Carbon Steel Spring in ABS Plastic Assy Corrosion 1

[OHSpringGuy]

We are seeing a Music Wire Spring ASTM A228 exhibiting rust very quickly during cycle testing. The spring is enclosed in an ABS housing. Small fines of ABS are present from wear. The spring when cycled by itself in ambient conditions, exhibits little or no rust. What is the Chemical reaction taking place that is causing ABS to rust carbon?

 

[CoryPad]

Was the spring/housing assembly exposed to ambient conditions? Any exposure to obvious corrodents like chloride ion? Any chance of water being trapped inside the housing?

If there is no obvious environmental corrodent, then maybe the ABS moisture content was sufficient to produce corrosion. This doesn't seem likely.

Perhaps there is an additive to the ABS that promoted the corrosion. Contact the ABS supplier and ask them about flame retardants, UV stabilizers, heat stabilizers, melt flow rate modifiers, etc.

 

[OHSpringGuy]

Both springs were exposed to the same ambient conditions. Only the one inside the housing exhibited rust. Do you or anyone know of particular additives to ABS that may be corrosive to carbon steel?

 

[CoryPad]

Historically, flame retardants have been inorganic salts, acids and clays, with the first two being possible steel corrodents. Modern flame retardants are halogenated, which can be chlorine based, and therefore, a potential corrodent.

 

[unclesyd]

Did the spring break while in the housing as shown in the photo?

 

[OHSpringGuy]

yes it did.The failure is due to stress and not the corrosion. It would take a great while for the superficial corrosion to affect the spring properties, as the oxidation is in very fine particulate. The problem lies in seepage of the rust color to the outside of the housing. Not favorable. Over hundreds of thousands of cycles the rust may come into play in the cycle life, but we have not found that to be the case so far.
I made the suggestions above to our molding people and thay are telling me there aren't any UV stabilizers, Fire retardants, or melt flow rate modifiers. I am NOT a chemical engineer or plastics engineer, but somehting in there is causing all this oxidation. The parts are molded in a very dry arid area of the country, so trapped humidity isn't a problem.. Does ABS have a curing time, or a time it should wait before being used?

 

[CoryPad]

I believe the failure is due to stress corrosion cracking (SCC) or hydrogen assisted stress cracking (HASC, also known as hydrogen embrittlement) or corrosion fatigue, so the corrosion is involved in the failure. Have you had a metallurgical evaluation done on this part? Using energy dispersive x-ray spectrometry (EDS) on the spring surface may detect elements that are responsible for the corrosion and help identify their source.

Are your molding people intimately familiar with the ABS material and its production method? Additives to plastics are very common. You can conduct fourier transform infrared spectrometry (FTIR) on the ABS to identify constituents, which may help identify a possible corrodent.

 

[OHSpringGuy]

Thanks Cory. The failure is due to stress only in four of the springs tested. Hydrogen embrittlement only occurs from a plating process called pickling where parts are electro cleaned using hydrochloric acid and subsequently plated trapping hydrogen withtin the grain and causing failure. In this case, parts were sent to a lab that failed both in assembly and not in assembly and they show the same stress fractures. Not to say that corrosion couldn't be a factor, but these spring are failing before corrosion becomes a factor in the failure. Our customer's requirement is 100,000 cycles and we are getting 150-250. Good idea on doing a chemical analysis on the surface rust. I'll check into that.

 

[EdStainless]

As a quick look you take a few plastic housings and age them at at max service temp (plus a little) for a few days and then see if you get the same results. Since this happens quickly it must be something volatile enough to outgas.

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Plymouth Tube

 

[CoryPad]

OHSpringGuy,

You are using an extremely limited definition of hydrogen embrittlement. The corrosion process generates hydrogen, which can enter the steel and weaken it. You can't say that the parts fail before corrosion is a problem - your parts are covered in rust, which could result in hydrogen assisted cracking (pick your acronym: HE, HASC, HISC, etc.), or fatigue initiation from a corrosion pit in less time than if the fatigue initiated from the wire surface. If you haven't looked at the spring surfaces with an electron microscope, you should.

 

[OHSpringGuy]

Thanks Cory. Use the lab used an electron microsope and noted that in their opinion, the corrosion did not play a factor in the failure. However it did contaminate the initial stress fracture.

 

[unclesyd]

If corrosion did contaminate the fracture surface it leaves open several possibilities for the initiation of the failure. There could have been a crack from the heat treating process, H2 could have been introduced by cleaning and the H2 could have initialed the failure prior to use while sitting on the self.

What is the hardness of your spring?
If you have some ready to install parts on the shelf you might want to do a magnetic particle test on them. If these are not already cracked you might be able to reclaim them by a bakeout.
Ask your lab if they can clean the fracture surface and look for fatigue striations.

 

[swall]

Hydrogen embrittlement would not be at the top of my list of possibilities. Music wire is generally not susceptible to hydrogen embrittlement the way oil tempered spring wire is.