Zinc Pest

[Il Grifone]

Having acquired an incomplete pre-war Dublo N2 chassis block, suffering from the pest (not too seriously - it's still in several pieces, rather than a pile of powder), I was investigating how to best deal with it . All the bits have been transferred to a post war block, which is now functional, but I thought to fit new wheels* and static mount it with a body repainted as a pre-war example - the genuine article is 1. hard to find and expensive and 2. liable to suffer the same fate. I'll probably do her as 2690 in LNER black, but N2 bodies are plentiful enough to ring the changes as 6699 (replacing the dome with the proper device!), 4917 or 2694, though the last should have green wheels. (Alternative numbers may be chosen to avoid the charge of faking....).

 

I thought of following the procedure outlined in 'Rivarossi Memory', but this involves a bath of ethylene trichloride, which has been banned from April 21st 2016.... I would assume some other degreasant would serve, as the idea is to remove all traces of moisture which apparently is the trigger for the breakdown of the alloy, with vibration and heat accelerating it. (It does seem to affect wheels in particular - all makes.)

 

http://www.rivarossi-memory.it/Tecnica/Curare_peste_zama.htm

 

Researching a substitute, I found the rather alarming claim that the pest is contagious and will result in the loss of an entire collection!!!!   I know the similar lead pest which affects lead soldiers et alia can spread from one item to another, but it is a corrosion phenomenon and isolation is sufficient to sop it AFAIK.

Does anyone know any more?

 

I found the following learned research into the matter, which may be of interest.

 

https://web.archive.org/web/20110716065232/http://www.nlr.nl/id~4210/lang~en.pdf

 

But that still leaves me with the problem of how to go about the restoration process. I don't expect the result to run on a layout, but I would like it stop it degrading further. The forty years claimed should be sufficient for me, I think.

 

Maybe I can find the nasty substance still on sale next time I go to Italy.... (Over there they sell - or did until recently - hydrofluoric acid as a rust remover in supermarkets  . I have a small bottle of this highly corrosive (especially to skin and the tissue beneath) substance and don't know how to safely get rid of it! - pouring it down the drain doesn't seem very public spirited even though it's only a small quantity.)

 

* For some inexplicable reason, it had been fitte. I have a bottle of this d with Triang wheels and converted to 2 rail.    I thought that the block had been bored out to fit Tri-ang axles, but this is not the case, as the wheels are on 1/8" axles. The Tri-ang gear it was fitted with has been bushed and the armature worm 'got at' in order to mesh with it - these are in the new block, but the gear needs rebushing as it wobbles slightly - in time....

[34theletterbetweenB&D]

Zinc pest is non-contagious, requires the contaminant that triggers the problem to be in the alloy melt when cast. All the evidence I have seen relating to the role of temperature is that it is low temperatures that accelerate the failure. (Somewhat counterintuitive, but the failure is caused by crystalline changes rather than a chemical action.)

 

Your local authority should be able to assist with disposal of HF. Alternatively if you have a friendly electroplating specialist somewhere in the vicinity, that's the type of business that could dispose of this chemical safely.

[bertiedog]

The only technique to salvage a cracked item is to reproduce it from the remains, by re-assembling as far as possible or taking casts of each part, in vinamold or silicon, and casting replacements in resin.

 

You could use the moulds to make a wax master, and have it cast in brass or bronze by the lost wax process.

 

The original parts do not suffer by the process of the mould making, and the moulds can be used to make multiple resin parts or wax positives for investment casting by the lost wax process.

 

In the sizes need for cast chassis the lost wax process is practical, but needs an art casting expert to do it for you. The resultant part or parts could be machined etc., or assembled from the bits, with silver solder, and then powder coated black to leave a pretty close copy of the original.

 

If resin casting is used, then really the whole thing needs reconstruction before the mould is made, and a metal loaded resin, art grade, used for the pour. It will be accurate, but lighter than the original.

 

The other modern way would be to have it 3d printed from a CAD file based on the casting, but 3d printing is only in plastic (for amateurs), and quite expensive. In theory castings could be scanned to produce the files, but scanning is only really done commercially, and again quite expensive. Frankly a lot of 3d work is rather poor in quality as well, needing a lot of hand finishing. The resultant parts would also be light in weight,

 

Stephen

[Il Grifone]

Zinc pest is non-contagious, requires the contaminant that triggers the problem to be in the alloy melt when cast. All the evidence I have seen relating to the role of temperature is that it is low temperatures that accelerate the failure. (Somewhat counterintuitive, but the failure is caused by crystalline changes rather than a chemical action.)

 

Your local authority should be able to assist with disposal of HF. Alternatively if you have a friendly electroplating specialist somewhere in the vicinity, that's the type of business that could dispose of this chemical safely.

 

That is what I thought. Thanks for the confirmation.

[Edley]

Personally , as an alternative to lost wax casting , you could have it cast in pewter.

 The advantage being that there is only one lot of shrinkage. Lost was has two. the wax shrinks and the  bronze shrinks. Pewter is cast directly into the silicon mould so only one lot of shrinkage . About 1%

 

 This  is similar to the material used by Leeds originally and is  the method used by them now to  make replacement motor blocks.

[CF MRC]

The lost wax process has the wax shrinkage compensated by expansion of the investment when heated to burn out the wax. It is an integral part of the process (at least for dental castings). Therefore only the shrinkage of the alloy on cooling.

 

Tim