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Hydrogen embrittlement in steel?

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fizzy02/03/2016 19:40:29
1836 forum posts
120 photos

I am planning on zinc plating my loco boiler before turning it black, mainly because I hate painting. Now I know the process has been proven to induce hydrogen embrittlement but would this just be confined to a few microns depth across the surface or is it likely to go deeper?

Howard Lewis02/03/2016 19:59:12
5751 forum posts
13 photos

Why not get it galvanised? Greater thickness of zinc, and no electrolytic problems.

Not sure that I'd like to be near any pressure vessel where the steel had any hydrogen embrittlement!


Brian Wood02/03/2016 20:26:14
2498 forum posts
39 photos

Without sounding alarmist, but to introduce a factor of weakness in a pressure vessel and on the outer surface where it will have maximum influence seems to be risky to say the least.

The effect cannot be quantified in any way and with cyclic pressure variations it may lead to crack propagation. There will no doubt be great tomes written somewhere on the subject in the pressure vessel regulations which might shed some light, but I suspect any boiler tester would take the course of least resistance and mark the vessel down as doubtful, having no wish to put his name on a certificate against a largely unknown factor influencing lifetime performance.

I think it might be safer altogether to stick to paint, even if it is a pain


Michael Cox 102/03/2016 22:12:14
548 forum posts
27 photos

If the article is heat treated after plating then the hydrogen will diffuse out. Normally heat treatment for a few hours at 200-250 degrees C will eliminate hydrogen from steels. This is well below melting point of zinc so there should be no problems. This temperature is easily accessible by normal domestic ovens.


Nigel McBurney 102/03/2016 22:41:57
965 forum posts
3 photos

I always understood that hydrogen embrittlement did not affect low carbon steels,if heat treatment is required then it should be done asap after completing the plating process,. I don't think that I would want a boiler that has been through the plating process, unless the access holes to the boiler had been totally sealed, corrosive plating salts always manage to find their way into hollow structures but its very difficult to get them out, why does a boiler need plating /painting ,it will need lagging and cleading,so most of a boiler is covered up.

oldvelo02/03/2016 23:02:22
280 forum posts
54 photos


Why would you want to a Zinc or Galvanised coating to a steam pressure vessel and run the risk of severe corrosion of any attached fittings.

Part working life was boiler overhauls and fitting and maintaining a factory steam supply Where the use of any Zinc or Galvanised fittings was prohibited because of the risk of failure from electrolosys.

Any exposed steel can be painted with a heat resistant paint.


Mark C02/03/2016 23:14:32
707 forum posts
1 photos

I don't know how big the thing is and can't comment on practicalities or otherwise but you should be able to plate with mechanical zinc if it will fit in the media container. Mechanical zinc = no issues with hydrogen and probably better protection.


Eugene03/03/2016 08:15:59
131 forum posts
12 photos

Hi Fizzy, as a one time electro-plating chemist / metallurgist with experience in post plating de-embrittlement and zinc blacking, it isn't a road I'd choose to go down. Given that the steel in your boiler is unlikely (but not impossible) to be a high tensile variety, the chances of hydrogen embrittlement are low, but that isn't the only consideration.

The plated layer will show up every minor scratch and dink in the surface finish; the process has no levelling power at all; this implies a pre polishing operation, something that in itself isn't without difficulties. To get a coating thick enough to take the subsequent blacking process will not be easy if the part has any recessed areas or re-entrants. The blacking process doesn't always (hardly ever really) give a true black, and its very soft. Any accidental scratch will cut through to the white zinc underneath and show up badly, and be very difficult to remediate. In industrial applications a protective lacquer is applied in an effort to alleviate this effect, not something I'd want to do on a part likely to be pretty hot.

"Mechanical" zinc is now a fairly unusual process; there are only a few companies doing it. II's not applicable to large parts with cavities, and the appearance is utilitarian at best, so not a practicable proposition in my view.

In short zinc plate and black will have safety implications, look terrible, and wear badly.

Paint pot time I'm afraid. sad


Martin Kyte03/03/2016 08:34:29
2638 forum posts
46 photos

Just to add to the confusion. Most things that are "galvanised" these days like greenhouses, lamp-posts etc are not plated at all but are hot dipped in a ruddy great bath of molten zinc.

regards Martin

Neil Wyatt03/03/2016 08:40:36
18899 forum posts
734 photos
80 articles

I suspect the 'blacking' process Fizzy has referred to involves coal and soot...


Muzzer03/03/2016 09:59:30
2904 forum posts
448 photos
Posted by Martin Kyte on 03/03/2016 08:34:29:

Just to add to the confusion. Most things that are "galvanised" these days like greenhouses, lamp-posts etc are not plated at all but are hot dipped in a ruddy great bath of molten zinc.

regards Martin

"Galvanising" usually involves immersion in molten zinc. "Plating" or "electrogalvanising" is generally a chemical or electrochemical process usually resulting in a thinner layer.

The protective effect resulting from zinc coating preventing rust causes consumption of the zinc layer where steel is exposed by damage or abrasion. The thickness determines how long the protective effect lasts. There are BS / EN / ISO specs to determine the appropriate thickness for different applications and section thicknesses.

Mark C03/03/2016 10:36:01
707 forum posts
1 photos

Eugene, Muzzer,

It occurred to me that mechanical zinc might be an option as the part is a boiler. As such I imagined you could plug the holes and then they would become a large round effectively solid object! I also don't know how big it is but you should be able to get something up to a foot long by 4 or 5 inch diameter in a media tank. All that is involved is tumbling the thing in zinc rich chemicals in a lot of glass beads (a bit like the stuff used for glass bead blasting). The beads impact the part and the energy causes the zinc to adhere to the part (simplistic description). I don't know why it is always reckoned to provide better resistance, maybe it is due to the lack of corrosive chemicals perhaps Eugene can enlighten me? I have designed quite a bit of plating equipment but I always did it in conjunction with a chemist/metallurgist so I only had to deal with the engineering.


PS. I got the impression it was a pre-treatment before painting or some other finish not that Fizzy wanted "black zinc" which as Eugene pointed out is a specific "electro" finish often favoured by automotive OEM's.

John McNamara03/03/2016 10:42:58
1331 forum posts
122 photos


Mechanical zinc coating is sometimes called sheradising.

Google Sheradising
or try the following link



Mark C03/03/2016 10:55:45
707 forum posts
1 photos


We are talking about "mechanical zinc" this is a cold, wet process. The zinc is in a solution/mixture (I don't know which) and relies on the rubbing/impact force of the glass balls (a bit like sand I suppose) to "hammer" the zinc onto the surface. I had a quick look and found this advert on youtube **LINK** which shows washers and such like being plated. It is most often used for small fasteners and such but there should be nothing to stop a bigger item being treated as long as it fits in the barrel comfortably and is free to move about in the media.


Howard Lewis03/03/2016 16:42:44
5751 forum posts
13 photos

Also, why paint it black anyway?

Black is a good radiator of heat, so silver paint would radiate less heat, and so aid the external insulation.


fizzy03/03/2016 18:22:48
1836 forum posts
120 photos

thanks to all for your input. Having had time to read up today it seems that only high tensile steel is adversley affected. The boiler plate is general purpose mild steel. It wont be lagged because the original full size was not lagged, and it needs to be black for the same reason. I was however under the impression that a black passivate would be a durable finish - if this isnt the case then the whole process stops there! As for plugging all holes whilst it is dipped, its a boiler so its completely sealed if i add plugs. There will be absolutely zero contact with steam or water when in use as this is externel plating in a solution. Please comment further if you are able.#

Tim Stevens03/03/2016 18:43:24
1517 forum posts

If you blank all your holes will the boiler sink in water? If not, this is something else to think about in any plating process.

Ditto if ever you think of real galvanising, except that molten zinc is much heavier than water so the extra mass to be added will be greater.

Cheers, Tim

Mark C03/03/2016 19:04:57
707 forum posts
1 photos

As long as it is a cold process you could simply fill it with fresh water and plug it up - metal tin full of water = guaranteed to sink!


John McNamara03/03/2016 23:04:25
1331 forum posts
122 photos

Sheradising is not a wet process.

From my previous link......(Wiki)

Sherardizing is a process of galvanization of ferrous metal surfaces, also called vapour galvanising and dry galvanizing. The process is named after the British metallurgist Sherard Osborn Cowper-Coles (son of naval inventorCowper Phipps Coles) who invented and patented the method ca. 1900.[1][2][3][4] This process involves heating the steel parts up to ca. 500°C in a closed rotating drum that also contains metallic zinc dust and possibly an inert filler, such as sand.[5] At temperatures above 300°C, zinc evaporates and diffuses into the steel substrate forming diffusion bonded Zn-Fe-phases.


Bouvet door handles France images

This company manufactures a large range steel and cast iron fittings. The sheradised Zinc plate they use is durable. A good example of the process.


Mark C03/03/2016 23:28:54
707 forum posts
1 photos


I never suggested it was. I was pointing out that mechanical zinc is a wet and cold process that produces a zinc plating akin to the electro zinc (also wet and cold) that was discussed at the very start. it is reputed to provide better protection than electro zinc (I don't know why that is) but with no problem associated with hydrogen.

I looked at your door handle link and did not know that sheradised parts could be polished/burnished to a bright finish. I have only ever associated the process with dark grey mat finish.

The other possibility for the boiler would be phosphate coating which would provide a very good key for any paint finish (and is also a nice dark grey to start with).


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