Pitting Corrosion - Magnetic Liquid Trap type 316 Stainless Steel in Tomato Application 3

[PM Process Co.]

I have a magnetic liquid trap (MLT) installed at a tomato processor. An MLT is a safety device to remove tramp ferrous materials from food (and other) products. It consists of rare earth magnets enclosed in a stainless housing. The tomato products (sauce, diced tomatoes, salsa, etc) are pumped through the MLT and the unwanted tramp metal collects on the magnets. Periodically the tramp ferrous is removed from the magnets. The housing surrounding the magnets is type 316 stainless. The stainless is corroding. I would describe it as pitting, but I'm not familiar with corrosion, so this might not be accurate. See attached photo.

Type 316 is used throughout this and other tomato processing facilities without problems. The facility in question has type 316 throughout (vessels, piping, etc.) and only sees corrosion on the MLTs. There are in excess of 100 MLTs in Northern California alone on similar tomato applications. A type 316 stainless housing (un-passivated) showed signs of corrosion within a few days. A passivated housing was then installed and lasted for a month or two before showing signs of corrosion.

Application details include:

- Ambient and tot tomato products (up to ~180 F). (The rare earth magnets are high temperature neodymium with operating temperatures up to ~300 F).
- Pressures up to a max of 220 psi (this is the rating of the MLT, in operation they usually see around around 30 psi)
- pH is 3.5 to 4.5
- Tomatoes have citric, ascorbic, acetic and malic acids
- The tomato products include salt, about a 3% salt solution
- The magnets are periodically cleaned with a hot caustic cleaning solution, but, so is all the other equipment in the facility and only the MLTs show signs of corrosion. The cleaning solutions are off the shelf solutions commonly used in food production. Nothing exotic.

Pieces of tramp ferrous will stick to the magnets for up to a day (the magnets are cleaned daily).

- Any thoughts on why 316 is corroding in this application?
- Why only at this facility?
- What materials are more appropriate for this environment?
- Can this be galvanic corrosion between the 316 and tramp ferrous? Is there some reason a magnetic field will accelerate galvanic corrosion?
- If you were having this problem, what material would you try next?

Thanks for your thoughts!

Peter

 

[EdStainless]

316 is totally unacceptable for long term use in tomato based products.
316 will not tolerate pH3.5 and 3% NaCl at room temp much less hot.
If the system is highly oxidizing you can get away with it for a while.
The passivation does not matter.
In you trap there will always be a film of material stuck to the surface, and this will lead to crevice corrosion (which is much more aggressive than pitting).

If it has worked count yourself lucky, and start planing the re-build.
All of the hot tomato product systems that I know of are built in AL-6XN.
I have supplied it for five or six producers that I know of.

Start with a trap in AL-6XN, and then start looking for where you have crevice corrosion other places in the system. Trust me, it is corroding.


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P.E. Metallurgy, Plymouth Tube

 

[PM Process Co.]

EdStainless,

Thank you very much! This is great info. There is a boatload of 316 stainless in tomato facilities in Northern California! I'm surprised the facilities are not dissolving away! Maybe it's because most of the processors are fresh tomato processors and much of the equipment is only used during the harvest season (3-4 months per year)? The rest of the year the equipment sits idle and can reform the oxide layer?

 

[moltenmetal]

Really lousy stainless steels will survive intermittent contact with boiling high chloride solutions- my cooking pots at home are witness to that.

Continuous contact with a hot 3% brine which is also acidic and rich in organic acids is another matter entirely!

AL6XN as recommended by EdStainless is an austenitic with high (6%) molybdenum and added nitrogen for pitting resistance and would be a good choice. A duplex might work, but is likely too ferromagnetic to work well for a magnet enclosure.

 

[EdStainless]

As I said I know of many plants that have been rebuilt in AL-6XN.
Some in CA and some in other locations.
Pitting failures normally start at the girth welds joining lengths of tube or in flanged connections where crevice corrosion is an issue.
In pasteurization/cooking applications CSCC usually causes 316 to crack and fail.

304 survives in cookware for a couple of reasons. First the temp is low, if you get a pan over 100C you have other problems. And we usually clean pans so that there is no risk of under deposit crevice corrosion. Secondly the stresses are non-existent, so CSCC doesn't happen.
My friend was cooking a pot of home made tomato sauce and she was called away by an emergency. She turned the stove off and ran out of the house. The next day she went back to clean up. The pot had a band around it at the fluid level with deep pits.

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P.E. Metallurgy, Plymouth Tube