BS 4921:1988 download free.British Standard Specification for Sherardized coatings on iron or steel.
1 Scope
BS 4921 specifies minimum thickness requirements for two classes of coatings consisting substantially of zinc applied by the sherardizing process to steel articles and to grey and malleable iron castings. It also specifies requirements for the zinc used during the sherardizing process (but see the foreword and the note to clause 4).
NOTE 1. Guidance is given on requirements which requir. agreement between the interested parties In appendix A and general guidance on sherardizirig is given In appendix B.
NOTE 2. The titles of the publications referred to In this standard are listed on the inside back cover.
2 Definitions
For the purposes of BS 4921, the following definitions apply.
NOTE. Delinltlons 2.1 to 2.7 are taken from BS 5411 : Part 1.
21 significant surface. That part of the article covered or to be covered by the coatings and for which the coating is essential for serviceability and/or appearance.
2.2 measuring area. The area of the significant surface over which a single measurement is made. ‘Measuring area’ for the following methods is defined as:
(a) for analytical methods: the area from which the coating is removed;
(b) for the anodic dissolution method: the area enclosed by the sealing ring of the cell;
(c) for the microscopical method: the place at which a single measurement is made;
(d) for non-destructive methods: the probe area or the area influencing the reading.
2.3 reference area. The area within which a specified
number of single measurements is required to be made.
2.4 local thickness. The mean of the thickness measure
ments, of which a specified number is made within a
reference area.
2.5 minimum local thickness. The lowest value of the local thicknesses found on the significant surface of a single article.
2.6 maximum local thickness. The highest value of the local thicknesses found on the significant surface of a single article.
2.7 average thickness. Either the value obtained by analytical methods or the mean value of a specified number of local thickness measurements that are evenly distributed over the significant surface.
2.8 sherardized coating. A coating of zinc-iron alloy layers obtained by heating iron or steel components in a container together with zinc dust (see clause 4).
3 Thickness
3.1 Thickness of coating
The minimum local thickness of the coating shall comply with the appropriate requirements of table 1.
3.2 Measurement of thickness
The thickness of the coating shall be measured in accordance with appendix C.
For reference purposes, the coating thickness shall be determined by the microscopical method given in C.2.
NOTE 1. For production control purposes, the local thickness of the coating may be measured by the means of electronic or magnetic measuring devices as specified in BS 5411: Parts 2 and 11. NOTE 2. Th. average coating thickness may be detrmined by the stripping method described in C.2.
4 Coating material
The zinc dust used shall have contained not less than 94 % by mass of metallic zinc, not more than 0.2 % by mass of lead and not more than 0.0005 % by mass of copper. The particle size shall have been such that 95 % of all particles passed through a 75 pm sieve complying with BS 410. NOTE. Attention Is drawn to the note on product certification given In the foreword.