corrosion rates of steel inserts in plastic overmolds  

Posting by mike on May 22, 2008 at 18:22:19.

I have a possible overmold application where i want to insert mold/ overmold a Silicon steel stator lamination in a pump application... the concern is the service life time of the stator as water is absorbed/ diffused/ permeates thru into plastic and thru to the this stator. The added complication is 110 C water temperatures/ Ph 1.5 to 11/ and 175 psi service pressures. is there any applications like this and what are the expected lives of the insert? any technical work done on the corrosion of metal inserts encapsulated in plastic, and the related lifetime prediction or expectation?

          follow up posts
    On May 27, 2008 at 13:30:37 gerd posts:
    Thank you for raising this issue. I am interested in some general equations and diffusion formulae that can describe the chemical surface corrosion and chemical resistance of a metal / steel with a plastic coating attached to it. Can we use fick's first and second law with a concentration dependent boundary condition for the coating - metal / steel interface? How to formulate the corrosion kinetics in this sort of composite materials? Are there some simple bootstrap equations available for very slow diffusion or very slow chemical reaction on the metal surface?

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      On May 27, 2008 at 20:13:03 gerd posts:
      Moreover, I am interested in acid and alkaline diffusion rate in polymer and composite (glass / carbon / kevlar fibre reinforced plastics) materials. Does an acid, like hydrochloric acid, sulphuric acid or nitric acid diffuse slower than water? And what about the permeation of sodium hydroxide in FRP?
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    On May 27, 2008 at 09:59:01 rodney posts:
    Poly Vinylidene Fluoride (PVDF) is indeed a good option, however it stress cracks under the load of very strong alkalies. Moreover, at 110 degrees Celsius the concentration of for example Sulfuric acid should be limited to approximately 40%. From the mechanical viewpoint: the abrasion resistance is comparable with that of ultrahigh molecular weight polyethylene (UHMWPE) and polyamide (PA) - at least at room temperature. More details on the composition of the mixture or mixtures, would help to provide a more detailed assessment of the diffusion, corrosion and chemical resistance properties of the plastic overmold on silicon based steel.

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    On May 24, 2008 at 10:44:38 composite agency posts: I think the fluoro based polymers (for example pvdf)- including an interfacial agent (an appropriate adhesive, polyurethane, epoxy based) - is an option for this type of environment and chemical exposure. However, do you have any indication of the sort of chemicals involved (e.g. in case of hydrofluoric acid, fluoro polymers are not a very good idea). Moreover, what is the exact requirement for the electrical conductivity of the plastic overmold?

    Furthermore, do the temperature and pressure change rapidly (temperature / pressure spiking - water absorption) or do they stay more or less constant?

    Is it possible to provide a picture of the overmold Silicon steel stator lamination? In case of an appropriate polymer, the life extension by the mold, will be substantial in the chemical environment described. Further to your information, it is possible to provide you with some initial quantitative water / acid / alkaline polymer diffusion with subsequent metal corrosion rate. Hence, the coating - metal service life can be compared to cast silicon steel.

    Best Regards,
    Composite Agency
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