A map of the standards that name electroless nickel

A specification that names a coating usually does it by designation, and an engineer who does not have the document on the desk is left guessing what each number covers. This note is a map, not a substitute: it lists the designations the public reference material names, states the scope in one line, and says where the public pages stop short.

Why a table and not a summary

Two habits make coating specifications hard to compare. The first is that the same coating family is called by several designations, some current, some superseded, some marked inactive. The second is that free public sources give the designation and a phrase of scope, while the actual requirements, thickness classes, test periods and acceptance criteria live inside the standards themselves, which are sold documents. Nothing in this note reproduces those requirements, because the public matter used here does not contain them. What it does contain is enough to sort the designations into families: electroless nickel-phosphorus plating, chromate conversion coatings, zinc plating with conversion, and the corrosion tests used to accept all of them. That sorting alone answers the question most often asked on a finishing line, which is whether the document on the purchase order is the right family at all. The electroless nickel-phosphorus plating reference page is the anchor for the first family and names the rest.

The documents that govern autocatalytic nickel

The Wikipedia article on electroless nickel-phosphorus plating lists eight designations under its standards heading, without describing their contents. They are AMS-C-26074, AMS-2404, ASTM B-733, ASTM-B-656, Mil-C-26074E, MIL-DTL-32119, IPC-4552 and IPC-7095. Two are named as covering the printed circuit board finish called electroless nickel immersion gold. One of the two ASTM entries, ASTM-B-656, is explicitly marked inactive in the source. That single word matters more than it seems: an inactive designation still circulates on old drawings, and a buyer can be holding a requirement that no longer corresponds to a live document.

The page does not publish the phosphorus ranges, thickness classes or test schedules of any of these documents, so this note does not either. What the page does give is the metallurgical background a specifier needs before reading them: coatings are grouped by phosphorus content, with low-phosphorus defined as up to 4% P, medium as 4 to 10% P and high as 10 to 14% P, though the page flags several of the associated hardness figures as needing a source.

Which alloy grade is the document about?

Where the classification helps is in matching the document to the service. The same public page states that high-phosphorus coatings, those with 10 to 14% P, are preferred for parts exposed to highly corrosive acidic environments such as oil drilling and coal mining, and that coatings with more than 11.2% P are non-magnetic. None of that is a requirement of AMS 2404 or ASTM B-733; it is context that tells the reader which grade a given requirement sheet is probably addressing. The page also notes that the Vickers hardness figure it gives for high-phosphorus deposits is not easily comparable to the Rockwell scale, and marks it as needing a source. A note on properties and testing covers how those two scales diverge.

The conversion coating designations

Chromate conversion, the passivation treatment that often follows plating on aluminium or zinc, has its own family of documents, and the public page on chromate conversion coating names the key ones with more context than the nickel page gives. The main standards for chromate conversion coating of aluminium are MIL-DTL-5541 in the US and Def Stan 03/18 in the UK. For chromate conversion on electroplated zinc and cadmium, the page names ISO 4520. On the zinc plating side, ASTM B633 Type II and Type III cover zinc plating plus chromate conversion on iron and steel parts, with a 2019 revision described as current, superseding the 2015 revision. The page adds that recent revisions of ASTM B633 defer to ASTM F1941 for zinc-plated mechanical fasteners such as bolts and nuts. Our companion note on chem film and chromate conversion follows the aluminium branch of that family.

The same page states two boundaries worth carrying into any specification review. Steel and iron cannot be chromated directly; only steel plated with zinc or zinc-aluminum alloy may be chromated. And chromating zinc-plated steel does not enhance the zinc layer's cathodic protection of the underlying steel, a point the Cronak process description makes explicitly.

What does a salt spray hours number mean?

The acceptance side of the map is the corrosion test, and here the public material is unusually clear about what the test is for. ASTM B117 was the first internationally recognized salt spray standard, originally published in 1939; ISO 9227, JIS Z 2371 and ASTM G85 are named as the other relevant standards. The page states plainly that salt spray testing is mostly deployed in a quality audit role, comparing actual against expected corrosion resistance, and that it has little application in predicting real-world corrosion because it does not replicate real-world conditions. The standards themselves, the page notes, give no test periods for particular coatings; durations are agreed between customer and manufacturer. A separate note examines what a salt spray test proves.

Adjacent ISO designations worth knowing

The salt spray page names a small ring of ISO documents that a coating specification can pull in by reference. ISO 4042 gives guidance for zinc and zinc-alloy plating; ISO 10683 covers zinc flake coatings not applied electrolytically; ISO 12944-6 covers painted surfaces over hot-dip galvanizing; ISO 1461 and ISO 10684 cover hot-dip galvanized surfaces, which the page says are not generally tested in salt spray; and ISO 9223 gives the guidelines for measuring the corrosion resistance of hot-dip galvanized specimens. ASTM D1141-98 is named as the practice for preparing substitute ocean water when synthetic seawater is specified. On the modified side, ASTM G85 contains five annexes, A1 through A5, covering acetic acid, acidified cyclic, seawater-acidified, SO2 and dilute electrolyte variants.

Reading a drawing against this map

The practical move this map supports is a check against the purchase order. Confirm the designation belongs to the family intended, that it is not the one entry marked inactive in the source, and that any salt spray duration quoted is an agreed requirement rather than something the test standard itself imposes. Then get the document, because every number that decides the batch lives there.