Steel in an aggressive environment.

[NikkiJ]

Hi, I'm an apprentice from an engineering firm in England. One of our products is currently being used in Bahrain, and is failing due to the nature of the environment. The item in question is cast steel block, with a deep hollow in the back, and a bored hole through the block.

At the moment we zinc plate this block before painting, which is good enough for countries such as England, but is no use to such harsh treatments as are found in the customer's countries. The plating solution sits in the hollow and this is causing problems. We've looked at investment casting in Stainless steel to prevent corrosion but the costs are not feasible. It doesn't seem possible to put in a draining hole either.

Can anyone recommend any other finishing techniques for this item? I have heard that Sheradizing (sp?) may be more suitable for this, but am unable to find any information on the process.

Thankyou in advance for your help.

 

[rustbuster1]

It's hard to tell from your description if the problem is with incomplete coverage with the zinc plating - which possibly could be remedied by hot-dip galvanizing the piece or using an organic or inorganic zinc-rich primer - or if the protective system is inadequate.

In relation to the expectedly hotter Mid-east service, poor results could occur because zinc changes from cathodic to anodic at temperatures above about 55C. Higher ambient temperatures also typically drive corrosion rates faster, although one also would expect lower "times of wetness" in the desert. You don't say how this is used.

If you want more help, try to be more descriptive of the problem

 

[NikkiJ]

The cast steel block is a lock body. There is a hole drilled and reamed vertically through the block. On the back of the block there is a deep well where the problem occurs. We have been told that the plating solution may sit in this well, as it cannot drain in the plating process.

It's used on gates with a high security padbar. They're used in high risk locations and if the locks seize, they are hard to break into. The locks have been known to hold gates together even after being blown off so that gives an idea on how difficult it is for the customer.

The block is plated to help the paint adhesion, then the vertical hole drilled. There are also thoughts that this well holds humid air in the day, which as the lock cools towards evening, condenses and compromises the system. Any other ideas which may aid me?

Thankyou for your help so far, it's very much appreciated!

 

[TVP]

It sounds like your problem is due to the fact that a hole is drilled after the plating is applied. Plate the part after the hole is drilled so that there is no unexposed areas.

My next suggestion would be to use a zinc-alloy plating instead of just zinc plating. A zinc-nickel alloy plated to the same thickness as conventional zinc plating will improve the corrosion protection (as measured by standard tests like ASTM B 117) by at least a factor of 4.

My last suggestion is to switch to a stainless steel instead of carbon or alloy steel if you want better inherent corrosion resistance.