Use of Sulfonic Acids in Dry Electrolytes to Remove Vapor Deposited And/Or Thermally Sprayed Coatings on Metal Surfaces

The present invention relates to a process for removing a coating from workpieces such as for example coated tools and in particular coated cutting tools. Processes for removing coatings are used for stripping coatings from worn or improperly coated workpieces and thus preparing them for recoating.

To remove the coating from worn and improperly coated workpieces, typically electrochemical processes are used, such as described for example in WO2008/028311. For this purpose, coat-stripping facilities are used for example that include a tank for containing a liquid electrolyte, wherein inside the tank a counter-electrode can be connected to one pole of a power supply device is provided. In general, the workpieces have to be individually contacted in such a way that they are connected as anodes and the counter-electrode as cathode to a power supply.

During use, the tank is filled with the electrolyte and the workpiece is immersed in the electrolyte. Between the workpiece and the counter-electrode, a predetermined constant voltage is applied that causes the workpiece's coating to be removed. The counter-electrode surface must be formed and positioned in such a way that the current flow is distributed as evenly as possible over the workpiece's surface areas whose coating is to be removed, in order to achieve a uniform removal of the coating and to avoid corrosion of the base body bearing the coating.

A further factor that influences the coating removal process is the conductivity of the layer or layers to be removed. One problem that can occur when removing in particular non-conducting layers is the damage to the surface of the base body such that the latter, after the coating removal according to the state of the art, is scarred with statistically distributed indentations.

In U.S. Pat. No. 9,512,539 a method is described how to avoid such indentations by starting with a low voltage and continuously increasing the voltage used for the decoating process.

Unfortunately the processes as described in WO2008/028311 and U.S. Pat. No. 9,512,539 involve a high degree of experience and in addition are quite expensive to process.

There is therefore a need for an inexpensive but reliable stripping process for vaper deposited and/or thermal sprayed coatings on metallic substrates.

Stripping is clearly to be distinguished from polishing. While stripping is a de-coating process, removing coating layers from a coated substrate, the small removal of material which is referred to polishing has the objective to smoothen the surface. According to DIN 8589, polishing is not a manufacturing process in its own right, but is carried out as polishing lapping, polishing honing or electrolytic polishing.

In this context EP3795722 exclusively relates to polishing. Accordingly the objective of EP3795722 is to develop an improved method to smooth and polish metals through ion transport by means of free solid bodies.

The use of sulfonic acids in free solid bodies or particles as described in EP3795722 to polish metal surfaces through ions transport has the advantages and characteristics that are explained in EP3795722 ad repeated in the following text.

A dry electrolyte comprises a set of porous particles with the capacity to retain a given amount of liquid and a given amount of electrically conductive liquid EP3795722 specifically refers to dry electrolytes that comprise porous particles with the capacity to retain a given amount of liquid, and a given amount of electrically conductive liquid that contains at least one sulfonic acid.

EP3795722 describes that the electrically conductive liquid comprises at least one sulfonic acid. The sulfonic acids are composed with a general formula RSOH, where R can be any organic substituent, either alkyl aromatic, another functional group or a halogen atom. This is the general structure of a sulfonic acid.

Preferably, the sulfonic acids used are those having a high solubility in water or another chosen dissolvent. In addition, preferably, those sulfonic acids that form soluble salts with the related metals. For example, can be used, but without limiting purposes, the sulfonic acids such as the methanesulfonic acid CHSOH, the trifluorosulfonic acid CFSOH, the fluorosulfonic acid FSOH, the chlorosulfonic acid ClSOH, the para-toluenesulfcnic acid 4-CHCHSOH and the sulfamic acid NHSOH, all of them thereafter represented

The sulfonic acids can be used pure in the event that they are liquid at the working temperature or in solution. The optimal concentration of sulfonic acid shall be empirically determined as it depends on the sulfonic acid chosen, the dissolvent and also the parameters of the part to be treated, such as the type of metal, the full surface and the shape. In solution, the preferred options of solvent are water or a polar solvent due to conductivity and solubility reasons. Preferably, the water is the chosen dissolvent. Concentrations of sulfonic acid in the conductive liquid from 1 to 70% demonstrated to be active in this process. Preferably, concentrations from 2 to 40%. These concentrations refer to the final concentration of the electrically conductive liquid in the dry electrolyte, regardless of how the dry electrolytes are prepared.

The sulfonic acids are strong acids, and their handling in liquids or in solutions, as for their use in the classic electropolishing, carried many handling risks. In liquid state or in solution, these sulfonic acids can produce an unwanted attack on the metal surfaces. Therefore, after using sulfonic acids in the classic electropolishing, often a further neutralizing step is required.

However, when it is limited to the porous particles, the handling becomes easier and the risks of unwanted attacks on the surface are prevented.

The sulfonic acids with an organic waste, such as, for example, without limiting purposes, methanesulfonic acid, trifluorosulfonic acid and para-toluenesulfonic acid, are much less polar than the inorganic acids. Therefore, the reduced localized polarity of these sulfonic acids facilitates their movement through the apolar resin. Namely, the smaller sulfonic acid that contains an organic waste, the methanesulfonic acid, will benefit of this effect while not sustaining steric hindrances.

In addition, other chemical compounds can be added to the conductive liquid, as complexing agents. These agents can capture the metal ions formed and increase the capacity to remove metal oxides and salts from the surface.

The complexing agents having more than one functional group are known as chelating agents. The effects of capturing and transferring metal ions would be even higher by the use of chelating agents, such as the citric acid, EDTA or phosphonates. The said agents would have a high affinity due to the metal ions formed on the surface and would help to carry the said ions to the particles.

In a preferred embodiment according to EP3795722, the complexing chelating agent is a polyether.

Polyether is defined as a compound including more than one ether group (C—O—C) in its structure, without prejudice that it can include in turn other functional groups such as esters, acids, amino, amide, etc.

It has been checked that adding polyethers to the formulation of the liquid contained in the particles increases the speed of transferring ion metals and therefore increases the speed of the polishing process.

In an even more preferred embodiment according to EP3795722, the polyether is a linear alkyl polyether. Within the group of the polyethers are specifically included to crown ethers and to alkylpolyethers. The alkylpolyethers can have different shapes, such as linear, star-shaped, branched or comb-shaped. For the electropolishing process we have found that the linear alkylpolyethers provide best results in the process, as they are more active at the moment of forming metal complexes.

Within the category of linear alkylpolyethers chelating complexing agents the polyethyleneglycol or PE is standing out, also called poly(oxy-1,2-ethinhediyl), poly (ethylene oxide), polyoxyethylene, polyethylene oxide and brands such as Carbowax or Macrogol;

Within the category of linear alkyl polyethers chelating complexing agents is also standing out the polypropyleneglycol or PPG.

Thereafter the PEG and the PPG are represented, wherein R can be any radical or functional group, preferably H or CH. The number “n” of repeating the repeating unit, is a significant factor. The metal complexes with polyethers mostly adopt a tetrahedral or octahedral conformation, that means, the metal ion is surrounded by four or six atoms of oxygen, respectively, that is why the optimal number of repetitions is located around n=6, because it covers both possibilities.

For the case of the PEG, the molecular weights of 200 to 500 Da are the preferred. Specifically PEG 300 is the most preferred.

Adding these polymers PEG and PPG to the electrically conductive liquids used in preparing dry electrolytes that contain sulfonic acids, produces electropolishing processes at high speeds and spectacular final finishes. This effect, absolutely not obvious, is due to a series of accumulated factors: they are soluble in the phase in which they find the sulfonic acids, they have the capacity to forming complexes with the metal ions removed, they act as phase transferring agents between the liquid retained in the particles and the gel phase of the particles themselves, they are stable to the voltages and intensities of current to which the process is submitted, and, in addition, they are biologically safe.

The given amount of electrically conductive liquid to impregnate the porous particles has to be sufficiently high to allow a measurable electric conductivity through the dry electrolyte. In addition, this amount has to be below the saturation point of the porous particle, in order there is no observable free liquid, being thus a “dry” electrolyte. Preferably, the amount of conductive liquid is close to but below the saturation point of the porous particle. This amount must be empirically determined because it depends on the sulfonic acid used, the type of resin, the temperature, the dissolvent and the concentration. As an example, AMBERLITE 252RFH with a water retention capacity from 52 to 58% the optimal amount of a conductive liquid that consists in 32% of methanesulfonic acid in water is ranging from 35 to 50% with respect to the resin absolutely dry weight.

The material of the porous particles used is preferably based on a sulfonate polymer, which means that it has active sulfonic acid groups RSOH or RSO-joined. Preferably, the porous particles sulfonate polymer is based in a styrenecopolymer and divinylbenzene. Specifically EP3795722 describes that the porous particles can be ion exchange resins, such as for example but without any limiting purpose, AMBERLITE 252RFH having an ion exchange capacity of 1.7 eq/I, a density of 1.24 g/ml, a diameter ranging from 0.6 to 0.8 mm, and a water retention capacity ranging from 52 to 58%.

The cooperative effect between the groups of sulfonic acid joined to the polymer and the sulfonic acids in the conductive liquid is interesting. It has been found, in a not obvious manner, that the fact that these groups have the same chemical structure, although in a different state, helps to the circulation of metal ions from the conductive liquid to the polymer matrix. The direct chemical environment of the metal ions in solution (complexed by sulfonates in solution) and in the polymer (complexed by sulfonates joined to the polymer) is similar. Therefore, the difference of energy levels between these states has to be very low, which presumably implies a state of transition with low energy, that is converted into a higher speed liquid-solid transfer. This has two positive effects for the process, on the one hand, it makes that the process is speedier and, on the other hand, it improves the general capacity of the resin to act as receptor of metal ions, that lengthens the useful life of the dry electrolyte.

Thus, the sum of these not obvious different effects when using sulfonic acids in dry electrolytes to polish metal surfaces through the ions transport allows that the speedier processes obtain spectacular results increasing at the same time the useful life of the dry electrolyte.

A dry electrolyte was prepared mixing and homogenizing 1.5 kg of ion exchange resin AMBERLITE 252RFH with 550 ml of a solution of methanesulfonic acid to 4% of water. This dry electrolyte is used to polish a part of iron alloy with the following composition expressed in % C (0.17-0.23) Si (0.40) Mn (0.65-0.95) V (0.025) S (0.050) Cr (0.35-0.70) Ni (0.40-0.70) Mo (0.15-0.55) Cu (0.35) Al (0.050) with a surface area of 5 cm2. The counter-electrode was a network of iridium on titanium. The current used was a positive wave of an electric current of 50 Hz at 20 V, that provided an intensity of 0.1 A. The part had a downwards/upwards movement at around 4 Hz and the dry electrolyte container was submitted to a vibration. After 5 minutes of this proceeding, the metal surface had acquired spectacular properties.

A dry electrolyte was prepared mixing and homogenizing 5.3 kg of ion exchange resin AMBERLITE 252RFH with 1950 ml of a methanesulfonic acid solution at 32% in water. This dry electrolyte is used to polish a part of iron alloy having the same composition as before with a surface area of 36 cm2. The counter-electrode was a network of iridium on titanium. The current used was a positive wave of an electric current of 50 Hz at 30 V. The part had an upwards/downwards movement at around 4 Hz and the dry electrolyte container was submitted to a vibration. After 10 minutes of this process, the metal surface had acquired spectacular properties.

A solution was prepared with 550 mL of methane sulfonic acid 70%, 160 mL PEG and 3000 mL of de-ionized water. This solution is mixed and homogenized with 6.7 kg of ion exchange resin AMBERLITE 252RFH to produce a dry electrolyte. This dry electrolyte was used to polish a part of carbon steel of 36 cm2. The counter-electrode used was a network of iridium on titanium. The current used was a positive wave of an electric current of 50 Hz at 30 V. The part had a downwards/upwards movement ca. 4 Hz and the dry electrolyte container was submitted to vibration. After 5 minutes of this process the metal surface had acquired spectacular properties.

Turning back to the present invention the inventors surprisingly realized that the polishing process as described in EP3795722 can as well efficiently be used to perform a stripping process.

The object of this invention refers to the method to strip vapor deposited coatings fand/or thermally sprayed coatings from a metallic surface of a substrate. The core of the invention is the use of ion transport that uses free solid bodies that contain sulfonic acids as electrolytes. All the aspects as described above related to EP3795722 can be applied to the stripping process according to the present invention.

Therefore according to the present invention sulfonic acids in free solid bodies or particles as described above are used to remove vapor deposited and/or thermally sprayed coatings from metal surfaces through ions transport. This can be used in the same way as described above for polishing. The vapor deposited coatings could be for example physical vapor deposited (PVD) coatings and/or chemical vapor deposited (CVD) coatings.

The invention will now be described in detail on the basis of examples.

As a first example a CrAlN-based coating (brand name “FORMERA”) which had been cathodic arc deposited onto an iron alloy. The thickness of the coating was about Sum. A dry electrolyte was prepared mixing and homogenizing 1.5 kg of ion exchange resin AMBERLITE 252RFH with 550 ml of a solution of methanesulfonic acid to 4% of water. This dry electrolyte is used to strip the iron alloy substrate. The counter-electrode was a network of iridium on titanium. The current used was a positive wave of an electric current of 50 Hz at 20 V, that provided an intensity of 0.1 A. The part had a downwards/upwards movement at around 4 Hz and the dry electrolyte container was submitted to a vibration. After 30 minutes of this proceeding, the coating was completely removed as can be seen in.shows in addition that the surface of the substrate is slightly roughened, which proposes that this process is different to a polishing process.

As a second example an AlCrN-based coating (brand name “ALNOVA”) which had been cathodic arc deposited onto an carbon steel. The thickness of the coating was again about 5 μm.

A solution was prepared with 550 mL of methane sulfonic acid 70%, 160 mL PEG and 3000 mL of de-ionized water. This solution is mixed and homogenized with 6.7 kg of ion exchange resin AMBERLITE 252RFH to produce a dry electrolyte. This dry electrolyte was used to strip the carbon steel substrate. The counter-electrode used was a network of iridium on titanium. The current used was a positive wave of an electric current of 50 Hz at 30 V. The part had a downwards/upwards movement ca. 4 Hz and the dry electrolyte container was submitted to vibration. After 30 minutes of this process the coating was completely removed as can be seen in.

The present invention relates to the use of dry electrolytes to strip vapor or thermal spray deposition coated metal surfaces through ion transport, characterized in that the conductive liquid of the dry electrolyte comprises at least a sulfonic acid.

The porous particles of the dry electrolyte can comprise sulfonate polymer.

The porous particles of the dry electrolyte can comprise ions exchange resins of polystyrene-divinylbenzene.

The conductive liquid of the dry electrolyte can comprise methane-sulfonic acid.

The concentration of sulfonic acid in relation to the solvent can ranging from 1 to 70%.

The conductive liquid of the dry electrolyte can comprises a complexing agent. The complexing agent can comprise a polyether. The polyether can be a linear alkyl.

The polyether can comprise or be polyethyleneglycol. The polyethyleneglycol can have a molecular weight ranging from 200 to 500 Da.