BS-1706-1990.pdf

BRITISH STANDARD BS 1706:1990 Incorporating Amendment No. 1 Method for specifying Electroplated coatings of zinc and cadmium on iron and steel UDC 669.1.056:669.587:669.738:006.3/.8 BS 1706:1990 This British Standard, having been prepared under the direction of the Surface Coatings (other than Paints) Standards Policy Committee, was published under the authority of the Board of BSI and comes into effect on 31 July 1990 © BSI 12-1998 First published March 1951 Second edition July 1960 Third edition July 1990 The following BSI references relate to the work on this standard: Committee reference SRC/1 Draft for comment 87/51808 DC ISBN 0 580 17248 1 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Surface Coatings (other than Paints) Standards Policy Committee (SRC/-) to Technical Committee SRC/1, upon which the following bodies were represented: British Constructional Steelwork Association Ltd. British Industrial Fasteners Federation British Lock Manufacturers Association British Malleable Tube Fittings Association British Metal Finishing Suppliers Association British Telecommunications plc Electricity Supply Industry in England and Wales Electronic Engineering Association Galvanizers Association Institute of Metal Finishing Metal Finishing Association Ministry of Defence Society of Motor Manufacturers and Traders Ltd. Spring Research and Manufacturers Association Telecommunication Engineering and Manufacturing Association Zinc Alloy Rust-proofing Co. Ltd. Zinc Development Association Amendments issued since publication Amd. No.DateComments 6731May 1991Indicated by a sideline in the margin BS 1706:1990 © BSI 12-1998i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Definitions1 3Information to be supplied by the purchaser to the electroplater1 4Basis material2 5Service conditions and classification code2 6Sampling3 7Coatings3 8Tests4 Appendix A Heat treatment4 Appendix B Test methods5 Table 1 Summary of information required 1 Table 2 Typical coating classification codes, thickness, service condition and service life3 Table 3 Classification of chromate coatings3 Table 4 Requirements for corrosion resistance of chromate coatings4 Table 5 Categories of steels and maximum tensile strengths corresponding to specified minimum tensile strengths4 Table 6 Recommendations for heat treatment for stress relief before electroplating5 Table 7 Recommendations for heat treatment for hydrogen embrittlement relief after electroplating 5 Publications referred toInside back cover BS 1706:1990 ii © BSI 12-1998 Foreword This British Standard has been prepared under the direction of the Surface Coatings (other than Paints) Standards Policy Committee. It is based on the international standards ISO 2081:1986 “Metallic coatings electroplated coatings of zinc on iron or steel”, and ISO 2082:1986 “Metallic coatings electroplated coatings of cadmium on iron or steel” which were prepared with the active participation of the United Kingdom by Subcommittee 3, Electrodeposited coatings and related finishes, of Technical committee 107, Metallic and other non-organic coatings, of the International Organization for Standardization (ISO). It describes a method for specifying zinc and cadmium coatings for the protection of iron and steel under various corrosive conditions. It revises and supersedes BS 1706:1960 which is withdrawn. BS 1706 was first published in 1951 and last revised in 1960. This revision of the standard brings the technical content up to date and in line with that of ISO 2081 and ISO 2082. It differs from the previous edition of BS 1706 in that it has been drafted as a method for specifying and not as a specification. For this reason it is no longer appropriate for a claim to be made that a coating complies with BS 1706:1988. The other main changes in the content of the standard are the different choice of coating thicknesses and the revised heat treatment conditions for minimizing the risk of hydrogen embrittlement failures of articles made from high strength steel. CAUTION. Cadmium vapour is toxic by inhalation. During heat treatment, therefore, all precautions should be taken to ensure that no person is exposed to it. Because of its toxicity, it is essential that cadmium is not employed as a coating for any object likely to come into contact with food or beverages. Attention is also drawn to the danger arising from welding, soldering or heating as its vapour is toxic. The surface condition of the basis material is not specified in this standard. Chromate conversion coatings on top of electroplated coatings of cadmium or zinc give additional protection against corrosion and should normally be specified. Zinc or cadmium electroplated articles to be painted may require alternative treatment such as phosphating to promote good adhesion. CAUTION. Chromates can cause dermatitis so skin contact with chromated components should be avoided as much as possible. Zinc and cadmium electroplated articles are subject to attack by certain organic materials, e.g. cardboard, wood or certain electrical insulating materials which emit reactive vapours. Cognizance of this should be taken when articles are packed, stored or transported. Attention is drawn to the fact that electroplated cadmium is more readily soldered than electroplated zinc. It is essential that the purchaser specifies all his requirements and provides at least all the essential information (see 3.1) to the electroplater; merely to ask for electroplating to be carried out in accordance with BS 1706 without this information is insufficient. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 6, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. BS 1706:1990 © BSI 12-19981 1 Scope This British Standard describes a method for specifying electroplated coatings of cadmium or zinc on iron or steel for protection against corrosion. It does not apply to coatings applied to sheet, strip or wire in the unfabricated form. It gives information to be supplied by the purchaser to the electroplater and information on coating thicknesses and chromate treatments. It also includes requirements for heat treatment both before and after electroplating. NOTE 1The coating thickness that can be applied to threaded components may be limited by dimensional requirements, including class or fit. Attention is drawn to BS 3382 which includes tables specifying the maximum thicknesses that can be applied to standard threads. NOTE 2The titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purpose of this British Standard the following definitions, as given in BS 5411-1, apply. 2.1 significant surface the part of the article covered or to be covered by the coating and for which the coating is essential for serviceability and/or appearance 2.2 local thickness the mean of the thickness measurements, of which a specified number is made within a reference area 2.3 minimum local thickness the lowest value of the local thicknesses found on the significant surface of a single article (see also 7.3.1) 2.4 reference area the area within which a specified number of single measurements is required to be made 3 Information to be supplied by the purchaser to the electroplater NOTEA summary of clause references pertaining to this information is given in Table 1. Table 1 Summary of information required Information requiredRelevant clausesOther related clauses Essential information Additional information required Reference (i.e. the number of this British Standard) 3.1 a) Coating classification code 3.1 b)5.2 Significant surface3.1 c)3.2 b)7.1, 7.2, 7.3, 8.1 Basis material3.1 d)Clause 4 Appearance3.1 e)7.2 Heat treatment3.1 f)7.6, appendix A Chromate or other conversion coatings 3.1 g)7.5 Corrosion resistance3.1 h)8.2 Sampling3.1 i)Clause 6 Area for thickness determination on small articles of complex shape 3.1 j)8.1, appendix B Contact marks3.2 a)7.2 Additional significant surfaces 3.2 b)7.1 BS 1706:1990 2 © BSI 12-1998 3.1 Essential information The information listed in a) to i) shall be supplied by the purchaser to the electroplater, if necessary in consultation with the electroplater. a) The number of this British Standard, i.e. BS 1706. b) The classification code (see clause 5). c) The significant surface indicated, for example, by drawings or by the provision of suitably marked samples (see 7.1). d) The nature and surface condition of the basis material (see clause 4). e) The surface appearance of the coating (see 7.2). f) Any requirements for heat treatment before or after electroplating and, for steels, a statement of the maximum tensile strength (see 7.6 and appendix A). g) A statement as to whether or not chromate coating is to be omitted and, if omitted, as to whether or not another type of conversion coating is to be applied (see 7.5). h) Whether or not the chromate coating, if applied, is to be subjected to a corrosion test (see 8.2). i) The sampling procedure to be used, any special inspection requirements and details of the sampling and acceptance levels (AQL) required (see clause 6). j) If appropriate, the area to be used for the determination of local thickness on small articles of complex shape and the method of measuring such an area (see 8.1 and appendix B). 3.2 Additional information The additional information listed in a) and b), as applicable, shall be supplied by the purchaser, if necessary in consultation with the electroplater. a) The location of areas of surface that can be used for connecting parts to jigs or racks and where contact marks are acceptable (see 7.2). b) An indication of those parts of the surface of the article that cannot be touched by a ball of diameter 20 mm but which are to be included in the significant surface (see 7.1). 4 Basis material The nature of the basis material shall be specified by the purchaser to the electroplater (see 3.1 d). NOTEThis British Standard does not specify requirements for the surface condition of the basis material before electroplating with zinc or cadmium but agreement should preferably be reached between the interested parties that the surface finish of the basis material is not too rough to prevent the appearance and/or the serviceability of the coating from being satisfactory. 5 Service conditions and classification code 5.1 The purchaser shall supply the following information to the electroplater by quoting the appropriate classification code described in 5.2 (see also 3.1 b) and 3.1 g) a) the chemical identity of the basis material; b) the chemical identity of the coating; c) the minimum local thickness of the coating; d) the identity of the chromate conversion coating or other type of conversion coating required, unless this is to be omitted. NOTEThe thickness of the coating required depends on the severity of the service conditions and on the service life required. Classes Fe/Cd 5 and Fe/Zn 5 (see 5.2) are recommended only for dry, indoor conditions. As the service conditions become more severe and/or the life required increases, coatings with greater thickness should be specified. 5.2 The classification code consists of the following parts, the first two of which are separated by an oblique stroke: a/b c d where a indicates the chemical symbol, i.e. Fe, for the basis material (iron or steel); b indicates the chemical symbol for cadmium, Cd, or for zinc, Zn, as appropriate; c indicates the minimum local thickness (in micrometres) of the cadmium or zinc coating; d indicates the chromate conversion coating required. This is indicated by a lower case c followed by the class and designation of the coating (see Table 3). It is omitted if a chromate conversion coating is not required. Details of other types of conversion coating are given separately. 5.3 A typical example of a complete classification code is Fe/Zn 8 c 1A i.e. a coating on steel of zinc of minimum local thickness 8 mm with a class 1A chromate conversion coating. 5.4 Table 2 shows the coating classification code and the corresponding minimum local thickness (see 7.3) of coatings prior to chromate treatment, if carried out, and an indication of their relationship to service conditions and to service life. It should be noted however that equal thicknesses of cadmium and zinc do not normally give the same service life under identical service conditions. BS 1706:1990 © BSI 12-19983 6 Sampling Unless otherwise specified, the sampling procedure shall be selected from BS 6001 or BS 6041. The sampling and acceptance levels (AQL) required shall be agreed between the purchaser and the electroplater. 7 Coatings 7.1 Significant surface Unless otherwise specified by the purchaser those parts of the surface of an article which cannot be touched by a ball of diameter 20 mm shall not be considered part of the significant surface (see 3.2 b). Table 2 Typical coating classification codes, thickness, service condition and service life 7.2 Appearance Over the significant surface, the electroplated article shall be free from clearly visible plating defects such as blisters, pits, roughness, cracks or unplated areas. On articles where a contact mark is unavoidable, its position shall, if appropriate, be specified by the purchaser, preferably by the provision of a suitably marked sample (see 3.2 a). The articles shall be clean and free from damage. Unless specified otherwise by the purchaser (see 3.1 e), the coatings shall be bright. NOTEHeat treatment may cause a slight dullness in the coating appearance. 7.3 Thickness of coatings 7.3.1 Minimum local thickness The minimum local thickness requirement of the coating, as indicated by the classification code (see 5.2), applies to any part of the significant surface (see 7.1). 7.3.2 Thickness of coatings on small articles In the case of articles having a significant surface area less than 100 mm2 the minimum local thickness is deemed to be the minimum value of the average thickness determined by the method specified in 8.1. 7.4 Adhesion The coating shall continue to adhere to the basis metal when subjected to the test described in B.2. 7.5 Application of conversion coatings Conversion coatings, particularly chromate coatings, enhance the corrosion resistance of the electrodeposited cadmium and zinc coatings, and chromate coatings shall only be omitted, or replaced by other conversion coatings, if specified by the purchaser (see 3.1 g)