430 SS Solenoid Armature Rusting... 1

[heathenx]

Hello,

i'm not use if i'm in the right forum...so be gentle. i have an off-the-shelf horton #ar7301 armature that is used in one of our water solenoids. th dimensions of this thing are about 1" long by about 1/4" in diameter. it's a fairly small part.

anyway, my problem is that this thing is in a chlorinated environment. it rusts after a few months os use...like a usty nail. 400 series stainless just doesn't hold up. 300 series stainless would be ideal but it's not magnetic and it will not work in the solenoid.

management (these guys are not engineers) gave me the suggestion of cutting the part in half. making one half out of 300 series and the other with 400 series, screwing them together. i can give about 5 immediate reasons why that is not practical but i'll refrain.

my question: fastening to dissimilar stainless steels, will that be a problem? obviously one half will rust and the other half will not...but since management thought of it then it's brilliant.

 

[TVP]

A better forum for this would be the stainless steel for engineers group. Many knowledgeable people participate there.

The coefficient of thermal expansion is different for ferritic grades vs. austenitic grades. Stresses due to thermal cycling can be quite significant, so you may need to consider this aspect. I would post this question in the other forum and then click on the "Inappropriate post?" link immediately below the text here in this forum to have this post erased.

 

[sreid]

This is a little late but how about nickel plating the 400 series SS.

 

[heathenx]

i tried:

high phos electroless nickel plate
electroless nickel with teflon plate
dry film (teflon) plate

from imagineering finishing in south bend, in

all three look like an Alka-Seltzer after about an hour or two in chlorox bleach.

the standard 430 ss un-plated lasted longer than my plated stuff.

 

[israelkk]

Are you sure that the rust is from the 430? it may come from the rusted water pipe lines and combined with the minerals in the water it settles on the armature and coats it. Are you using fitered water?

Does the ugly look affects the magnetic and mechanical work of the solenoid?

 

[heathenx]

i placed several parts in glass beakers. seperate beakers were filled with chlorox bleach (6% chlorine) and liquid swimming pool shock (12% chlorine). nothing was diluted. after 24 hours in the solutions it looked terrible.

go here and download my pdf with pictures so that you guys can see what's happening.

http://mysite.verizon.net/mabystry/index.htm

 

[TVP]

Wow, the images definitely tell the story. Coating stainless steel is usually not a good idea for this very reason-- the passive film does not exist when there is something else on the surface. I don't recall seeing this question in the stainless steel for engineers forum, and apparently none of those gurus routinely check this forum. The Corrosion engineering forum is another area where mcguire and EdStainless routinely hang out.

Anyway, one question is how much magnetism is needed? Can a highly cold worked grade like 301 be used? 301 still does not have outstanding corrosion resistance, but it would be an improvement over 430. Have you called anyone at Carpenter to discuss the specifics of this application? They would definitely be able to assist with material selection.

 

[heathenx]

actually, i have been so busy with other projects that i have not had time to call the mfg of this armature. horton is the company that makes it (

http://www.hortonvalves.com/).

perhaps a 301 would work but i have not looked into it.

 

[kenvlach]

1) Verify that you're not using 430F, the free-machining grade that contains corrosion-inducing sulfide.

2) Talk to your plater re quality. What specs & thicknesses did you use for the EN? And, the Teflon coating must be porous and/or poorly bonded to the substrate.

3) I guess is a ferritic microstructure is needed for the electromagnetic properties. Try 434 SS. About the same properties as 430, but with 1% Mo for improved corrosion resistance.

 

[heathenx]

thanks kenvlach. i did end up calling the manufacture about this. they told me that they had a bad lot of material about a couple of years ago but has been improved since then.

their take on it was that they have not had a single complaint about the armatures rusting except for us. they have sold millions.

they do in fact use a 430F ss. to verify your statement, were can i get facts supporting the corrosion-inducing sulfide?

you got me interested in the 434 ss. maybe i can have them try this. is there a particular 434 ss?

thank-you for the tip. i had just about given up.

 

[kenvlach]

I've seen 430F corrode before. Proper passivation may help. See Figure 3 & the section 'Free-Machining Stainless Grades' in the article
HOW TO PASSIVATE STAINLESS STEEL PARTS from Carpenter Technology.

http://crswnew.cartech.com/wnew/techarticles/TA00042.html

 

[heathenx]

they are already passivated...

 

[kenvlach]

Could be inadequate passivation, a metallurgy problem pertaining to the sulfide inclusions, or simply that a more corrosion-resistant alloy is required.

Re inadequate passivation: most metal finishers do not perform the pre- and post-passivation alkaline treatments prescribed by Carpenter. Such treated are required as necessary to pass performance tests. The post-passivation alkaline treatment is in ASTM A967, Paragraph 9, Note 1.
See also Paragraph 10:
"10.2 When specified, within one hour after the final water
rinse as required in 6.2, 7.2, or 8.3, all ferritic and martensitic steel parts shall be immersed in an aqueous solution containing 4 to 6 weight percent of sodium dichromate at a temperature in the range from 140 to 160°F (60 to 71°C) for a minimum of 30 min, followed by a rinse in accordance with 6.2, 7.2, or 8.3.
The parts shall then be thoroughly dried.
10.3 The purchaser may specify other post-cleaning treatments."

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/A967.htm?L+mystore+asmo2435

The average passivator performs neither the pre- and post-passivation treatments, nor performance testing, unless specifically required to do so. However, an OEM is much more likely to properly passivate, which may explain why some of your as-received parts were better than those plated.

Re metallurgy. A major factor in the corrosion of sulfurized (free-machining) SS is the morphology of the sulfides. This perhaps explain the 'bad batch' mentioned by the OEM. The effect of sulfide morphology is covered in Sulfide Inclusions in Steel, American Society for Metals (1975). Briefly: acicular sulfides -- really bad, globular -- not-so-bad, none -- best.
mcguire has newer info on the role of sulfides in the corrosion of SS.

Re more corrosion-resistant alloy. 430 would be better than 430F, 434 (with 1% Mo) is the most direct substitute with significantly improved corrosion resistance.

Carpenter has a free-machining alloy with 1.75% Mo (0.3% S as in 430F, but with higher Mo than 434, plus a bit of Nb):
"Chrome Core® 18-FM stainless is a soft magnetic ferritic material designed for operation in more corrosive environments than tolerated by 18 percent Cr-Fe Type 430 stainless...with generally similar magnetic properties."

http://www.cartech.com/sao_products/wr_sao_product.html?leftn=sao_products&lefto=empty&lefti=sao_ps0

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