Problem With Corrosion Of Stainless Steel

[Bob RodgersonParticipant @bobrodgerson97362]

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?

[Bob RodgersonParticipant @bobrodgerson97362]

[David JuppParticipant @davidjupp51506]

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.

​

[Michael GilliganParticipant @michaelgilligan61133]

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.

.

[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

[Bob RodgersonParticipant @bobrodgerson97362]

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).

[Bob RodgersonParticipant @bobrodgerson97362]

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).

[not done it yetParticipant @notdoneityet]

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.

[Tractor manParticipant @tractorman]

From my knifemaking knowledge most stainless steels will corrode until they are polished. So my advice would be to have a try at getting the very best surface finish possible. I use a scotchbite wheel To get rid of machining marks then a felt wheel with a cutting compound before finishing on a cotton polishing wheel.
I can achieve a mirror finish but I think a satin finish might solve your problem. Unless the steel is indeed not what it says on the certificate which is possible.
Old Sheffield test for stainless is to clean the surface and paint it with copper sulphate solution, if it stays bright it’s stainless, if it plates with copper it’s carbon steel.

[duncan webster 1Participant @duncanwebster1]

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.

[Tractor manParticipant @tractorman]

By the way a local firm were making items for an oil platform which needed some heavy duty inspection at every stage of production for the Lloyds insurance. They were perfect apart from the steel was not as specified. Certificates were produced from steel supplier in Sheffield and it turned out China had supplied low grade steel with falsified papers.

[Martin DowingParticipant @martindowing58466]

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.

[roy entwistleParticipant @royentwistle24699]

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

Roy

[MuzzerParticipant @muzzer]

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

[duncan webster 1Participant @duncanwebster1]

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

[Bob RodgersonParticipant @bobrodgerson97362]

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.

[Brian WoodParticipant @brianwood45127]

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

[Martin DowingParticipant @martindowing58466]

Posted by roy entwistle on 10/03/2018 15:53:43:

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

Roy

"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

[WingoParticipant @wingo]

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

[J HancockParticipant @jhancock95746]

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.

[Martin DowingParticipant @martindowing58466]

Posted by Brian Wood on 10/03/2018 17:11:55:

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

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.

[WindyParticipant @windy30762]

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.

[SamsarandaParticipant @samsaranda]

Bob, you state that customer is a nuclear power station, one would assume that they know their metallurgy and would have specified accordingly, if they are of the opinion that 400 grade “stainless” is not going to rust then they are either deluded or have made a major error in specification. I have worked in companies who contract supply to nuclear customers and my experience is that they are meticulous in specifying materials and processes, if they required a passivation process then my experience is that it would have been clearly documented as required. It is nearly 30 years since I was involved in making for nuclear customers and they were always very thorough in their dealings with contractors, unless things have changed drastically then in your case they have specified incorrectly. If this is an error on their part then one wonders how many other components, perhaps critical components, have also suffered from specification errors.

Dave W

[Chris Evans 6Participant @chrisevans6]

I have used a lot of 400 series stainless steel when wire eroding parts for injection mould tools and extrusion dies. Normal tool steels would rust in the wire eroding process if done overnight hence the switch to 420 stainless. It would still rust if left overnight on the machine though not excessively .

[Martin DowingParticipant @martindowing58466]

Posted by Windy on 10/03/2018 19:52:04:

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.

There is solution for that and it takes a form of silicon nitride bearing balls which are for all practical purposes indestructible in use which you are describing and you can buy these cheaply from manufacturer here:

http://lozyskaceramiczne.pl/index.php

They speak english over phone and will send via mail order.

Amazon also sells these but watch out – I have seen ripoffs.

If after a long time of use they collected stone, you can get rid of it by washing with hydrochloric acid (viniger will also do with more patience).

Martin

[lug lordParticipant @luglord]

I make moulds out of off cuts of stainless tube and i have found contact with fairy liquid which i use to stop the silicon sticking makes the stainless rust like mad 304 and even 316 gets rust spots.

I have also found that even 316 rusts where its been worked on cut drilled etc mostly

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