Problem With Corrosion Of Stainless Steel

Three months back I was asked if I could make 1500 or so Magnetic Stainless Steel buttons from 416 Stainless that were to be attached to various electric motors in a Nuclear power station to enable a magnetic sensor to be attached and removed as required for monitoring purposes.

The job was very simple, consisting of a 23mm diameter X approx 6 mm thick disc with a rough finish on one side and a smooth finish on the other.

First problem encountered after making them was that the powers to be were not happy with the parting pip that had been left, even though they had been dressed off there was still a little raised pip in the centre. They were bought back to me and I re faced all of them with a facing tool and eliminated the problem.

The next problem was very serious and that was that the buttons were rusting, some of them alarmingly so. Fortunately I had the mill certificate for the material so at least I couldn't be accused of substituting stainless with plain mild steel.

Some of the ones fitted showed a complete cover of rust and also some were starting to show signs of rusting in the bag that they were stored in, however one batch of 500 that were completed a few days after the initial batch showed no signs of rust forming. I think though can't be certain that I washed one batch, possibly the first to remove any grease from them because they were to be glued to the motor and I wanted to give it the best chance, I washed them in a strong detergent (Fairy Liquid) and dried them in the oven, it was at the end of this process that I noticed some very small signs of pitting here and there amongst them so I wiped them all clean and sent them off

I thad the ones that were showing signs of rust sent back to me and I took another skim off them.

Yesterday I experimented with various buttons, I soaked two in a mix of table salt and washing up liquid, another couple I soaked for an hour or two in lime juice in an attempt to passivate them, another couple had been laying around the workshop for a couple of months, having been found when sweeping up and another batch I took straight from the machine and placed all of them outside open to the elements.

The results are not promising, the batch that were soaked in washing up liquid and salt had pretty much rusted right over the face, The ones that came straight off the machine were showing signs of slight rust after a few hours, The ones that had been machined a while back were a little less corroded than the freshly machined ones and the ones that I soaked in lime juice were better than the rest other than a slight bit of light rust starting to show on one of them.

The problem looks to be corrosion of free iron after the machining process that has not been removed properly. Would prosper passivating or electroplating improve the corrosion resistance and which would be best?

From a supplier web site

With its higher sulphur content 416 stainless steel has lower corrosion resistance to that of 410 stainless and all other 400 series stainless steel grades. It provides optimum corrosion resistance in the hardened and tempered condition. Allowing oxygen to circulate freely on the surface 416 stainless steel will form an oxide film which protects the surface. Keeping the surface free of scale and foreign particles improves corrosion resistance, finished components should be passivated.

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Bob,

This is probably the best brief summary of stainless steel grades that I have seen: **LINK**

http://sesco.biz/sesco-food-service-blog/sois_stainless_steel_really_stainless

Chromium Oxide would appear to be your friend

MichaelG.

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[quote]

How does stainless steel work?

The chromium in the steel combines with oxygen in the atmosphere to form a thin layer of chromium oxide, which acts as a passive film. The chromium atoms and their oxides are similar so they pack tightly together on the surface of the metal forming a layer only a few atoms in thickness. If the metal is cut or scratched and this passive film is disrupted more oxide will quickly form and recover the exposed surface. Essentially it continually “self repairs” itself. It requires oxygen to do this so if the stainless steel is in a low oxygen (think underwater or wet) or poor circulation environment….it could rust.

[/quote]

Edited By Michael Gilligan on 10/03/2018 14:11:31

I will try hardening and tempering a few tonight in the furnace. There seems to be a fair bit of difference in recommendations, some say passivate some Harden and Temper and others Electroplating (Not actual deposition of metal such as nickel but altering the surface of the metal by passing current through an electrolyte to alter the surface finish).

I will try hardening and tempering a few tonight in the furnace. There seems to be a fair bit of difference in recommendations, some say passivate some Harden and Temper and others Electroplating (Not actual deposition of metal such as nickel but altering the surface of the metal by passing current through an electrolyte to alter the surface finish).

Two things come to mind. Use of steel cutters and any contact with salt. Neither are good for stainless steel and either might have exacerbated the rusting.

Doesn’t appear that 416 was the best choice of material.

as you can show that you have made them out of the material specified this is the customer's problem not yours. It's up to him to come up with a solution, so I hope you're billing him for your time.

400 series of stainless are known as "rustless" steel. They will rust easier than austenitic 300 series which are proper "stainless".

Contact with HSS cutting tool is known to cause sometimes troubles with stainless.

You may try to degrease your steel with detergent, wash with water and then passivate it with concentrated nitric acid. Acid must be 60% and more. Make a sample first.

I might be totally wrong but is rustless the same as stainless as far as steel is concerned ?

Roy

There's probably not much you can do about the existing parts. If anything, hardening them risks damaging the surface - if you manage to add nitrogen or carbon to the chromium in the process you will lose more of the corrosion resistance by forming chromium nitrides or carbides.

The problem seems to be the desire to have parts that are both "magnetic" and "stainless" and is presumably down to naivety on the part of the customer. Whilst it's good to try to help to recover the situation, the bottom line is you supplied what was specified and presumably did not offer yourself as an authority on stainless steels.

I'm not familiar with passivation of stainless steels but it appears that citric and nitric acids are commonly used. That might stop any initial corrosion but perhaps the issue here is down to the bulk material itself. You don't say if there is any solid, liquid, corrosive etc material in contact during operation.

Murray

There are companies arounf who specialise in passivating stainless, I wouldn't go messing with concentrated nitric acid

Thanks for the responses so far,

T.M. one sample I have out in the open air is one that I polished and dunked in citric acid in the form of lime juice for a few hours and so far it seems to be OK. The problem I would have would be polishing 1500 of them quickly thus keeping costs down.

Hello Bob,

Just to add to your concerns, I believe washing up liquid is loaded with salt to thicken the detergent so it is nowhere near as benign as the cuddly lambs suggest. I do have to agree with Muzzer in that they specified the material and you have complied with that requirement to the letter. The problem is of their making.

You are at the moment trying to accommodate their mistake and Duncan Webster is quite right to point that out as an added cost you are currently exposing yourself to. Make quite sure the cost of the whole fiasco doesn't get dumped on you

Regards

Brian

"Rustless" is lower grade stainless, it is devoid of nickel and remains magnetic. These are 400 series of steel.

It has much lower resistance to corrosion than proper stainless. Stainless are austenitic steels, non magnetic or only slightly magnetic, 300 series.

Martin

Hello Bob,

New to the forum but to add my comments to your problem...

In the food and pharma industries passivation is carried out on new Stainless equipment as welding and other workings such as cutting exposes iron on the surface. This can mark as a rust stain or be "rusted off" the material and carried to where the flow drops it, normally tank inlets etc. You then get a red stain on the surface from the rusting iron deposit. No what is required when you want a perfect chemical concoction for your drug that needs to be “pure”, for the most heavly regulated of business! Try telling the FDA it does not matter!

Most of the marerials used are 316 or in some cases 304 which should be better than the 416 you are using in the first place.

Passivation eats the iron from the surface leaving just chromium etc to be the contact material, thus no rust like indications. ! Nitric or Citric acid can be used, never been directly involved so I am unsure of concentrations etc.

Regards

Michael

Problem is, you will not be aware of the full circumstances of use.

If it is in a highly irradiated area all sorts of restrictions apply.

If you were told to provide 416, that is what they got.

Both you and Duncan are right but I think, OP wants to keep a customer, so he needs to be cooperative or at least look like someone who is.

I used hardened magnetic stainless balls in my water pump on the hydro soon found they were no good and lost there sealing due to surface defects caused by corrosion.

316 balls were the solution but they are not very hard some can be drilled but have had no problems with them.