Patent Publication Number: US-2010126878-A1

Title: Method for Electrolytic Stripping of Spray Metal Coated Substrate

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method for electrochemically stripping a spray metal coating from an electrically conductive material. 
     BACKGROUND OF THE INVENTION 
     Surface treatment of a substrate is known to control friction and wear, improve corrosion resistance, appearance, and/or alter the dimensions of the substrate. There are a variety of surface treatments available, including painting, laminating, physical vapor deposition and chemical vapor deposition. In addition, two common surface treatments used on metal substrates are electroplating and thermal spray coating (also known as hardfacing). 
     Electroplating or plating is an electrochemical process in which a metal coating is deposited on a substrate by passing a current through the substrate placed in an electrolytic bath containing metal ions. Electroplated coatings are widely used for depositing a corrosion- or wear-resistant metal on the substrate. 
     Thermal spraying is a coating process that consists of a heat source and a coating material feedstock. The coating material can often be in a powder- or wire-form, and is sprayed onto a substrate in a molten state. The thermal spraying process welds the coating material to the substrate. Thermal spraying methods using high velocity processes such as flame spray, plasma, high velocity oxygen fuel (HVOF), electric-arc, and detonation gun (D-gun), are widely used. 
     The plasma spray, HVOF and detonation gun (D-gun) processes, use different approaches for melting a metal powder, and propelling the resulting metal droplets onto the surface to be coated. They produce a coating layer made of consecutive layers of solidified metal droplets. The result is a rather porous coating with different degrees of coating oxides included in the coating. 
     Flame spray, also known as oxy-acetylene combustion spray, uses the basic principles of a welding torch to propel molten particles onto the substrate. The coating material can be either a wire- or powder-form. Usually nitrogen is used as a carrier to conduct metal powder into the centre of the combustion zone in the torch. 
     Thermal spray metal coatings are naturally somewhat porous or permeable, since the molten metal droplets upon impact with the substrate don&#39;t necessarily merge into a continuous, impermeable coating. To achieve an impermeable coating, thermal spray metal coatings are often fused after being applied to enhance bond strengths and coating density. The fusing process at least partially melts and bonds coating particles to each other, resulting in a non-porous coating virtually free from coating oxides found in non-fused spray coatings. This optional step changes the process from a “cold spray” method to a “spray-and-fuse” method. Cold spray coatings exhibit lower bond strengths than most other thermal spray processes. Spray-and-fuse coatings are used in applications where excessive wear and/or corrosion combined with high stresses on the coating/substrate are a problem. 
     Methods for the removal of electroplated coatings essentially involve a process known as reverse electroplating, the electrical reversal of the plating process. Reverse electroplating involves applying an electric current through an electrolytic bath, wherein the positive electrical potential is applied to a coated metallic substrate and a negative electrical potential is applied to a cathode. A direct current is generated between the substrate and the cathode, removing the coating. Electroplated coatings can usually be completely removed without damage to the substrate, given sufficient care and attention. 
     The prior art describes electrochemical methods for removing electroplated coatings. U.S. Pat. No. 4,356,069 (Cunningham) describes a method wherein electroplated chrome and nickel coatings can be stripped using an electrochemical process. The patent discloses a stripping composition which aids in the removal of chromium and nickel ions from the surface of a base metal, such as zinc, steel, aluminum, brass or copper which had been previously plated with chrome and/or nickel“. 
     Electrochemical processes have generally been accepted in the art as unsuited for the removal of fused thermal spray coatings since the coatings were considered insufficiently porous to be susceptible to the known stripping compositions. In fact, Cunningham explicitly states that the process is for use on metals previously plated (column  2 , line  35 ). The term “plated” is known in the art to refer to the deposition of a coating of metal using electrolysis. Cunningham does not mention thermal spray coatings in general, or flame spray-and-fuse coatings in particular. Furthermore, the harsh stripping compositions generally used were expected to result in the corrosion of the metal substrate, especially iron substrates. 
     Up to this point, there has been no reason to expect that the fused thermal spray coatings would be susceptible to stripping composition attack due to their lack of porosity and that the extremely harsh chemical stripping composition necessary for electrochemical removal of fused coatings would not damage the underlying substrate if used for removing fused thermal spray metal coatings from soft metal substrates. The prior art does describe electrochemical methods for removing thermal spray metal coatings, but not fused coatings, and the disclosed methods are limited to situations where a person of skill in the art would expect that the metal substrate would not be corroded by an acidic electrolytic bath. 
     Thermal spray or electroplated coatings generally become worn during use, and the substrates are often of sufficient value that it is desirable to reuse or recycle them. For reuse, it is necessary to strip the coating, re-coat the substrate, and place it back in service. This requires a means of economically and quickly stripping the coating without damaging the base metal substrate of the object it is also sometimes desirable to strip the coating and re-coat the substrate if, in the process of coating a part, the part no longer meets specifications. This, also, must be accomplished without damaging the substrate. 
     Traditionally, removal of thermal spray coatings from a substrate has been accomplished using grinding or other mechanical means, such as sand, shot, or grit blasting. However, parts with complex shape cannot be ground within the desired tolerances, and the other mechanical methods are also not sufficiently precise. Moreover, the removal of thermal spray coatings using mechanical means can result in irreparable damage to the substrate. 
     It is frequently necessary or desirable to roughen the surface of a substrate rather deeply before thermal spray metal coating in order to achieve the necessary bonding strength of the coating to the substrate. Therefore, coatings must be completely removed before the substrate is re-coated with thermal spray coatings. Since the thermal spray coating cannot be completely removed by mechanical means without also removing a considerable amount of the substrate material, serious and often unacceptable changes to the dimensions of the substrate can occur. 
     U.S. Pat. No. 4,128,463 (Formanik) discloses a process wherein titanium or titanium alloy substrates coated with flame-spray metal WC coatings are stripped using an electrolytic cell. Titanium and titanium alloys are known in the art to be highly resistant to corrosion and would not be expected to be damaged during the reverse electroplating process. It is, therefore, not unexpected that reverse electroplating could be used to remove a spray metal coating without damaging the titanium substrate. However, Formanik does not discuss or even suggest that the process could be used for other substrates. The process disclosed by Formanik is only described for use in removing WC coatings from a titanium or a titanium alloy, substrate, and no soft metal substrates such as iron or steel. More importantly, Formanik never discusses fused spray metal coatings and whether the disclosed process could be used for the removal of fused thermal spray metal coatings. 
     U.S. Pat. No. 4,886,588 (Curfman) discloses a process wherein spray metal coatings can be electrochemically removed from a soft metal substrate, in this case aluminum. However, aluminum is not as corrosion resistant as titanium. Thus, it is not surprising that the focus of the process described by Curfman is to prevent corrosion of the substrate by using aluminum corrosion inhibitors in the reverse electroplating process. It is taught by Curfman that preventing the corrosion of the metal substrate by the highly corrosive electrolytic bath used for reverse electroplating is of great importance (see column 2, lines 7-11). 
     Neither Formanik nor Curfman disclose a process for the removal of fused thermal spray coatings or other coatings of low permeability (e.g. flame spray-and-fuse coatings, plasma spray coatings, or HVOF coatings). In fact, Formanik describes the process as mechanically loosening the coatings by electrolytically generating hydrogen gas on the surface of the substrate (see column 1, lines 61-64). For this mechanical loosening to occur, the coating must be permeable. Formanik and Curfman neither disclose nor suggest the electrochemical removal of low permeability, fused coatings from steel, mild steel, common steel or iron. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obviate or mitigate at least one disadvantage of the references discussed above. 
     It has been surprisingly discovered that electrochemical removal of fused thermal spray metal coatings is possible. 
     It is surprising that electrochemical conditions were discovered that completely remove flame spray-and-fused coatings economically from a soft metal substrate, without altering the substrate. 
     In a first aspect, the present invention provides a method for stripping a fused spray metal coating from the surface of a soft metallic substrate. The steps for the process comprise: immersing the coated metallic substrate as an anode in an aqueous solution of chromic acid, peroxide, and sulphuric acid or phosphoric acid; immersing a metal cathode in the aqueous solution; applying a positive potential to the fused spray metal coated substrate and a negative potential to the metal cathode to generate a direct current between the substrate and the cathode; whereby the current is preferably applied for a sufficient time to completely remove the coating. 
     In various embodiments, the spray metal coating is a NiCr, WC or MoS 2  alloy. In various embodiments, the metallic substrate is steel, common steel or iron. In various embodiments, the aqueous solution is made up of: 6 to 38 parts by volume of a 20 to 25 wt % aqueous chromic acid solution; 2 to 8 parts by volume of about a 35 wt % aqueous peroxide solution; and 100 parts by volume of 55°-35° Baume sulphuric acid. While any peroxide is acceptable, the aqueous solution preferably comprises hydrogen peroxide or barium peroxide. In further embodiments, the chromic acid is H 2 CrO 4 , and the positive potential applied generates a current of 20 to 80 mA/cm 2  (milli Amperes per square centimeter) (20 to 80 A/ft 2 ). 
     In another aspect, the aqueous solution further includes a catalyst, which, in the embodiment above, is peroxide. In a further aspect, the temperature of the aqueous solution is maintained at a temperature between 30° C. and 80° C., (90° F. and 180° F.) and preferably between 50° and 60° C. (120° F. and 140° F.). 
     In a further embodiment, the cathode is solid lead, carbon, titanium or any other conductor inert to the electrolyte solution. The cathode preferably has a conductive material, the conductive material having a non-conductive backing and a coating of lead, carbon, titanium or other conductor inert to the electrolyte solution which has been electrochemically deposited on the conductive material. Cathodes of this embodiment are lighter and more easily maneuvered than solid metal cathodes. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention. 
    
    
     DETAILED DESCRIPTION 
     Generally, the present invention provides a method and system for electrochemically removing fused spray metal coatings from metallic substrates. For the purpose of illustrating the preferred embodiment, there will be discussed a method for the use of a stripping composition, which is adapted for the electrochemical removal of chrome and/or nickel spray metal coatings. However, other alloys such as WC or MoS 2  can also be removed by this process. 
     The electrochemical stripping of a coating depends upon the object being placed in a bath containing chemicals such that one of the coating components is either soluble or will react with a chemical in the bath to form a soluble compound. Preferably, the reaction between the chemicals in the bath and the coating should be slow, otherwise the process would be difficult to control and there would be greater risk of damaging the substrate. 
     In some cases, the reaction will not proceed because compounds are formed that coat the surface and stop the reaction. In others, the process would be uneven because the reaction would be catalyzed by impurities in places. The application of an electrical potential overcomes these problems with the correct placement of the electrodes. 
     The application of an electrical potential enables a weak reaction to be accelerated and controlled to prevent damage to the substrate. In some cases, corrosion inhibitors are helpful to protect the substrate. 
     A final advantage of electrochemical stripping is that the concentration of the chemicals in the bath are maintained because the material being removed is redeposited at the other electrode and the chemical stripper is thereby rejuvenated. This also helps in the control of the process because the rate of stripping is more or less constant. 
     The stripping composition of the present invention is an aqueous, electrolyte solution having the following ingredients: chromic acid (CrO 3 ), a peroxide, preferably hydrogen peroxide (H 2 O 2 ), a second acid, preferably sulfuric acid (H 2 SO 4 ) or phosphoric acid (H 3 PO 4 ), and water (H 2 O). The method of preparing the stripping composition is also important since exothermic reactions are involved and the order of addition of the ingredients is significant. 
     Thus, it is preferred that the chromic acid solution be initially formed having a 20-25% CrO 3  concentration by weight in water. In the presence of water, CrO 3  forms H 2 CrO 4 . Preferably 225-1500 ml (6-40 parts by volume) of this chromic acid solution is added to each gallon (100 parts by volume) of sulfuric acid. In terms of dry weight of CrO 3  added to each gallon of H 2 SO 4 , this ranges from 2-10 ounces, and is preferably 24 ounces. The sulfuric acid itself is an aqueous solution of H 2 SO 4  and water. Preferred is a 50° Baume solution, i.e. one having a 62.2% H 2 SO 4  concentration by weight and a specific gravity of 1.53. Other sulfuric acids in the 55°-35° Baume range may be used as well. Phosphoric acid at this concentration may be used as well. The chromic acid solution is added to the sulfuric acid preferably at between room temperature and 50° C. 
     Because an elevated temperature may be used and since the reaction between CrO 3  and H 2 SO 4  is exothermic, the solution should thereafter be cooled. Once cooled, the peroxide is added. Preferred is a hydrogen peroxide which has a 35% H 2 O 2  concentration by weight in water. Technical grades of hydrogen peroxide at this concentration are available. Other concentrations may also be used, as may other peroxides, such as barium peroxide. The preferred amount is 100-300 ml (2.5-8 parts by volume) of this peroxide solution. Although more may be added without deleterious effect, it is not necessary to achieve the result desired. 
     The stripping solution is now complete in terms of active ingredients. Clearly, it may contain amounts of any number of inactive ingredients as well. The exact chemistry of the products within the stripping solution after mixing is not known. In the preferred embodiment, however, it is believed that a trivalent form of chromium such as chromium sulfate or chromium dichromate is formed and that the trivalent chromium is oxidized at the anode to form a hexavalent chromium. In the process, the chromium metal plated on the base metal substrate of the anode will be stripped therefrom. The peroxide is believed in the process to form an intermediate oxide layer on the surface of the base metal substrate which will protect the substrate metal from attack by the sulfuric acid. 
     A chromic acid solution is prepared having a CrO 3  concentration of 25%. 20 parts per volume of this solution are gradually added to 50° Baume sulfuric acid at room temperature. The rate of addition is controlled to prevent overheating of the resulting composition. Alternatively, the composition is cooled during addition of the chromic acid solution. 4 parts per volume of a 35% aqueous hydrogen peroxide solution are added once the composition is at room temperature to finish the stripping composition. A lead cathode is inserted into the stripping composition and a steel PCP pump rotor with a fused thermal spray coating of NiCr is suspended in the stripping composition. A positive electrical potential is applied to the coated rotor and a negative electrical potential is applied to the lead cathode. A current is applied for  30  minutes for complete removal of the fused NiCr coating. 
     In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments of the invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the invention. 
     The above-described embodiments of the invention are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.