Yellow zinc passivation of steel

[Steerpike]

Hi Bulgin
That is useful info, anodising the aluminium front is going to have to happen sometime soon too!
I know from other posts on that topic that BLACK is difficult to do - any progress in that colour?

Grand Poobah: Yes! Good point about the sacrificial aspect - I hadn't though of that. Maybe I will just leave it as is....

 

[eeka chu]

Ships with steel hulls exploit the same trick. Zinc blocks are bolted to the hull. The zinc scavanges the corroding electrons from the steel, rotting it's self away into the water. The zinc isn't cost free, but it's a lot cheaper than dragging a ship into dry dock and replacing the entire hull, or having it sink, or having an underwater welder try to patch up continually respawning holes. You can actually see the blocks sometimes if the boat is in clear, shallow water. They look like hand sized lumps under the water.

 

[AndrewT]

and the manufacturers of our steel bodied cars could do the same.
I wonder why they don't?

A zinc block bolted under each wheel arch would probably make the painted car last at least twice as long and possibly a lot longer than that.

 

[eeka chu]

and the manufacturers of our steel bodied cars could do the same.
I wonder why they don't?

A zinc block bolted under each wheel arch would probably make the painted car last at least twice as long and possibly a lot longer than that.

Good question, three possible answers. The first is cost, but galvanising has to be more expensive. The second is that people would probably never check them. The third, which I suspect is the correct one, is that the zinc blocks on a ship are constantly connected via a gigantic salty electrode to Earth. I don't know if the block option works quite so well if this isn't the case.

 

[auplater]

Corrosion protection

Ships with steel hulls exploit the same trick. Zinc blocks are bolted to the hull. The zinc scavanges the corroding electrons from the steel, rotting it's self away into the water. The zinc isn't cost free, but it's a lot cheaper than dragging a ship into dry dock and replacing the entire hull, or having it sink, or having an underwater welder try to patch up continually respawning holes. You can actually see the blocks sometimes if the boat is in clear, shallow water. They look like hand sized lumps under the water.

Actually, the zinc supplies electrons preferentially to the corrosion reaction, forcing the steel hull to behave as a cathode instead of an anode and hence receiving the corrosion current instead of sourcing it. External cathodic protection also works this way, i.e.
a cathodic charge can be impressed on the hull with countering insoluble anodes located externally.

Zn -> Zn+2 + 2 e- +0.76 V

Fe -> Fe+2 + 2 e- +0.44 V

hence the zinc behaves anodically to the iron, protecting it

Note, other metals occurring higher in the series could be used, but zinc is the cheapest that fits the bill

http://www.chemguide.co.uk/physical/redoxeqia/ecs.html

http://en.wikipedia.org/wiki/Cathodic_protection


John L.

 

[auplater]

Auto corrosion

and the manufacturers of our steel bodied cars could do the same.
I wonder why they don't?

A zinc block bolted under each wheel arch would probably make the painted car last at least twice as long and possibly a lot longer than that.

There are many reasons why this would be ineffective. the zinc would only protect a limited area around it, for one... applying enough zinc to protect the entire car would be cost prohibitive.

Eeka hints at this referring to the salty electrolyte of the sea providing the cunduction path for cathodic protection... doesn't exist on a car.

galvanizing the panels is costly, and it is difficult to make paint stick to zinc.

A second bigger problem is differential aeration that occurs anywhere the paint has a scratch or defect exposing the underlying steel. When this happens, rapid corrosion will ensue, regardless of other protection schemes (including cathodic protection)

http://www.azom.com/details.asp?ArticleID=96

John L.

 

[AndrewT]

Good question, three possible answers. The first is cost, but galvanising has to be more expensive. The second is that people would probably never check them. The third, which I suspect is the correct one, is that the zinc blocks on a ship are constantly connected via a gigantic salty electrode to Earth. I don't know if the block option works quite so well if this isn't the case.

the wheel arches of UK cars are often wet and sometimes salty.
We have MOT tests annually, it would be very easy to add zinc block inspection to all the other things, both safety related and non safety. But that's another flee up my nose. Bodywork corrosion is a safety related matter.

The real reason, for not installing sacrificial anode protection, is that car manufacturers would sell fewer cars if proper corrosion protection were invoked.


Auplater,
thanks for the link.

 

[eeka chu]

Re: Auto corrosion

galvanizing the panels is costly, and it is difficult to make paint stick to zinc.

Thanks for the correction, I knew it was something along those lines but have managed to forget that area of A Level chemistry.

I remember working for a steel fabricators and we had an order to install some kind of fence posts for an airport I think. We'd made them and had them galvanised, then bought huge quantities of very expensive acid based stuff (I think) to prepare the surface for the paint. Turned out it could be diluted X number of times and we had enough of this pricey stuff to do every airport in the UK. Luckily, I was on work experience and not the one paying for said surface preparation. Rather than the experts I was learning from alongside, it was the spray shop who read the instructions and worked out it could be diluted - probably still turned up on the bill.

I also worked making custom gates and railings that we usually galvinised. I don't know if it helped much with paint application, since I wasn't doing it for more than a few months, but the red oxide undercoat usually stuck nicely. But then, I wasn't producing mirror finished either, most people were perfectly happy so long as it looked semi-decent from more than a few feet away.

 

[dnsey]

and the manufacturers of our steel bodied cars could do the same.

Some do!

There are two processes used - either galvanised steel is used to manufacture the body panels, or the assembled body is galvanised.

 

[bananaslug86]

The pictures look a lot like a coating I would put on aluminum, strontium chromate. It was a two part coating which would be used as a base before applying paint or other finishes. It was more active than zinc chromate so it would protect aluminum where zinc wouldn't. I got it at auto paint stores. All the usual warnings apply, use breathing protection, apply in a well vented space, use gloves and dispose of carefully. The nice thing about it is unused portions harden and are easier to dispose.

 

[dsb]

Cars' monocoque body construction are primarily welded. Galvanized steel doesn't weld. Post weld galvanizing produces too uneven coating thickness to support automotive class A finish, notwithstanding poor paint adhesion to zinc. Also, its a time consuming process requiring inspection: expensive.

On example posted, as described it is a chromate conversion coating. This is one of the most common coating on formed and stamped sheet metal chassis components, others being "Satincoat" (pre-galvanized sheet metal, i.e. all PC chassis) and electroless nickel. It can be a clear chromate conversion, colour is added, usually by default now if not otherwise specified, to visually indicate it's there. For example, a clear chromate conversion coating on aluminum resembles very closely clear anodizing. Where the colour is of interest is that the anodized surface is non conductive.

More than you ever wanted to know about metal coating...

 

[bulgin]

Yellow zinc passivation...

Hi All

@ auplater

I do seal the things in boiling deionised water for a time equal to the usual 40mins anodising time. I also have acetic acid which I use to adjust the tint ph. I haven't used it in the sealing bath yet. Will try. Thanks

@ steerpike

I never got the black the way I want it. I will soon get the cost lowdown on professional black tint from my supplier as Dylon Ebony General purpose dye doesn't seem to be around any more.

@ Elsewhere in this thread a member mentioned about painting of galvanised steel. I made such a steel fence around 1988. After galvanising, I scoured the lot with coarse rouge and pieces of hessian cloth. What a job

The paint job lasted well more than 14 years without peeling.

Everything is in the preparation.

bulgin