8 µm of zinc: 200 hours plated, 120 mechanical

The source is Deposition processes and properties of coatings on steel fasteners – A review, in Friction, volume 7, issue 5, pages 389–416, 2019, by three authors at Swinburne University of Technology. It is open access under a Creative Commons licence and it compares electroplating, hot-dip galvanizing, ion vapour deposition and mechanical plating.

Alloying moves the number further than thickness does

The first table holds electroplated zinc alloys at a fixed 8 µm with iridescent yellow chromate, so the only thing changing is what is in the coating.

Eight micrometres throughout. A fifth of a percent of cobalt doubles the hours, and eight percent of nickel takes them to four or five times the plain zinc figure. If you have been treating coating specification as a thickness decision, this table is the argument that the alloy is the larger lever.

That Zn–Ni row is the same family as the bolts in the baking study, where the coating was alkaline zinc-nickel at about 14 % nickel. The corrosion performance is why anyone puts it on a 12.9 bolt; the hydrogen is the price.

The mechanical plating table, and a disagreement inside it

The second table is mechanically plated steel washers in 5 % neutral salt spray to ASTM B-117.

Zinc appears at 20 µm twice, from two different references, at 180 and 212 hours. Same metal, same nominal thickness, two sources, eighteen percent apart. Nothing is wrong with the table; that is the ordinary spread between laboratories, and it is the best short argument this site has yet found for not writing hours on a drawing.

The 75 µm row is worth a second look for the opposite reason. It clears six hundred hours without chromate passivation, which is the one row where the coating is doing the work alone.

Why 200 and 120 cannot simply be subtracted

Both rows say zinc, eight micrometres, yellow chromate. Four things behind them are not the same.

So this is not a controlled experiment with the deposition process as the single variable. It is two compilations of other people's results, printed a few pages apart by authors who were not claiming to have run them against each other.

What it supports is a direction, not a ratio: at the same nominal thickness and the same post-treatment, mechanically deposited zinc came out lower than electroplated zinc in the numbers this review collected. That is consistent with something ISO 12683 says about itself, which has its own page here: a mechanically deposited coating tends to be thinner at exposed edges and sharp projections, and thicker on flat and shielded areas. A nominal thickness on a washer and a nominal thickness on a thread are not distributed the same way, and red rust starts where the coating is thinnest.

What to do with a table like this

Not put it on a drawing. This site's position on that has not changed and the eighteen percent spread above is the reason: hours are an outcome of a cabinet, a specimen and a laboratory, and nobody will convert them into a service life. What the table is good for is choosing which lever to pull before the specification is written.

• If the requirement is more protection at the same thickness, change the alloy. The first table moves from 200 to 1000 hours without adding a micrometre.
• If the thickness has to stay low because of the thread, the alloy is the only lever left. Below the small diameters the coating is a dimension before it is a finish.
• If the process is being changed, do not carry the hours across. The same nominal thickness deposited a different way is not the same distribution of metal on the part.
• Ask for the test on the finished fastener, not on a washer or a panel, which is the ordinary reason a catalogue figure and an incoming inspection disagree.

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

How many salt spray hours does 8 µm of zinc give?

It depends on how it was deposited and what is on top of it, and the published figures for the same nominal thickness differ. A 2019 review of fastener coatings lists electroplated zinc at 8 µm with iridescent yellow chromate at 200 to 250 hours to red rust, and mechanically plated zinc at 8 µm with yellow chromate at 120 hours, in two separate tables drawn from different sources with different test pieces. Those are collected figures rather than one experiment, so treat the difference as a direction rather than a ratio.

Does alloying zinc help more than making it thicker?

In the electroplated table it does. At a fixed 8 µm with the same chromate, plain zinc gives 200 to 250 hours, zinc with 1 % iron gives 350, zinc with 0,8 % cobalt gives 500, and zinc with 8 % nickel gives 1000. Four to five times the hours with no extra thickness, which matters on small threads where thickness is limited by the tolerance before it is limited by anything else.

Why do two sources give different hours for the same coating?

Because the number is an outcome of a specimen, a cabinet and a laboratory rather than a property of the coating. In the mechanical plating table, zinc at 20 µm appears twice from two references, at 180 and 212 hours, eighteen percent apart. That is ordinary, and it is the reason a drawing should specify the coating standard and thickness class rather than a number of hours.

Is mechanical plating worse than electroplating?

The figures this review collected are lower for mechanical plating at the same nominal thickness, but they come from different sources with different test pieces, so they do not establish a ratio. There is a mechanism that fits the direction: ISO 12683 notes that mechanically deposited coatings tend to be thinner at exposed edges and sharp projections and thicker on flat and shielded areas, and red rust begins where the coating is thinnest. Mechanical plating is also chosen for a reason electroplating cannot offer, which is that no current is involved.

References

• Chung P. P., Wang J., Durandet Y. — Deposition processes and properties of coatings on steel fasteners: a review. Friction 7(5) 389–416, 2019. Tables 5 and 6, and the sections on corrosion performance
• ISO 12683 — mechanically deposited coatings of zinc; the note on thickness at edges and shielded areas

The review was read in full as the published open-access version, not from its abstract. It is a review, which means the two tables quoted here are compilations of other people’s results and not measurements the authors made. Table 5 is reproduced in the review from an ASM International source dated 1994 and Table 6 draws on three separate references; none of those originals was read for this page and none is quoted. The two tables use different test pieces, one unstated and one steel washers, and describe their tests differently, one as neutral salt spray and one as 5 % neutral salt spray to ASTM B-117. For that reason the difference between 200–250 and 120 hours is presented here as a direction and never as a ratio or a conversion factor. The figures for hot-dip galvanizing and ion vapour deposition that also appear in the review are not reproduced, because they sit in running text under varying conditions and collecting them into a table would be this page inventing a comparison. The friction coefficients in the review’s section 5 are sliding values and are not used here, because they are not the thread and bearing-face friction that a tightening torque depends on. This page recommends no thickness, no alloy and no number of hours.