Quality of 316 stainless

I am having a discussion on a yachting forum about the rusting of 316 stainless on a boat. A member has claimed that modern 316 is "junk". I pointed out that if he bought from an ISO registered company they would have systems for ensuring that the material ordered was what was supplied. His answer was that 316 is not the same quality that it was years ago. He cited the point that 316 cleats etc on his boat fitted years ago have retained their brightness. However, 316 supplied in the last few years continually show sign of rust staining in spite of repeated polishing

Can anyone in the know tell me if the quality of 316 has been down graded I know that it has a different classification 1.4404 I think- but is that because the structure has altered.

Is the other forumite correct in his "junk" assessment, albeit a bit over the top. Or is he wrong & there is no change

My second question is about cast 316- This is because the thread arose when someone wanted a stainless steel elbow to replace a cast iron one.

Does the structure of 316 change much if cast instead of being welded. The Op for the thread has actually ordered a cast 304 ss one & has been told that it will rust very quickly. Something I doubt, not being in water

Any comments please?

In my humble opinion chaps who complain about the quality of metal are on thin ice because the word quality is meaningless in this context.

Before drawing conclusions, you have to know the metal's specification. Otherwise off to a bad start by not comparing like with like. Not easy - there are a least 6 alloys with 316 in the title, and without looking them up, it's not known how close the specifications of the several alternatives to 316 are.

One possibility is the chap is comparing 316 and 316L: the difference is 0.05% Carbon, but that makes 316L more salt resistant and expensive to buy. 316L might be considered better quality by an owner focussed on corrosion, but an engineer will know that 316 is markedly stronger. On something like a yacht cleat, which matters most: persistent shininess or grubby strength? Alloys have many qualities, not just one.

Many reasons why metals perform more-or-less badly, but I think the idea that metal was generally better made in the past is unlikely.

304 is a poor choice for anything exposed near the sea - it's pin-holed by Chloride ions in the air.

I don't know about casting versus welding.

Unless you have a friendly metallurgist I would think it would be costly to have the metal analysis done. It may cost more but a stockholder with the correct paper trail to assure conformance with the specification you require could be worth buying from. It would be interesting to test both materials to establish if they are different. The metallurgist in our works lab was a very knowledgeable guy and was most helpful in suggesting a superior performing material when we had multiple failures of a shaft. The Culham laboratory machine shop manufactured a batch and no more trouble.

Mike

I make a lot of motorcycle parts in st/st and mostly use 303 or304 and have been for 35 plus years without any corrosion problems, one bike has had the bits on for that long. corrosion resistance can be increased by several hundred percent by high lustre polishing. have also used 316 when forced to but find it difficult to get good machined surface finish and therefore hard work to polish.

Until I retired I spent the last 14 years of work working with an Italian company who made large and small industrial washing machines, the largest big enough to take a car, everything including pipework and the machine bodies were made from 316 and all welds etc were cleaned and polished for finish, the washing chemicals used were Alcaline or Acid depending what was to be washed, the machines didn't tarnish or rust.

Martin P

Another small point is that stainless steels need passivating to avoid rust spots when exposed to corrosive conditions. Pickling in sulfuric and or nitric acid is a good way to get rid of iron on the surface, which leads to rust spots..

Mark brings up a very good point there. A classic example is where there is a stainless steel urinal in a workshop where plenty of mild steel is worked. Particles of mild steel find their way onto the surface of the stainless and start it corroding. Once the corrosion is started like that, the stainless will rust just like mild steel. The protection of stainless is due to the surface layer of chromium oxide that forms, if that gets disturbed the iron part can easily corrode. (There can be nickel and other alloying elements in the stainless, but that is the basic idea)

It is also important to only work stainless with cutters reserved for it, particularly with grinding or sanding. Smearing a layer of ordinary mild steel over the surface from a used grinder is a wonderful way to set off corrosion.

It is generally best to avoid stainless, unless the conditions really call for it. It is troublesome to work, and loves to gall and seize up when in contact with itself, as with fasteners. Best to use anti-seize compound when assembling.

John

I would look for sources of contamination by iron/steel. Has it been near someone using a grinder for instance? Is the polish and polishing cloth used entirely on stainless? Are the ropes being used on it coming into contact with iron rings at the quayside then transferring contaminants to the stainless steel?

If you see a weld in a stainless steel pipe that has been brushed with a plain steel wire brush after welding then exposed to rain the rust appears very quickly and means the pipe has to be stripped out and treated to get it back to a serviceable state. We had to buy stainless steel wire brushes for our welders to avoid this. We also has a stainless only pipe shop well away from any other machining operations to avoid contamination. Grinders, band saws, drills, files, hacksaws and vice jaws were all potential sources of iron or steel particles that would cause problems. We sub-contracted any non-stainless pipes as a result, it was only a small percentage of the work so it was no great loss and was only for heritage spares. The machine shops were having frequent problems with corrosion on stainless parts. It was traced to grinders being used for mixed materials. A magnet placed in the coolant flow picked up large quantities of iron filings that were getting through the filters.

Martin C

There are two grades of 316. There is the common or garden variety and there is 316L. The L denotes a lower carbon content.

What makes stainless steel "stainless" is the chromium content of the steel. If the steel is heated to between about 250 - 530 degC. (casting, welding etc) for any length of time, the chromium combines with the carbon to form chromium carbide. This depletes local areas of the protective chromium and the steel rusts! (Sensitization)

316L contains the lower carbon content. As such it sensitizes the steel to a lesser extent and the corrosion resistant quality of the steel is not compromised.

316 is fine for general use (cutlery etc) but for more aggressive marine applications it's 316L.

But be prepared to pay more for it!

This is not a case of defective materials, it's probably a misunderstanding between customer and supplier.

Keith

PS if you want to create rust spots on stainless steel knives, braze the blade to the handle with a low silver content (cheaper) alloy and use oxy-acetylene. Seldom fails!

Edited By Keith Hale on 04/07/2022 11:58:04