Dear Mr Jenkins
Thanks for opinion and convincing reasoning for” the effect of Higher Fe in Monel weld”
The reason given by consultant and end user (Refinery) that, increase in Fe , accelerates the corrosion and life is reduced.
Of course I have seen the effect of Fe in Monel weld overlay ,which was exposed to brine solution in soda ash manufacturing unit. Where Fe was restricted to 5 %. In a month the weld overlay started corroding . (scattered Pitting)
Some time free iron on weld overlay also causes the corrosion. it is recommended to check with Ferroxile test .
We can achieve the %Fe in Monel overlay below 2.5 % with increase in overlay thickness say up to 6-7mm There is a cost implication.
What is the concentration, pH, pressure and temperature of the acid? HCl is, of course, extremely aggressive but also has a tendency to be reducing in nature, so it will dissolve some passive oxide films without replenishing it oxidatively in the way that concentrated nitric and sulphuric acids can.
Alloy 400 has the following composition approximately:
63 - 70
Clearly, the Fe content is 2.5%., so that is almost certainly why they wish to keep the Fe content to the same maximum figure in the weld repair overlay. If you don't, you will get preferential attack in the weld repair areas, as they will be anodic to the main area of the cladding. You will thus have a small anode and large cathode and a hole in your vessel pretty quickly.
You also have the possible problems of pitting and leaching .
Where you get dissolved oxygen in the acid, you will end up with ferric ions in solution, which are very effective oxidising agents - ferric chloride is commonly used as an etchant, so you may end up with an autocatalytic corrosion process.
Have you asked the specifier why they want the lower Fe content? It's well worth doing for two reasons -
1. He should know the answer.
2. If he doesn't, he needs to find out why they've been specifying it!
In my experience (which goes back 40 years), people often include things in specifications because they always have done and not because they understand why. This sometimes leads to some grossly inappropriate specs. being issued. In this case, though, they have a very clear case for specifying the lower Fe content and I would endorse their requirement.
----- Original Message -----
From: PB HEGDE
Sent: Monday, June 08, 2009 6:11 AM
Subject: [MW:2344] Effect of %Fe in NiCu7 weld overlay
I have a query i.e.
For Monel 400 clad restoration by weld overlay of Nicu7 on SA 516 Gr 70 base , Now a days Process licenser allows of Max 2.5% Fe (earlier they were allowing up to 5% Fe) The equipment is used in refinery for HCL fluid service.
i)What must be the reason for making stringent requirement
ii) What is the effect of Fe in monel weld for chloride service
I will be thankful to you for guidance in this regard
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