HF acid corrosion

if you are talking about valves then you need to press the issue with the factory. they have a lot of resources for such problems.

with HF you need to be more specific as to the service conditions. in operating process units the conditions are pretty severe and materials selection is the big issue as opposed to heat treating lesser grade materials.

Factory support is not an option. We are an independant buisness for the most part. We are an authorized repair center for Durco acid valves I was just looking for some info or suggestions I haven't heard before. We have done this a long time and have a good handle on it.

I just recently had a customer reaquest a valve in solid monel because acid is eating the steel one (steel is common in most places of the alky unit for valve body material they just have monel trim) The particular valve size is not currently availible anywhere and I am trying to find out if I can make the steel one last longer.

body attack can be caused be excessive velocities and cavitation.

short of going to a higher grade of materials about all you can do is limit the velocities in the body by proper sizing, and attempt to control the cavitation with limits on valve d/p.

In the Oil & Gas industry we regularly supply steel valves for various services which have seal areas and wetted areas weld overlayed with corrosion resistant materials to reduce corrosion in service.

This is more cost effective than supplying a complete valve in Ni alloy etc. However, the ability apply the weld overlay depaends on the type of valve and the design.

We do our own overlay in house. In this particular situation the steel body valve is being severly attacked and a monel replacement is not readily availible. I'm looking for specific treatment to WCB steel to increase corrosion resistance (such as annealing).

ndh78

I don't believe there is any heat treatment or simple surface treatment that you can apply to WCB which will improve its corrosion reistance in HF service. Hence my original posting referring to overlay.

How about coating it with a fluoropolymer.

As HF is an extremely bad actor if it escapes, you are looking to accept more liability than most people. The safe response to you is replace this valve with the proper material of construction. Anything else is exposing your customer to an unnecessary premature failure.

First, ASTM A216 steel castings are heat treated (annealed, normalized, or N & Tempered), so I feel we are working with carbon steel microstructures that should provide the best overall corrosion resistance.

Our experience from laboratory corrosion testing and piping components from HF alkylation units around the world showed very definitively with wrought carbon steel, at least, that keeping the cumulative level of residual elements (copper + nickel + chromium) to less than 0.20 wt. % was beneficial to minimizing corrosion of carbon steel in HF acid service. The concept was published by NACE in 1993 (Paper 623). Note the permissible level of these same elements per ASTM A216 is 1.0% max. Of course, in general, carbon steel should not be used beyond its generally accepted limit in anhydrous HF, namely metal temperature below 150F. Fluid velocity also is an important factor in the corrosion rate.

To answer your specific question ... other than what has previously been offered in this forum (monel weld overlay, for instance) I know of no treatment to significantly improve the corrosion resistance of carbon steel castings to anhydrous HF acid.

We are having success treating parts immersed in smelt at 2000 F with free sulphur present. Results are quite impressive. We would like to try our approach with HF and we are willing to treat a couple of test valves w/o charge. We use Chromium diffusion with a proprietory CVD process. This process promotes a diffusion of Chromium into the base metal. Please contact us for details
marciogerep

Chromium is the worst thing we can have in the steel so I don't think it would work. We actually nuclear analyze our wcb bodies to make sure the trace elements are at a minimum