ISO 15726 Zinc Nickel/Cobalt/Iron Alloy Coated fasteners
ISO 15726 Standard covers the requirements for electrodeposited zinc alloy coatings containing either nickel, cobalt or iron. It provides a method of designating the electrodeposited coatings, chromate conversion coatings and other supplementary treatments.Electrodeposited zinc alloy coatings containing nickel, cobalt or iron are significantly more corrosion-resistant than electrodeposited zinc coatings of equivalent thickness. The alloy coatings are anodic to steel, but less so than pure zinc. Although originally developed for the continuous coating of steel for the fabrication of automobile body panels, zinc alloy electroplating processes have become available that are suitable for the rack and barrel electroplating of individual components.
In the case of zinc-nickel alloys, proprietary acid and alkaline electroplating processes exist that yield deposits with controlled nickel contents. Zinc Nickel Coatings containing either 8 % or 12 % nickel are most widely used, and are often considered possible substitutes for cadmium coatings. Zinc-nickel alloy coatings can reportedly be applied by brush electroplating techniques.
The alloy coatings are often used in combination with chromate conversion coatings, organic sealants and other supplementary treatments to further enhance corrosion resistance, and often serve as the base for applying organic coatings. Chromate conversion coatings formulated specifically for use with zinc-cobalt and zinc-nickel alloy coatings are available for producing yellow- to bronze-coloured surfaces.
The electrodeposited zinc alloy coating shall be designated by the chemical symbols given in below table followed by a number in parentheses giving the composition of the alloy coating within 0,5 % mass fraction (w), followed by a number giving the minimum local thickness of the coating, in micrometres. For example, ZnNi(12)5 designates a zinc alloy coating containing 12 % nickel that is 5 μm thick.
Below Table gives typical thickness values specified for zinc alloy coatings deposited on steel, as guidance.
- Examples if Zinc Alloy Coating Systems
-
EXAMPLE 1 - A coating 8 µm thick containing 10 % nickel, balance zinc, on steel (Fe), with a supplementary chromate
conversion coating that is iridescent yellow (C) has the following designation:
Electrodeposited coating ISO 15726 - Fe/ZnNi(10)8/C
EXAMPLE 2 - A coating 5 µm thick containing 8 % nickel, balance zinc, on steel, with a supplementary chromate
conversion coating that is colourless (blue bright) and that has received a subsequent organic sealant has the following
designation:
Electrodeposited coating ISO 15726 - Fe/ZnNi(8)5/A/T2
EXAMPLE 3 - A coating on high-strength steel that is stress-relieved at 190 °C for 1 h before electroplating with a
coating 5 µm thick containing 1,0 % cobalt, balance zinc, with a supplementary chromate conversion coating that is black
(F), and that is heat-treated at 200 °C for 6 h after coating to minimize the risk of hydrogen embrittlement has the following
designation:
Electrodeposited coating ISO 15726 - Fe/[SR(190)1]/ZnCo(1)5/F/[ER(200)6]
- Appearance
-
The coating on the significant surfaces of the product shall be smooth and free of visual defects such as
blisters, pits, roughness, cracks, flaking, burned deposits, and non-coated areas. The boundaries of
electroplating that cover only a portion of the surface shall, after finishing as indicated in the drawings, be free
of beads, nodules, jagged edges and other detrimental irregularities. Imperfections and surface conditions of
the basis metal (scratches, pores, roll marks, inclusions) that persist in the finish despite the observance of
good metal finishing practices shall not be cause for rejection.
Approved samples of artefacts shall be used for comparison purposes to control the final appearance of the
coating.
Electrodeposited finishes generally perform better when the substrate over which they are applied is smooth
and free from deep scratches, torn metal, pores, inclusions and other defects. It is recommended that the
specifications that cover the unfinished product provide limits for these defects. A metal finisher can often
remove defects through special treatments such as grinding, polishing, abrasive blasting, and special
chemical treatments. However, these are not normal treatment steps.
NOTE Zinc alloy coatings are commonly used in automotive applications where subsequent forming, bending and
crimping operations are commonly performed. These operations will necessarily detract from the performance of coatings.
Some cracking of coatings may be unavoidable.
Flaking of the coating after the operations described in the Note above shall be cause for rejection.
- Thickness
-
The coating thickness specified in the article designation shall be the minimum local thickness. The minimum
local thickness of an electrodeposited coating shall be measured at any point on the significant surface that
can be touched by a ball of 20 mm in diameter, unless otherwise specified.
The minimum local thickness of the zinc alloy coatings shall be measured by one of the methods given in A.1.
NOTE - Variations in the minimum local thickness from point to point on an article, and from article to article in a
production lot, are intrinsic to electroplating. If all the articles in a production lot are to meet minimum local thickness
requirements, the average coating thickness of the entire production lot will be greater than the specified minimum.
- Adhesion
-
The coating shall withstand normal handling and storage conditions without chipping, flaking, or other coating
damage, and shall conform to the minimum requirements.
It is the responsibility of the electroplater to ensure that the method for surface preparation used prior to
electroplating results in a surface capable of meeting the requirements of this subclause.
- Porosity
-
The porosity shall be measured by one of the methods. The specific test method to be used, and the
number and location of acceptable pores, shall be specified by the purchaser.
- Composition of coating
-
The composition of the coating shall be verified by atomic absorption spectrophotometry (AA), inductively
coupled plasma (ICP), directly coupled plasma (DCP), or any other method with a measurement uncertainty of
less than 10 % determined on standard reference materials.
- Corrosion performance in neutral salt spray
-
Corrosion performance of the coated parts shall be evaluated by exposure to neutral salt spray testing in
accordance with ASTM B117. Annex B gives the minimum number of hours during which the coating should
withstand corrosion after continuous exposure to neutral salt spray. The requirements for corrosion
performance shall be specified by the purchaser.
The time to zinc alloy coating corrosion is for rack-processed parts. For parts processed in
barrels, the time to zinc alloy coating corrosion will be less.
NOTE - The duration and results of artificial atmosphere corrosion tests may bear little relationship to the service life of
the coated article and, therefore, the results obtained are not to be regarded as a direct guide to the corrosion resistance
of the tested coatings in all environments where these coatings may be used.
- Stress-relief heat treatments
-
Steel parts that have an ultimate tensile strength equal to or greater than 1 000 MPa (31 HRc) and that
contain tensile stresses caused by machining, grinding, straightening, or cold-forming operations shall be
given a stress-relief heat treatment prior to cleaning and metal deposition, according to the procedures and
classes of ISO 9587, unless otherwise specified by the purchaser.
Steels with oxide or scale shall be cleaned before application of the coatings. For high-strength steels, nonelectrolytic alkaline and anodic alkaline cleaners, as well as mechanical cleaning procedures, are preferred to
avoid the risk of producing hydrogen embrittlement during cleaning operations. The possibility of overheating
should be considered in the case of mechanical cleaning of high-strength steel with a tensile strength greater
than 1 400 MPa.
- Hydrogen-embrittlement-relief heat treatments
-
Steel parts having an ultimate tensile strength equal to or greater than 1 000 MPa (31 HRC), as well as
surface-hardened parts, shall receive hydrogen-embrittlement-relief heat treatment according to the
procedures and classes of ISO 9588, unless otherwise specified by the purchaser.
Electroplated springs or other parts subject to flexure shall not be flexed before the
hydrogen-embrittlement-relief heat treatment is carried out.
The effectiveness of the hydrogen-embrittlement-relief treatement shall be determined in accordance with
ISO 10587 for testing threaded articles for residual hydrogen-relief heat treatement and with ISO 15724 for
measuring the relative, diffusible hydrogen concentration in steels, unless otherwise specified by the
purchaser.