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Anodising and Passivating

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jon hill 313/04/2021 19:51:32
121 forum posts
24 photos

Can anyone explain the difference and what properties these processes add to commonly used metals?

Clive Foster13/04/2021 22:29:03
3105 forum posts
107 photos

Anodising is for aluminium and light alloys. It allows you to trap dye in the oxide layer to give a near permanent colour. It also improves surface hardness in greater of lesser degree depending on process details.

Hard Anodising, unsurprisingly, produces the hardest surface as that what its intended for. Colours tend not to be as deep or even. Often left natural.

Although ordinary anodising is usually done for decorative purposes some process / material combinations are surprisingly hard.

Passivating is usally associated with steel. Various simple chemical and electrochemical processes that make it much ahrder for rusting or other corrosion to get started. Many have a characteristic colour. Usually the process is identified by a trade name.


Nigel Graham 213/04/2021 22:31:39
2056 forum posts
28 photos

Anodising is for treating aluminium and its alloys only, by giving it a hard but very thin oxide skin that can be coloured with special dyes. It is not plating, and is not entirely protective. There is a form called "hard anodising" that gives the surface some measure of wear resistance.

Passivating is a further treatment for zinc-plating on steel.

Clive Hartland13/04/2021 22:33:39
2812 forum posts
40 photos

Passivating from my early days was a zinc plating and a dip into a bath that turned it a gold/yellow colour.

Anodising is done to give varying depth and hardness , AA5. AA10. AA15 and AA25. the latter usually associated with exterior window frames etc. Longer in the etch bath gives a harder surface.

Remember anodising is non conductive to any connection must be made to the base metal.

SillyOldDuffer14/04/2021 09:10:16
8513 forum posts
1914 photos

I think the difference is simply down to how the protective layer is created. Anodising is an electrical process producing a protective oxide layer; Passivating is a chemical process, including oxidisation by heat, but also paint, and plating.

Anodising is most useful on Aluminium because the Oxide is tough, firmly attached, corrosion resistant and can be dyed. Applying the same process to steel produces rust, which is weak, flakes off, holds water, and plug ugly. Anodising is a rotten way of protecting steel, fortunately steel can be passivated in other ways. And many passivation methods good for steel, like bluing, don't work on Aluminium.


Bo'sun14/04/2021 09:23:34
602 forum posts
2 photos

Passivation give a micron thin layer of protection to steel, but nowhere near as good as galvanizing. While it provides a decorative finish and some degree of corrosion protection, it doesn't seem to fare too well outdoors. Someone told me years ago that it was slightly porous. Also, passivation can suffer from "white corrosion", which is a sort of powdery deposit, I believe caused by a salty atmosphere.

Neil Lickfold14/04/2021 10:50:34
836 forum posts
166 photos

Anodising has an electrical insulating effect as well. It is used as the insulating medium for aluminium made glow plug bodies. The post must have a non anodised area for the plug wire and for the electrical connection. Like wise the body needs either on the threads or the body seating area no anodising as well. An interesting benefit from the anodising.

Adrian R214/04/2021 10:58:48
161 forum posts
5 photos

"Passivated zinc" is a common finish for car bits, it keeps them looking nice and presentable in storage so they can be sold, but doesn't last very long once fitted to the vehicle and exposed to the elements. Of course this is probably done to carefully specified minimum standards and "fit for purpose".

Bo'sun14/04/2021 11:08:23
602 forum posts
2 photos
Posted by Adrian R2 on 14/04/2021 10:58:48:

"Passivated zinc" is a common finish for car bits, it keeps them looking nice and presentable in storage so they can be sold, but doesn't last very long once fitted to the vehicle and exposed to the elements. Of course this is probably done to carefully specified minimum standards and "fit for purpose".

I guess that's what I was eluding too. Looks good, but doesn't last. We used to waxoil exposed hardware on tractors that were being shipped overseas "on deck" to make it last a little longer.

Howard Lewis14/04/2021 12:30:15
6032 forum posts
14 photos

The danger with any electrolytic treatment of highly stressed steel components is Hydrogen embrittlement.

Thus Galvanising entails dipping the ferrous components into a bath of molten Zinc, (Tapped holes, or studs can be protected by filling/wrapping them with masking tape. To my surprise, this is what Lotus did with their chassis ).

Sherardising entails tumbling the parts with metallic Zinc, whilst in a heated enclosure.

In this way, no electrolysis is involved.

Aluminium can be anodised by electrolysis in a bath of Chromic acid (As the name suggests, the component is the Anode )

I am not advocating this, but Aluminium, or steel, can be passivated by dipping into a bath of concentrated Nitric acid. Nitric acid is such a powerful oxidising agent that it produces a protective oxide film on the surface.

Concentrated Nitric acid is dangerous!


martin haysom14/04/2021 12:54:26
90 forum posts
Posted by Howard Lewis on 14/04/2021 12:30:15:

Aluminium can be anodised by electrolysis in a bath of Chromic acid (As the name suggests, the component is the Anode )


can also be done with sulfuric acid

Dave Smith 1414/04/2021 13:43:24
212 forum posts
43 photos

Until the advent of REACH there were two processes and three types of anodising used for Aluminium and its Alloys.

Sulphuric which be used to give either a very hard coating (hard anodised) or dyed to give colour and commonly known as sulphuric anodising. Sulphuric anodising you need to specify the coating thickness as the build up on the outside of the surface will effect close tolerance dimensions.

Chromic anodising gives either a clear or grey finish, but more important is the build up of the anodising on the outside of the surface is small enough that in most cases it does not have any effect on the part dimension. Chromic anodising gives better corrosion resistance at temps up to about 150 deg C. after that Sulphuric gives better protection. This is important in defining the best process with regards to protecting aircraft parts.

Chromic anodising is now banned under REACH, It is not the finished product that is dangerous only the process. Most Aerospace companies still have a waiver to use Chromic for now, , Sulphuric or modified sulphuric process are now being used or developed but when I retired three years ago they were not giving the same level of protection as Chromic. Boeing did some some tests about 10 years ago using purely sulphuric building 3 aircraft. They found after a years service the corrosion present was more than they would expect in three years of service using chromic.

Oily Rag14/04/2021 14:16:00
540 forum posts
184 photos


This is the same problem as faced by the process of 'black chromate' (but sometimes gold coloured as well ) anti corrosion treatment for magnesium castings. As the process used Hexachrome which is now recognised as highly carcinogenic. Waivers apply for aerospace and defence industries but these are due to 'expire' soon. This is a major problem for magnesium components used in Manx Nortons, 7R's and G50 Matchless engine castings as well as Cosworth DFV and such like front covers and cam covers. This is likely also to affect in future 'Hard Chroming' as this also uses Hexachromium, decorative chromium is trivalent chrome so should be safe for a while yet.

Magnesium has only one object in life - that is to return to its original state, magnesium oxide!


Clive Hartland14/04/2021 15:20:39
2812 forum posts
40 photos

My Anodiser used Tin. in the vats and Sulphuric acid.

One of his main jobs was anodising shop front aluminium frames to withstand all weathers, They came out in a dark shiny bronze colour.

I tried to do it for a military job but it came out all mottled, as a non payer I could not press him to do it for me.

Another process was powder coating on top of anodising, this for skirting panels in all sorts of colours.

Re. Zinc plating, I found that zincplating if heated came up with a lovely black surface for engraving.

A small side story here, we had a heated vat of Magnu 577 if I remeber correctly and cast and steel put in came out pristine from all the gunge and detritus of carbon and burnt oil.

Another was Black chrome which was used to treat parts inside of artillery recuperators to stop corrosion from the oil.

ChrisB14/04/2021 16:00:19
659 forum posts
212 photos

At work (aircraft MRO) we use chromic acid to protect aluminium alloys, it gives a golden colour to the metal and the process is the base for corrosion protection. Anodizing on it's own is not sufficient. The process is normally followed with etchprimer and top coat. The only areas which are exposed are made from clad aluminium alloy.

For steel parts we send them to the playing shop for cadmium plating which also gives the metal a golden matt colour. We used to repair damaged areas with brush type plating, but the material is now banned.

Dave Smith 1414/04/2021 19:41:33
212 forum posts
43 photos

The gold coloured Chromic Acid processes are more commonly known as Alodine (USA) and Alocrom (UK). As Martin said these are also banned on the REACH list and ChrisB's company will be using them on waiver. We never used it on finished parts and stopped using it on bared edges and repairs about 6 years ago prior to the original REACH sunset dates replacing it with Surtec 650 Pens.

Cadnium plating has been banned by all the western Aircraft OEM's for new projects for about the last 15 years and ChrisB's company will only be able to use it via Grandfather clause on parts qualified using it. Although at present there is nothing that can touch it for corrosion protection of steel components on aircraft!

Jouke van der Veen14/04/2021 21:24:33
171 forum posts
17 photos

Passivation is even applied on stainless steel to further improve corrosion resistance.

It is done with, e.g., hot nitric acid so that iron is removed from the surface and chromium enriched to form chromium oxyde.

I remember this process for components to be applied in aerospace machinery.

Stuart Bridger14/04/2021 21:24:41
538 forum posts
29 photos

I will have to dig out my apprentice log from my tour of the process shop at BAe Weybridge in 1981. Lots of "nasties". As others have mentioned, Chromic acid, cyanide salts, trichloroethylene vapour phase degreasers, molten salt baths for heat treating aluminium alloys, etc. Cadmium plated fasteners were standard specification back then. All my steel apprentice pieces were phosphated and are still rust free after 40 years.

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