Macro rust spots.

This is my diversion from Raphael’s Macro thread which commenced here … **LINK**

I have wondered about this pattern of rust spots. They are on the inside edge of the 1.5mm gauge plate marked CSt, see inset. 

Gauge plate has a probable carbon content of around 1.3-1.5%.

The rest of the block is made up of several hundred stainless steel 'shims', marked SSt in the inset.

The surface is rough, a result of the EDM wire cutting erosion.

What puzzles me is why are the rust spots that shape, variegated in colour, and loosely distributed the way they are?

Shouldn’t the rust appear uniform?

By the way, the hole (2mm dia) at the bottom of the image was were they initially threaded the EDM wire.

For reference, here's the block again ...

Sam

Edited By Sam Stones on 20/05/2020 22:45:56

Sam,

I have seen patterns like that before.

First thing it reminded me of is how rust forms under paint. Where a small hole in the paint lets water and oxygen in and allows the rust to start. I then understand that the difference in oxygen levels between the hole and edge of the rust spot causes a small electric potential. The potential then promotes rusting.

The other is rust spots on the stainless kitchen sink. This was caused by very small iron filings, from a angle grinder. They stuck to the sink surface then over night rusted and caused blob stains like above. This is rusting is promoted by galvanic action due to the dissimilar metals.

So my guesses are either there is a coating on it that is imperfect and electrolytic rusting is happening. Or the surface is contaminated by very small metallic particles which are causing galvanic rusting. You say it was cut using EDM, well that creates a lot of very small metallic particles. Both the metal being cut and from the copper electrode.

Adrian

I also had a similar pattern on a stainless sink that I flushed some tiny Potassium Permanganate crystals down. I assume it acted like a strong oxidiser with the pattern where the tiny crystals had lingered. EDM I assume will have similar electron movements very dependant on steel composition and spark pattern??

pgk

When my company was having some research undertaken by a local University we checked on the various experiments every day just to make sure that proper procedures were being followed and thing were going in the right direction. I went in one morning and there was a strange smell hanging around. Everything was fine with our research but I thought the smell was getting stronger and following my nose I moved to the next door Lab. Well somebody had left some concoction in a stainless sink about 2 inches deep and it was fuming. We called out the fire service who called out de-contamination experts and the lab was closed for more than a week until it was made safe. When the sink was removed it was found to be only about 20% of its original thickness and absolutely brown with rust and this was a proper laboratory sink.

I don't know what was in the sink but I have never seen anything like it defore or since. I have seen powerful acid in a stainless container and that just leaves a slight gum. They were very lucky it didn't go down the drain or who knows what damage it would have done.

I understand that rust will appear on the surface of stainless steel if the surface oxide coat, which prevents chemical reaction taking place with the underlying stainless alloy taking place, is breached chemically. A good example of this is where stainless cooks knives put into a dishwasher will sometimes be left with rust spots in random places, this doesn’t happen every time they are washed but only when something chemically breaches the stainless protective oxide layer. Metallurgists will be able to explain much better than I can.
Dave W

When I took a large collection of stainless to the scrap merchant, first thing he did was apply a permanent magnet to separate the good stuff containing 'no iron' which he paid me for and I had to take away the rest. The rest was attracted by the magnet and contained iron which he said was worthless. The yachting fraternity had trouble with stainless screw failure, due to being always wet in the timber hull and wet with highly corrosive seawater. Screws looked OK outside but rotted away in the wood. Gone back to Bronze screws. Back in the days when LBSC was busy loco building and construction notes, he often referred to using "Rustless" steel. I've often wondered about that and if it was Austenitic non - magnetic type.

Rustless steel? stainless in german is Rostfrei, free of rust? but not always!

The wire cut EDM process is done with water. Either power flushed or submerged. If the distillation of the water is slightly out or the part left a while after cutting you get the results shown. As far as I am aware only the very early Japanese "Sodik" brand machines used a paraffin type dielectric like a normal solid sink EDM (spark erosion) machine At the end of the day it is down to slight impurities.

Hi all

I am a biologist rather than a chemist but I have always understood that "stainless steel" is resistant to corrosion because a very thin layer of chromium oxide forms "instantly" on the surface of an object - when in the presence of oxygen. This layer is impervious to water and it prevents any water around the object contacting the iron in the steel, so no corrosion can occur.

This layer is normally self-repairing if the object is rubbed, but the repair process requires oxygen, so without oxygen it stops, and corrosion starts if there is water around. In the sea and sometimes in fresh water or underground there is normally at least some dissolved oxygen but this can get used up by, for example, plankton or bacteria - so, no oxygen, but some water = corrosion.

So stainless steel is not at all reliable in marine environments, and can prove unreliable in some other places , including the bottom of chemists' sinks!

I notice the mention of the earlier name of "rustless iron" I think that stainless steel was developed in Sheffield to make cutlery, especially knife blades, which did not rust as the usual knive blades did. So perhaps the original name of the new metal was "rust less iron" ie metal which did not rust as much ( or as often) as the metal previously used?

Rod

Edited By Rod Renshaw on 21/05/2020 12:52:28

No iron? If so it's not steel. 304 (austenitic) is otherwise known as 18/8, meaning 18% chromium, 8% nickel. The rest (apart from a trace of carbon) is iron. The clue is in the name, if it didn't contain iron it wouldn't be steel.

All sorts of things can make stainless rust, amongst these are cold working (bending or drawing), welding, getting contaminated with non stainless (from grinding in the vicinity)

Methinks that scrappy was away with the fairies?

The magnet test doesn't detect high levels of Iron. It detects the crystalline structure of the alloy, which is only loosely related to the value of the metal. On the face of it, the magnet is a poor test for this purpose because it would wrongly reject some high Chromium alloys and wrongly accept some low Chromium alloys. Very hit and miss. And as almost all stainless steels contain between 11.3 and 22% Chromium, I'd have thought any scrap would have value.

I suggest this Scrap Merchant should ask Father Christmas for a nice Spectrometer and use his old magnet to make a Rife Device.

Dave

We used stainless steel cup plugs in core holes in finish machined castings. They were of a grade which did not rust, but were faintly magnetic. So a magnet is not always a reliable check.

Howard

Stainless steel requires passivation with acid to dissolve the ferrous particles from the surface. Pickling in citric acid will work well and is more environmentally friendly than some of the other methods. Passivation is particularly important when the SS is in a salt water setting, and the chemicals in an EDM would not be much nicer.

In my example about the rust spots in the sink, I actually used acid to remove the iron. Incidentally I also got caught out by scrubbing the toilet bowl u bend with a brillo pad. It left a grey patch, which I removed with citric acid toilet cleaner. Thankfully before my wife saw it.

Adrian