. 13

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