Zinc, Zinc Nickel, Cadmium Plated fasteners
ISO 4042 Specification provides requirements for electroplated coatings and coating systems on steel fasteners. The requirements related to dimensional properties also apply to fasteners made of copper or copper alloys.
It also specifies requirements and gives recommendations to minimize the risk of hydrogen
It mainly applies to zinc and zinc alloy coating systems (zinc, zinc-nickel, zinc-iron) and cadmium, primarily intended for corrosion protection and other functional properties:
- with or without conversion coating
- with or without sealant
- with or without top coat
- with or without lubricant (integral lubricant and/or subsequently added lubricant).
Designation of ISO 4042 Zinc Electroplated fasteners
|Conversion coating (passivation)
||Transparent, clear to b lu ishc
||Transparent, clear to iridescent
||Black, dark iridescent permitted
||Yellow to yellow iridescent
|No conversion coating
||No conversion coating
||Sealant (see also A.1.3)
||Sealant shall be applied, it may be with or without integral lubricant c .
||A top coat shall be applied.
Top coat is o ften used to achieve particular characteristics, e.g. chemical resistance, colour.
Top coat may be with or without integral lubricant c .
|No sealant, no top coat
||Sealant and top coat shall not be applied for a certain application(e.g. for adherence, conductivity, electric contact, welding).
||Integral lubricant shall not be present (this code shall be addedto T2 or T7, as relevant).
||Subsequently added lubricant
||Lubricant or wax shall be applied.
Lubricant can be applied directly on the metal layer, or on thepassivation, or on a sealant, or on a top coat c .
- NOTE For more in formation about sealants and top coats,
- a Coding for fasteners was developed to be consistent with ISO 2081 and ISO 19598.
- b Sealant and top coat may be organic or inorganic, or a combination o f both.
- c When torque/clamp force relationship (e.g. range o f friction coe fficient) is specified, integral lubricant or subsequently added lubricant shall be applied as appropriate.
- d When a subsequently added lubricant is specified, the code T4 shall be placed a fter a slash and directly after the designation of the sealant or top coat
- ISO 4042 for the electroplated coating system in accordance with this document;
- The material of the coating;
- The thickness of the metal layer(s) , or minimum neutral salt spray test duration in accordance with;
- The conversion coating in accordance with (the suffix n indicating that it does not contain hexavalent chromium) or with ;
- The sealant, top coat and/or subsequently added lubricant, if any, in accordance with .
- EXAMPLE [fastener designation] - ISO 4042/ZnNi8/Fn/T7/T4
|Zinc based coating system
||Minimum neutral salt spray test duration for barrel coatinga
|No coating metal corrosion (white corrosion)
||No basis metal corrosion (red rust)
|Zn, transparent passivatedb
|Zn, iridescent passivatedb
|Zn, iridescent passivated, sealedb
|Zn, black passivated, sealed
|ZnFe, iridescent passivatedb
|ZnFe, iridescent passivated, sealedb
|ZnFe, black passivated, sealed
|ZnNi, silver grey, passivated
|ZnNi, silver grey passivated, sealed
|ZnNi, black passivated
|ZnNi, black passivated, sealed
- Minimum neutral salt spray test duration shall no t be regarded as a direct guide for the corrosion resistance of coated fasteners in all environments where they might be used . See ISO 9227.
- With a rack plating process, the effect of possible damage to coating is reduced and therefore increased corrosion resistance can be achieved.
- Black spots shall not be cause for rej ection because they do not impair corrosion resistance,
- White haze is not considered as white corrosion,
- Typically higher corrosion resistance is achieved, however for the purpose of this document, corrosion tests are stopped after 720 h.
Electroplated coating systems shall comply with the provisions of ISO 1456, ISO 2081, ISO 2082 , ISO 2093, ISO 4521, ISO 15726, ISO 19598 (see also) for the coating concerned in respect of appearance, adhesion and ductility, together with the requirements specified into.
The coated fastener shall be free from blisters, delamination and uncoated areas which can adversely affect the corrosion protection. Local excess of coating (e.g. in case of top coats) shall not impair functional properties (gaugeability, torque/clamp force relationship, etc.) .
It is possible to add dyes/pigments to the passivation or chromatation solution in order to give a coloured aspect to the conversion layer, often used for distinction purposes. Dyes/pigments may also be added to sealants/top coats to obtain coloured surfaces.
- Corrosion resistance related to Temperature
Elevated temperature can affect the corrosion protection of coated fasteners.
When agreed at the time of the order, the corrosion resistance may be tested after a specified heating cycle. Temperature and duration shall be agreed upon, e.g. 1 h at 120 °C , 24 h at 120 °C , 1 h at 150 °C . For zinc-based coatings, after heating the fasteners at part temperature for a specified cycle, the corrosion
resistance requirements specified in shall still be met.
When required, torque/clamp force relationship may be determined for fasteners with ISO metric threads with electroplated coating systems including sealants and/or top coats with integral lubricant and/or subsequently added lubricant.
The test method shall be agreed between the supplier and the purchaser, in accordance with ISO 160 47 for fasteners with ISO metric thread, and/or in accordance with other relevant technical specifications.
The requirements for torque/clamp force relationship shall be agreed between the supplier and the
purchaser. See for information.
When required, the presence or absence of Cr(VI ) shall be determined in accordance with ISO 3613: 2010, 5. 5.2 .
This International Standard specifies dimensional requirements for electroplated fasteners of steel or copper alloy. It specifies coating thicknesses and gives recommendations for hydrogen embrittlement relief for fasteners with high tensile strength or hardness and for surface-hardened fasteners.
This International Standard primarily concerns the electroplating of threaded fasteners, but it may also be applied to other threaded parts. For the applicability to screws that cut or form their own mating threads, see clause 8.
The specifications given in this International Standard may also be applied to non-threaded parts such as washers and pins.
Dimensional requirements before electroplating
Before coating, parts shall comply with the relevant International Standards if applicable or other standards as specified, except where threads or other features are specifically manufactured to allow, for functional reasons, the application of thicker coatings than are possible on normal threads.
Coating thicknesses which can be applied on ISO metric threads in accordance with ISO 965-1, ISO 965-2 and ISO 965-3 depend on the fundamental deviation available, which itself depends on the screw thread and the following tolerance positions:
- g, f, e for external threads;
- G for internal threads or H if required.
The tolerance positions apply prior to application of the electroplated coating.
- Dimensional requirements after electroplating
After coating, ISO metric screw threads shall be gauged in accordance with ISO 1502 with a GO gauge of tolerance position h for external threads and H for internal threads.
Other product dimensions apply only before coating.
NOTE : Care should be exercised where relatively thick coatings may affect dimensions with small tolerances as in the case of internal drives; in these cases an agreement should be made between the supplier and the purchaser.
The applicability of the recommended coatings to ISO metric threads is limited by the fundamental deviation of the threads concerned and hence, by the pitch and tolerance positions. The coating shall not cause the zero line (basic size) to be exceeded in the case of external threads, nor shall it fall below this line in the case of internal threads. This means that for an internal thread of tolerance position H, a measurable coating thickness can only be applied to the threads if the tolerance zone is not taken up to the zero line (basic size).
- Other coating requirements
The electroplated coating shall comply with the provisions of the relevant International Standards (ISO 1456, ISO 1458, ISO 2081, ISO 2082) for the coating concerned in respect of appearance, adhesion, ductility, corrosion resistance, etc.
- Hydrogen embrittlement relief
When the core or surface hardness is above 320 HV, process investigation shall be conducted using a test to detect hydrogen embrittlement, for example the "Parallel bearing surface method" in accordance with ISO 15330, to be sure that the process with regard to embrittlement is under control. If embrittlement is discovered, modification of the manufacturing process will be necessary, such as the inclusion of a baking process (see informative annex A for more information).
For fasteners of hardness in excess of 365 HV, a written agreement should exist between the customer and manufacturer to define how to manage the risk. If written agreement does not exist, the manufacturer shall process the parts in accordance with his recommended practices to reduce the risk of hydrogen embrittlement.
Complete elimination of hydrogen embrittlement cannot be assured. If a reduced probability of encountering hydrogen embrittlement is desired, alternative procedures should be evaluated.
NOTE : Investigations are proceeding to develop methods for the reduction of hydrogen embrittlement.
In cases of parts
- with high tensile strength or hardness or which have been surface hardened,
- which have absorbed hydrogen and are under tensile stress there is the risk of failure due to hydrogen embrittlement.
- Corrosion protection
The corrosion protection of an electroplated coating depends to a considerable extent on its thickness. In addition to greater coating thickness, a chromate conversion treatment can be specified for increased corrosion protection on zinc and cadmium coatings.
Contact with other metals and materials, the frequency and duration of wetting and service temperatures may influence the protective performance of coatings and expert advice is essential when uncertainties of choice arise.
Coatings of Zn and Cd applied to ferrous substrates are less electropositive than the steel base metal and consequently provide cathodic protection. In contrast, Ni and Cr coatings are more electropositive than the steel base metal and may intensify part corrosion where the coating is damaged or pitted.
Cadmium coatings are dealt with in ISO 2082. Zinc coatings are dealt with in ISO 2081.
Nickel coatings are dealt with in ISO 1458.
Nickel + chromium and copper + nickel + chromium coatings are dealt with in ISO 1456. Chromate conversion treatments are dealt with in ISO 4520.
NOTE : Information on salt spray corrosion protection performance of metallic coatings is given in informative annex B.
- Applicability to fasteners that cut or form their own mating threas
All recommended coatings may be applied to screws that cut or form their own mating threads such as wood screws, self tapping screws, self drilling screws and thread forming screws. The maximum value for batch average thickness given in Table 1 may be ignored unless otherwise specified.
- Specification of coating thickness
|Nominal coating thickness
||Effective coating thickness
|Local a min.
The local and batch average thicknesses corresponding to the nominal coating thicknesses recommended in the relevant International Standards for electroplating are given in Table below.
In order to reduce the risk of interference on assembly of threads with electroplated coatings, the coating thickness shall not exceed one-quarter of the fundamental deviation of the thread. These values are specified in .
NOTE : For accommodation of thick coatings guidance is given in informative annex C.
The effective coating thicknesses measured according to one of the methods specified in clause 10 shall comply with the values specified in Table below