Thermal spray stripping methods

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In a previous post, we dealt with the reasons for stripping thermal sprayed coatings. You may have concluded from there that the reasons for stripping thermal spray coatings are not necessarily the fault of the flame spray coatings operator, but sometimes stripping and thermal spray re-coating of hardware is by design as in the case of overhaul and repair components used in the aerospace industry.

In this post, however, we will deal with the options available for the stripping of thermal spray coatings, giving the thermal spray engineer a choice in this procedure. Some options may be simply ruled out from a technical standpoint, some may be ruled out from a facilities stand point and some method of coatings stripping may be somewhat better economically as opposed to others.

Method number one: Grit blast stripping of thermal spray coatings.

As one might very well know, grit blast is used as a surface preparation method prior to the application of thermal spray coatings. While this is true, grit blasting procedures can also be used to strip thermal spray coatings. Obviously, the key characteristic to be considered is the uncoated dimension of the part not to be violated. Thus it is essential that the thermal spray engineer read the blue print and indicate to the grit blast operator and in-process inspector to measure the part dimensions at intervals to ensure that the part dimensions are not violated.

A key problem occurs when using this procedure to strip thermal spray coatings from thin walled components because they may tend to warp during the stripping process. Additionally if you are a quality thermal spray coatings house, then you must ensure that excessive grit blast inclusions are not resulting in the actual hardware because if this happens then the subsequent re-application of the thermal spray coating may violate the grit entrapment limits of the applicable specifications. This is not to be taken lightly by any means. This means that the grit blast stripping parameters need to be not excessively aggressive.

To overcome this, every thermal spray shop that plans to use grit blast stripping of thermal spray coatings, must develop such parameters on various commonly used substrates and commonly applied coatings for their customers ahead of time, and evaluate them in the metallurgical laboratory for effective coatings removal without excessive grit entrapment, so that when the actual time comes to strip real hardware, one knows what he is doing.

Method number two: Grind stripping of thermal spray coatings

Grind stripping of thermal spray coatings is another option at the disposal of the engineering team. While this may seem like an easy alternative, it comes with its own set of headaches and parameter controls. As before, dimensional controls are very important and must be adhered to. Additionally, proper coolant usage is imperative if called for. Wheel dressing requirements, proper wheel designations, etc are a given. Thermal spray shops sometimes perform coat and grind operations in-house and in such cases the quality controls are much superior because they do this day in and day out. If this is going to get outsourced then ensure that the outside vendor is has the necessary experience and expertise dealing with grinding thermal spray coatings, otherwise there might be nothing but problems to deal with.


Method number three: Chemical stripping of thermal spray coatings

Chemical stripping of thermal spray coatings is quite an effective method depending upon the chemistry of the coating AND the chemistry of the substrate. While the chemical solution might be quite effective in eating away the coating material, it MIGHT eat away at the substrate material too. I know this previous statement sounded quite non-technical, but that is the best way I could drive home the point that the combination of the coating material AND the substrate material needs to be considered when using this procedure.

Chemical stripping of thermal spray coatings is quite a detailed topic in itself and we will be dealing with it quite extensively in future posts. This is an introductory article for the stripping methods and hence just a cursory glance is what we are interested in presenting here.

There are two basic kinds of chemical stripping: one is standard dipping of the parts in a chemical solution that is sometimes heated and the other is electrolytic stripping where there is electric current passed through the solution wherein the part itself forms an electrode. There is an array of safety procedures and quality control aspects of the stripping solution itself that need to be watched out for. Additionally, intergranular attack of the base metal by the chemical solution must be evaluated for. Such intergranular attack can cause a weakening of the hardware and is many times not acceptable per specifications. More about this in future articles.

In summary, there are options available to the thermal spray engineer when considering stripping of coatings. These methods can be used on actual hardware as well in hard tooling components. Needless to say sophisticated analysis of intergranular attack and such are not needed for hard tooling components but are definitely needed for actual customer hardware, especially rotating parts.