Title of Invention

A PROCESS FOR PULSE HARD ANODISING OF ALUMINIUM AND ITS ALLOYS

Abstract

The present invention provides a process for pulse hard-anodizing of aluminum substrates, said process comprising the steps of; degreasing the substrate by immersing in a solvent and agitating the solution initially at a room temperature and thereafter to a higher temperature followed by air-drying of the substrate, treating the dried substrate with an alkali, acid cleaning and de-smutting in an acid solution of nitric acid, sulfuric acid and hydrofluoric acid, at an ambient temperature, neutralizing the substrate and rinsing with water, and pulse hard-anodizing the substrate in an electrolyte solution.

Full Text

A PROCESS FOR PULSE HARD ANODIZING OF ALUMINUM AND ITS ALLOYS Technical Field The present invention relates to a process for pulse hard anodizing of aluminum substrates. Background and Related art Hard anodic coatings on aluminum are widely used in aerospace and allied industries to improve the tribological properties of the components. These coating are also used in automobile industries and kitchenware to improve their durability, corrosion resistance and heat radiation characteristics. Hard-anodizing is an electrochemical process where hard and thick coating of metal oxide/hydroxide is produced on the anodic job/substrate in a suitable electrolyte. The conventional hard anodizing process employs a high operating current density and subzero temperature of electrolyte. The cooling is required to reduce the solvency power of electrolyte to achieve higher coating thickness. The problems associated with conventional hard-anodizing process are high cooling requirement and burning of jobs due to high operating voltage. Since, the hard anodizing process proceeds at a constant current density, the electrode voltage increases due to the insulating nature of coating formed. Further, at sub-zero temperatures the effective conductivity of the electrolyte is reduced substantially, thereby necessitating a vigorous electrolyte/substrate agitation to dissipate the heat generated and to transport the depleted reactants at the interface of substrate and electrolyte. This leads to the defects and non-uniformity in hard anodic films. In a known hard-anodizing process, an electrolyte solution of sulfuric acid, oxalic acid is used at very low temperature of 0 to -10 °C and at a constant current density of 20-40 A/ft2 with DC power supply for 60-120 minutes. Further, a process of cooling is required to reduce the solvency power of electrolyte, which helps in shifting the equilibrium of forming and dissolution of coating to obtain higher thickness. One of the limitations of such process is, due to cooling the effective conductivity of the electrolyte is reduced. Also as the process proceeds, a very high voltage (due to insulating nature of coating) is required to maintain a constant current. This high voltage leads to local high temperature at the electrolyte-substrate interface. Consequently, the burning problems (Joule's heat) are often encountered and coating formation is non-uniform on the substrate. The local rise in temperature enhances the concentration of the current and due to associated chemical field an increased dissolution of the oxide coating occurs, which phenomenon is known as burning. Further, in conventional hard anodizing processes the thickness of the hard-anodic coating that can be obtained is limited, which is further compounded by powdery deposits on the coating. The powdery deposition on the coating is as a result of use of high average constant current throughout the coating formation, which often leads to depletion of reactants at the substrate-electrolyte interface resulting into coarse powdery and loose deposits (due to higher dissolution rate of coating). Objects of the present invention The main object of the present invention is to provide a process for pulse hard anodizing of aluminum substrates. An object of the present invention is to provide a process for pulse hard anodizing of aluminum substrates that controls the total amount of heat produced during hard-anodizing process. Another object of the present invention is to provide a process for pulse hard anodizing of aluminum substrates at a moderate temperature and to increase the heat dispersion at the bottom of the pores of substrates. Yet another object of the present invention is to provide a process for pulse hard-anodizing of aluminum substrates by milli second pulsing of the applied current between base to peak values to provide enough time for replenishment of reactant's concentration at the substrate-electrolyte interface for uniform coating deposition with improved morphology. Summary of the present invention A process for pulse hard-anodizing of aluminum substrates, said process comprising the steps of; degreasing the substrate by immersing in a solvent and agitating the solution initially at a room temperature and thereafter to a higher temperature followed by air-drying of the substrate, treating the dried substrate with an alkali, acid cleaning and de-smutting/ neutralizing in an acid solution of nitric acid, sulfuric acid and hydrofluoric acid, at an ambient temperature and rinsing with water, and pulse hard-anodizing the substrate in an electrolyte solution. Brief description of the accompanied diagrams Fig 1 illustrates an exemplary schematic depiction of the apparatus used to provide pulse hard anodizing of substrate. Fig 2 depicts a variation of micro-hardness of the hard anodic coating formed with respect to duty cycle at different solution temperatures for a current density of 70 A/ft . Fig 3 depicts a variation of micro-hardness of the hard anodic coating formed with respect to duty cycle (at 10°C solution temperatures for a current density of 60 A/ft ) for three different frequencies. Detailed description of the present invention Accordingly, the present invention provides a process for pulse hard-anodizing of aluminum substrate. Initially, by referring to Fig 1 of the accompanied diagrams, a schematic depiction of the apparatus used in the present invention for the pulse hard anodizing of aluminum substrate is described. It is to be noted here that the apparatus as shown in Fig 1 is as an exemplary embodiment and the apparatus can be suitably modified or adapted to perform the process steps of the present invention. The apparatus as shown in Fig 1 comprises a Lead lined Mild steel tank 3 of suitable capacity with built in lead coils 4. The pulse hard anodising solution or an electrolyte 2 is filled in the tank 3 and chilled brine supplied from brine (45% poly ethylene glycol in water) chilling unit 6, is circulated in the lead cooling coil 4 to cool the hard anodising solution 2 to a required temperature. An air agitation pipe 5 is disposed to cause turbulence and to provide an oil free air agitation of the solution 2 in the tank 3. A substrate 1, which is to be pulse hard anodized, is dipped in the solution 2 and connected to the positive terminal of pulse power supply 7, whereas the negative terminal is connected to the tank body. Applying a desired current through the power supply circuit the pulse hard anodising process of the selected substrate 1 is performed. The process steps of the present invention are described in the form of following sequence of operations: The selected substrate is initially subjected to a process of solvent degreasing by immersing the substrate in a degreasing solvent solution of trichloroethylene or 2-propanol or per chloro ethylene. The substrate is immersed in said solvent and agitated manually or ultrasonically. The substrate is alternatively degreased in a vapour degreaser by placing the substrate in the vapour path. The degreasing process is performed initially at an ambient temperature and thereafter progressively at a higher temperature for about 2-10 minutes, preferably for about 2-5 minutes. The desired higher temperature depends on the boiling temperature of the solvent used. In the present invention, when trichloroethylene is used as a degreasing solvent the higher temperature thus provided is in the range of 86-88°C. Further, it is also understood here that the treatment time for degreasing of the substrate depends on the degree of contamination that is present on the surface of the selected substrate. Thereafter, if the selected substrate is either a machined or non-corroded one, the dried and degreased substrate is treated with an alkali, in a solution of sodium carbonate in the range of 10-40 g/L, preferably about 25g/L, tri sodium orthophosphate in the range of 20-50 g/L, preferably about 25 g/L, and sodium meta silicate in the range of 10-40 g/L, preferably about 20 g/L. It is to be noted here that the alkali treatment is optionally performed in the presence of a wetting agent. The wetting agent used in the present invention is Sodium lauryl sulphate at a temperature in the range of 30-80°C for about 2-10 minutes, preferably for about 2-5 minutes to improve the wettability of the substrate. The cleaning of the degreased substrate is performed by way of a mild and mechanical agitation. The substrate is further rinsed with water to prevent the alkaline cleaning solution adhering to component surface from entering and contaminating the acid cleaning tank. However, if the selected substrate is either a corroded one or heat-treated substrate, the process of cleaning is performed by an alkaline etching process by agitating the substrate in a solution of sodium hydroxide in the range of 50-150 g/L, preferably about 75 g/L, sodium hydrogen fluoride 10-30 g/L, preferably about 25g/L and tri sodium orthophosphate 1-3 g/L, preferably about 2 g/L, at a temperature in the range of 30-80°C, preferably at about 60°C for about 2-10 minutes. The substrate thus obtained is rinsed with water. Further, if required the substrate is deoxidized, by immersing and agitating the substrate in a solution of sulfuric acid (p=1.84) 50-200 ml/L, preferably about 75 ml/L, chromium trioxide, 30-90 g/L, preferably about 40 ml/L, at a temperature of about 40-80°C, preferably at about 60°C for about 2-10 minutes. The substrate thus obtained is rinsed with water. The substrate is then cleaned with an acid in a tank, preferably a polypropylene tank having a solution of nitric acid (70%) in the range of 10-35 ml/L, preferably about 25 ml/L, sulfuric acid (p= 1.84) in the range of 5-25 ml/L, preferably about 15 ml/L and hydrofluoric acid (40%) in the range of 5-25 ml/L, preferably about 15 ml/L at an ambient temperature of about 25°C (20-30°C) for about 2-10 minutes. This step is generally referred to as desmutting or neutralization. The neutralized substrate is again rinsed with water. The cleaned substrate is then subjected to a pulse hard-anodization process, by placing the cleaned substrate in a mild steel tank whose internal surface is lined with a lead. The container is fitted with temperature controllers and equipped with a DC Pulse Power Supply, to provide a desired pulsation of current and voltage. The tank is filled with a solution of electrolyte comprising sulfuric acid in the range of 70-150 ml/L, oxalic acid in the range of 5-25 g/L at a temperature in the range of 5- 20°C. In the pulse hard-anodization of the present invention, the solution is cooled by circulating the coolant in lead coil and the solution is agitated with air. In the present invention as an exemplary embodiment the coolant used is 45% poly ethylene glycol in water. After attaining the desired temperature of about 5-20°C, the substrate to be pulse hard anodized is dipped in the solution and is connected to the positive terminal of the pulse power supply output whereas the negative terminal is connected to the body of the tank to complete the circuit. The substrate is then subjected to pulse mode hard anodisation at a constant current density of 30-70 A/ft2, preferably 40-60 A/ft2 with pulsed power supply duty cycle of 40-90%. The current pulse frequency is in the range of 5-10 CPS, in a reduced time of about 30-40 minutes, at a desired DC voltage of 20- 36 volts. The anodized substrate is then removed from the solution and is rinsed with water. In an embodiment of the present invention the power supply duty cycle is in the range of 50-70%. It is also an embodiment of the present invention, wherein the preferred hard anodizing temperature is in the range of 5-10°C. In the present invention the duty cycle is calculated by dividing the on time by the sum of on time and off time of current pulse. In the present invention the use of Pulse power supply provides pulsed current (high level current amplitude) super imposed on a base level current (low level current amplitude). The pulsation allows the surface to recover from the momentary high current heating effects and thus keeps the surface temperature lower. The pulsed current occurs many times a second which allows the coating to form uniformly and quickly, even over high current density areas such as corners without burning. Also, the pulsation allows the replenishment of depleted ions near the electrode surface. The net result of all these benefits are increased production without rejection. In the pulse hard anodizing, the current is pulsed many times a second i.e. from a base line to peak current. The base line recovery period leads to the formation of improved morphology coating at higher electrolyte temperature without burning. Further, by interrupting the power supply, continuous heat dissipation is prevented, resulting in better dispersion of heat to the electrolyte. Since, the anodic coatings have a porous structure heat dispersion takes place at the bottom of the pores. Also, this process minimizes the amount of tank refrigeration required. The pulse hard-anodized substrate is sealed, optionally in de-mineralized water containing 0-1 g/L ammonium acetate operating at 80-98°C. The sealing of anodized substrate is used to increase the insulation value/corrosion resistance of the anodic coating. In the present invention, as an exemplary embodiment, the substrates that are used are Aluminum alloys AA-1100 (also known as commercial Aluminum), AA-6061 and AA-7075 alloys and AA-7075 alloys. Now by referring to Fig 2 & 3 of the accompanied diagrams, the micro hardness of the coating of the substrate that has been achieved by using the process steps of the present invention is depicted in the form of a graphical representation, by using process parameters like duty cycle, frequency, current density, time and temperature. The above Figures clearly show the micro hardness of the coating of the substrate obtained by pulse hardness process of the present invention is higher and is in the range of 300-500 VHN, resulting in harder and abrasion resistant surface of the selected substrate. Fig 2 shows a typical relationship among the duty cycle and micro hardness at different temperatures. The increase in duty cycle results in higher current density hence the higher values of thickness are obtained. However, the maximum values of micro hardness are obtained at 50-70 % duty cycle (at 10°C ). It is also observed that above 80% duty cycle, the property of the duty cycle decreases slowly as a result of lack of influence of pulse effect, i.e., the benefit of relaxation and recovery effect is not significant due to shorter 'Off period. Therefore, lower duty cycles reduce the current density and hence the time required to build up required thickness increases considerably resulting in decreased micro hardness. Fig 3 depicts the variation of micro hardness with respect to change in duty cycle at 10°C. The problem like burning and powdery coatings is encountered at higher frequencies. Accordingly, lower frequencies in the range of 5 to 10 CPS are preferred for all subsequent trials. The properties of the pulse hard-anodized aluminum substrate that is obtained by using the process steps of the present invention are subjected to the quality tests and the results are tabulated in Table 1. A comparative statement of features of known process steps vis-a-vis pulse hard- anodization of the present invention is provided in the following Table 1. present invention, the anodisation temperature is about + 10°C as against the hard anodisation temperature of-10 to 0°C as used in known processes. Therefore, the saving in terms of cooling load in the present invention is quite significant. It is to be noted here that the quantum of saving on cooling load depends on the tank size and the total number of substrates that are processed. The pulse hard-anodized substrate as obtained by using the process steps of the present invention, is subjected to standard tests to evaluate the pulse-hard anodic coating and the results are tabulated in the following Table 2. 1. The pulse hard-anodization process provides hard anodizing at relatively elevated temperature 10-20°C with improved properties than the conventional hard anodizing and as a result, the extent of refrigeration is minimized. 2. The pulse hard-anodization process eliminates the problem of burning and powdering, which are frequently encountered in case of conventional hard anodizing. 3. By adopting the process steps of the present invention, the time taken to build-up the required coating thickness is significantly reduced by more than half as compared to conventional hard anodizing process. 4. The hard anodic coatings obtained by using the process steps of the present invention have improved properties such as solar absorptance (-0.90), infrared emittance(~0.85) and insulation value (30MΩ-1.5 GΩ in 10-100 V range). 5. The substrates treated with process steps of the present invention are very useful for thermal control (improving the heat radiation characteristics) and electrical insulation applications. 6. By using the process steps of the present invention, an increased surface thickness of the substrate in the range of 30-35% is obtained without any significant increase in surface roughness. We claim: 1. A process for pulse hard anodizing of aluminum substrates, said process comprising the steps of; degreasing the substrate by immersing in a solvent and agitating the solution initially at a room temperature and thereafter to a higher temperature followed by air-drying of the substrate, treating the dried substrate with an alkali, acid cleaning and de-smutting in an acid solution of nitric acid, sulfuric acid and hydrofluoric acid, at an ambient temperature, neutralizing the substrate and rinsing with water, and pulse hard anodizing the substrate in an electrolyte solution. 2. The process as claimed in claim 1, said process of pulse hard anodizing further comprising the steps of: cooling the electrolyte solution of sulfuric acid and oxalic acid to a pulse anodizing temperature range of 5-20°C, disposing the substrate in said solution and applying a desired DC voltage, pulse hard anodizing the substrate in a reduced time-frame by applying a constant current density in a pulsed power supply duty cycle, and rinsing the pulse-anodized substrate with water to obtain a pulse hard anodized substrate. 3. The process as claimed in claim 1, wherein the pulse hard anodizing temperature is intherangeof5-10°C. 4. The process as claimed in claim 1, wherein the DC voltage is in the range of 20-36 volts. 5. The process as claimed in claim 1, wherein the reduced pulse hard-anodizing process time is in the range of 30-40 minutes. 6. The process as claimed in claim 1, wherein the constant current density is in the range of 30-70 A/ft2. 7. The process as claimed in claim 1, wherein the pulsed power supply duty cycle is in the range of 50-70%. 8. The process as claimed in claim 1, wherein the hard anodized substrate is optionally sealed to increase the insulation value and/or corrosion resistance.

Documents:

1311-CHE-2005 AMENDED CLAIMS 28-10-2014..pdf

1311-CHE-2005 FORM-13 04-10-2010.pdf

1311-CHE-2005 FORM-3 28-10-2014..pdf

1311-CHE-2005 FORM-5 28-10-2014..pdf

1311-CHE-2005 EXAMINATION REPORT REPLY RECIEVED 28-10-2014..pdf

1311-CHE-2005 POWER OF ATTORNEY 28-10-2014..pdf

1311-CHE-2005 AMENDED PAGES OF SPECIFICATION 28-10-2014..pdf

1311-che-2005-abstract.pdf

1311-che-2005-claims.pdf

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1311-che-2005-form 5.pdf

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