The Coating Designation Has a Symbol for the Step You Skipped

The source is ISO 4527:2003, Metallic coatings, autocatalytic (electroless) nickel-phosphorus alloy coatings, second edition, reviewed and confirmed in 2021. This page uses the free preview, which carries the introduction, the scope, the ordering information, the whole of the designation clause and clause 6 as far as corrosion resistance.

A coating that plates itself

The introduction explains the chemistry in three sentences. The coatings are produced by the catalytic reduction of nickel ions in hot, usually mildly acidic solutions at atmospheric pressure, using hypophosphite ion as the reducing agent. And then the part that gives the process its name: because the deposited nickel alloy is itself a catalyst for the reaction, the process is self-sustaining.

No current. That has one consequence the standard states immediately, and it is the reason people specify this finish. The coatings are uniform in thickness on irregularly shaped parts, if the processing solution circulates freely over their surfaces. Nothing is thicker at the edge because the current crowded there, and nothing is thinner in the recess because the field could not reach.

That is worth setting beside the other coating standards this site has read, because each of them answers the same question differently.

Two of the three claim uniformity and one is candid about where it goes thin. The mechanically plated case is on our page about the zinc that is hammered on by glass beads.

One knob, and it trades corrosion against wear

The deposit is described as a thermodynamically metastable, supersaturated solid solution of phosphorus in nickel, containing up to 14 % mass fraction phosphorus. Its properties depend on that composition, on the plating solution, on the pre-treatment and quality of the substrate, and on heat treatment after deposition.

Then the trade-off, stated plainly. Corrosion performance is significantly improved as the phosphorus content is increased to 8 % mass fraction or higher, whereas wear resistance is improved as the phosphorus content is decreased below that level. One number, moving two properties in opposite directions, with the crossover named.

And a partial way out of the trade: with suitable heat treatment, coatings with high phosphorus contents display greatly improved microhardness, and hence wear resistance. What that heat treatment is falls outside the free preview, in a normative annex, so this page does not give conditions for it. What matters for a specification is that the phosphorus content is a choice with consequences on both sides, and that it goes in the designation.

Reading the designation

The standard gives its own worked example, and decoding it is the fastest way to see how much of a process a finish designation can carry.

The three letters are SR for stress relief, HT for heat treatment to increase hardness or adhesion, and ER for hydrogen embrittlement relief. The temperature in the parentheses is a minimum, not a target.

Now the same coating on aluminium, where the standard says no heat treatment is required:

Two double slashes, in the two places where the steel version had heat treatments. The notation does not simply leave the fields out. It writes them as empty. Anyone reading the string later can see that a stress relief slot and an embrittlement relief slot existed and were not filled, which is a very different piece of information from a designation that never mentioned heat treatment at all.

These are the standard’s own illustrative examples, not requirements. The clauses that set out when each treatment is actually required are outside the free preview and nothing is quoted from them here.

Where the thickness applies: a 20 mm ball

Clause 6.4 answers the question that every coating standard has to answer somehow, and it answers it with an object.

The thickness in the designation is the minimum local thickness. And that minimum local thickness shall be measured at any point on the significant surface that can be touched by a ball 20 mm in diameter, unless the purchaser specifies otherwise.

Pick up a part and imagine rolling a 20 mm ball over it. Wherever the ball can touch, the number applies. Wherever it cannot reach, a small internal corner, a narrow slot, the root of a fine thread, the requirement does not reach either. It is a geometric test that anybody can apply without a discussion, and it is unusually concrete for a definition of scope.

The three coating standards this site has read give three different answers to the same question, which is worth keeping in mind when comparing certificates.

Two of those are minima at a point and one is an average over a group. A thickness on a certificate does not mean the same thing in each case, and the sherardizing page covers why the averaged one is averaged.

What can and cannot be rejected

Clause 6.2 sorts appearance into three groups, and the sorting is sharper than most.

• Not a reason to reject: imperfections and variations arising from the surface condition of the basis metal, named as scratches, pores, roll marks and inclusions, that persist despite good metal finishing practice. Also stains and coloured oxides formed by post-coating heat treatment, unless a special heat treatment atmosphere was specified
• A reason to reject: blisters or cracks visible to the naked eye that result from heat treatment performed by the coater
• Not to be coated at all: the standard says flatly that damaged basis metals shall not be coated

The dividing line is who caused it. Defects that came in with the part are the purchaser’s, and the standard adds a warning worth passing on: defects present in the basis metal before coating, including hidden defects, may be reproduced by the coating. Defects the coater created by heat treating are the coater’s.

Appearance itself is bright, semi-bright or dull as the purchaser specifies, judged against approved samples. And the ordering clause adds the caveat that makes sample-based judgement honest: approved samples may deteriorate over time and may need to be replaced at regular intervals. This site has met appearance-by-sample before, in the standard that never says how black; this one at least admits the reference decays.

One more expectation is managed in a note. The surface finish of an electroless nickel coating is not usually superior to that of the substrate before coating, except where the substrate is extremely smooth and micro-levelling occurs. Plating does not tidy up the part underneath.

And a note about salt spray

Clause 6.8 leaves corrosion resistance and its test method to the purchaser, points at ISO 10289 for acceptance criteria, and mentions acetic acid and copper accelerated salt spray for evaluating pitting. Then it adds a note:

A salt spray figure is a comparison between processes under one artificial condition. The standard says so itself, in the clause that offers the test. What a thickness does buy in service, and on what basis, is the subject of our page on estimating coating life.

What has to be on the order

Clause 4.1 lists fourteen items the purchaser shall supply in writing, as part of the contract, the purchase order, the product specification or the drawings. Four are worth pulling out because they are the ones most often left implicit:

• The tensile strength of the part, together with the requirements for any heat treatment before and after coating. The strength of the part is an input the coater needs, not something to be inferred from the drawing
• The significant surface, indicated by drawings of the articles or by suitably marked samples. Since the thickness requirement is defined on the significant surface, a designation without one has not said where it applies
• Whether a maximum thickness is measured before or after machining of the coating, which the standard raises specifically for building up worn or over-machined parts
• Whether steel parts need degaussing before coating, listed under additional information, to minimise the inclusion of magnetic particles in the coating

The pattern across every finishing standard on this site is the same by now. The specification is a description of a process, most of the decisions inside it belong to the purchaser, and the standard’s main service is to give those decisions names and a place to be written down. ISO 4527 goes one step further than the others: it gives a name to the decisions that were never made.

This page covers step 5, the finish. The whole order is substrate, thread, head, drive, finish, documentation, and why doing it out of order is rework rather than a tweak is in specifying a screw.

Common questions

What does the double slash mean in an ISO 4527 designation?

That a step was omitted or never specified. Clause 5.1 states that double separators shall be used to indicate that a step or operation has either not been specified or has been omitted. So a designation reading Al//NiP(10)15/Cr0,5// shows two empty slots where the steel version of the same coating carries stress relief and embrittlement relief heat treatments.

What is electroless nickel plating?

ISO 4527 describes autocatalytic nickel-phosphorus coatings as produced by the catalytic reduction of nickel ions in hot, usually mildly acidic solutions at atmospheric pressure, using hypophosphite ion as the reducing agent. Because the deposited nickel alloy is itself a catalyst for the reaction, the process is self-sustaining. No electric current is involved, and the coatings are uniform in thickness on irregularly shaped parts if the solution circulates freely over their surfaces.

Does more phosphorus make an electroless nickel coating better?

It depends which property you want. The introduction to ISO 4527 states that corrosion performance is significantly improved as the phosphorus content is increased to 8 % mass fraction or higher, whereas wear resistance is improved as the phosphorus content is decreased below that level. It adds that with suitable heat treatment, high phosphorus coatings display greatly improved microhardness and hence wear resistance.

Where does the coating thickness requirement apply?

Anywhere a 20 mm ball can touch. Clause 6.4 states that the thickness specified in the designation is the minimum local thickness, and that it shall be measured at any point on the significant surface that can be touched by a ball 20 millimetres in diameter, unless the purchaser specifies otherwise.

Can a coater reject parts for scratches that were already there?

No, and the standard also says those parts should not have been coated if they were damaged. Imperfections and variations arising from the surface condition of the basis metal, named as scratches, pores, roll marks and inclusions, shall not be cause for rejection. Separately the standard states that damaged basis metals shall not be coated, and warns that defects present before coating, including hidden ones, may be reproduced by the coating.

Will electroless nickel improve the surface finish of a part?

Generally not. A note in clause 6.3 states that the surface finish of autocatalytic nickel coatings is not usually superior to that of the substrate before coating, except when the surface of the substrate is extremely smooth and micro-levelling occurs.

Does a salt spray result predict how long the coating will last?

The standard says it does not necessarily. A note to clause 6.8 states that corrosion testing in artificial atmospheres does not necessarily relate to the service life or performance of the finished article. The clause leaves the corrosion test and its acceptance criteria to the purchaser.

References

• ISO 4527:2003, Metallic coatings, autocatalytic (electroless) nickel-phosphorus alloy coatings, specification and test methods. Introduction, clause 1, clause 4, clause 5 and clause 6 as far as 6.8

ISO 4527:2003 was read from the publicly available preview, which for this standard carries the front matter and content to the end of 6.8 on page 6. Clauses 6.9 to 6.17, clause 7 on sampling, and Annexes A to D are outside the preview, and nothing is quoted from them. That includes the normative annex on heat treatment, the guidance on thickness and composition, and the sampling procedure. No heat treatment temperature or duration is given here as a requirement; the figures inside the designation examples are the standard’s own illustrations of the notation and are presented only as that. No thickness value is recommended, the guidance being in an annex outside the preview. The clause on hydrogen embrittlement relief after coating is likewise outside the preview, so this page states only that the clause exists, that the designation reserves an ER field, and that the purchaser must supply the tensile strength of the part. No claim is made about the suitability of this coating for high strength fasteners, and its corrosion performance is not compared numerically with any zinc coating. The standard is at stage 90.20, under systematic review, and was last reviewed and confirmed in 2021; it is the second edition and replaces ISO 4527:1987.