Etching rate determining method and apparatus

Abstract: The etching ability of an etchant for a metal part is monitored by accurately measuring the etching rate with a relatively simple arrangement. The etching rate is determined by channelling a portion of the etchant from an etching tank (4) to a reaction chamber (1), etching a specimen (8) of the same material as the metal part with the etchant portion in the reaction chamber (1), collecting hydrogen gas generated during etching of the specimen, measuring the time taken until a predetermined quantity of hydrogen gas is generated, and computing the etching rate from the measured time.

Description: This invention relates to a method and apparatus for determining etching rate. More particularly, it relates to a method and apparatus for determining the etching rate at which light metals and alloys thereof such as aluminum, aluminum alloys, magnesium and magnesium alloys are etched with acid or alkali etchants. In the prior art, aluminum parts are often subject to a pretreatment by immersing them in acid or alkali liquid etchants for etching the surface thereof prior to customary surface treatments like anodizing and plating. For such a pretreatment, it is desired in view of etchant management to monitor the etching ability of the etchant by measuring the etching rate. Several techniques are known for the measurement of etching rate, including a weighing technique of measuring the quantity (in mg unit) of metal etched away per unit time, and a coulostatic technique of applying potential to an electrode immersed in etchant and measuring the time taken until the potential becomes constant. However, the weighing technique has the problem that since measurement cannot be carried out in a reaction system for actual treatment, another system containing a measurement specimen is used for measurement from which the etching rate associated with actual etching is presumed. This can lead to a time lag and requires a large size of measuring system. The coulostatic technique also requires a large size of apparatus and increases the cost for etching rate measurement. SU-A-1272099 (KAPRALOV LV) concerns the control of electrochemical metal-processing by the cyclic venting of gas from a measurement chamber. In this respect the chemical milling of e.g. Al or Ni is conducted to a desired degree by calculating the volume of gas, such as hydrogen, emitted and vented. SU-A-905327 (LAZUTINA VA) concerns generally an etching process for magnesium alloys. Therefore, an object of the present invention is to provide a method for accurately determining an etching rate with minimal measurement errors such as time lag. Another object is to provide a relatively simple and inexpensive apparatus capable of such accurate determination of etching rate. Seeking a method for measuring the etching rate at which a metal part is etched with an acid or alkali etchant in an etching tank, the metal generating hydrogen gas when etched with said etchant, we have found that the etching rate can be determined by sampling a portion of the etchant from the etching tank, etching a specimen of the same material as the metal part with the sampled portion of the etchant, collecting hydrogen gas generated from the specimen as a result of this etching, measuring the time taken until the quantity of hydrogen gas generated reaches a predetermined level, and computing the etching rate from the measured time. The means for collecting the hydrogen gas generated with the progress of etching and measuring the time taken until the predetermined quantity of hydrogen gas is generated comprises a reaction column of the closed type adapted to carry out etching of the specimen and a metering column for measuring the quantity of hydrogen gas generated, with both the columns connected for fluid communication. With this arrangement, the etchant is first channeled from the etching tank into the reaction column where the specimen is subject to etching with the etchant. The etchant is then channeled from the reaction column into the metering column due to an increase in the interior pressure of the reaction column as a result of hydrogen gas generation during etching. There is further provided a means for measuring the time taken until the etchant in the metering column has increased by a predetermined quantity. Then the time taken for the generation of a unit quantity of hydrogen gas can be accurately measured by a relatively simple arrangement. Accordingly, the present invention provides a method for determining the etching rate at which a metal part is etched with an acid or alkali etchant in an etching tank, the metal generating hydrogen gas when etched with the etchant. The method comprises the steps set out in the accompanying claim 1. In a second aspect, the invention provides an apparatus for determining the etching rate at which a metal part is etched with an acid or alkali etchant in an etching tank, the metal generating hydrogen gas when etched with the etchant as set out in the accompanying claim 4. A reaction column of the closed type is adapted to accommodate therein a specimen of the same material as the metal part and receive the etchant from the etching tank whereby the specimen is subject to etching with the etchant in the reaction column. A metering column is connected to the reaction column such that the etchant is introduced into the metering column from the reaction column as the interior pressure of the reaction column increases. A flow control mechanism for controlling the inflow and outflow of the etchant into and from the respective columns operates such that the etchant is channeled from the etching tank to the reaction column, whereby the specimen is subject to etching with the etchant in the reaction column and the etchant is then channeled from the reaction column into the metering column due to an increase in the interior pressure of the reaction column as a result of hydrogen gas generation during etching. The metering column is provided with means for measuring the time taken until the etchant in the metering column reaches a predetermined quantity. FIG. 1 is a schematic view of an etching rate determining apparatus according to one embodiment of the invention. FIG. 2 is a schematic view of the air pump used in the apparatus of FIG. 1. The etching rate determining method of the present invention is useful for determining the etching rate associated with a process of etching parts of metallic material such as aluminum, aluminum alloys, magnesium and magnesium alloys with an acid or alkali liquid etchant in an etching tank, the metallic material generating hydrogen gas when etched with such etchant. A specimen formed of the same material as the metal part is etched with the etchant sampled from the etching tank. The time taken until a predetermined unit quantity of hydrogen gas is generated is measured (which time is referred to as a unit quantity hydrogen release time, hereinafter). The etching rate is computed from the measured time. Referring to FIG. 1, there is illustrated an etching rate determining arrangement according to one embodiment of the invention, by which the unit quantity hydrogen release time can be measured in a simple manner. The apparatus generally includes a reaction column 1, a metering column 2 and a priming or suction column 3. An etching tank 4 is partially illustrated as containing a liquid etchant 5 to be examined for etching rate so that metallic parts (not shown) may be actually etched in the tank 4. The reaction column 1 is of the closed box type and half immersed in the etchant 5 in the tank 4. To the bottom of the reaction column 1 is connected an etchant inlet line 7 having a valve 6. The distal end of the line 7 is immersed in the etchant 5 in the tank 4. In the reaction column 1 is disposed a specimen 8 which is formed of the same material as the metallic parts to be etched in the tank 4. The metering column 2 is a cylinder column which at the bottom is in communication with the bottom of the reaction column 1 through a feed line 10 having a valve 9. A return line 11 is inserted into the metering column 2 such that its proximal end is located at a lower level b2 in the metering column 2. The return line 11 has a valve 12 at an intermediate and a distal end disposed above the etching tank 4. Disposed in the metering column 2 is a liquid level sensor 13 which has a sensitive end located at an upper level b1 in the metering column 2 and a common level 15 located enough deep in the metering column 2. Further a first air pump 14 is coupled to the top of the metering column 2. The priming column 3 at the bottom is in fluid communication with the top of the reaction column 1 through a line 17 having a valve 16. Disposed in the priming column 3 are a first liquid level sensor 18 having a sensitive end located at a lower level in the column and a second liquid level sensor 19 having a sensitive end located at an upper level in the column and a common level 21 located enough deep in the column 3. A second air pump 20 is coupled to the top of the priming column 3. The first and second air pumps 14 and 20 each are of the arrangement shown in FIG. 2. The pump includes a pump body 22 having suction and discharge ends connected through a loop line 23. Valves 24 and 25 are disposed in the loop line 23 near the suction and discharge ends, respectively, and a coupling line 26 is connected to the loop line 23 intermediate the valves 24 and 25. The coupling line 26 is connected to the metering or priming column 2 or 3. An air venting valve 27 is connected to the coupling line 26. With the vent valve 27 closed, the air pump of this arrangement can be a suction pump when the valve 25 is closed and the valve 24 is opened, but a discharge pump when the valve 25 is opened and the valve 24 is closed. It is to be noted that the manipulation of these valves during operation of the first and second air pumps 14 and 20 for suction or discharge of air is omitted from the following description. With this arrangement, the unit quantity hydrogen release time is measured by first actuating the second air pump 20 for sucking and evacuating the air from the priming column 3 with the valves 6 and 16 opened. Then the liquid etchant 5 in the etching tank 4 is channeled to the reaction column 1 through the inlet line 7 and after the reaction column 1 is filled with the etchant, the etchant is further channeled to the priming column 3 through the line 17. When the first level sensor 18 detects the surface of the etchant, the second air pump 20 is interrupted and the valve 16 is closed. Then the reaction column 1 is full of the etchant to be examined. Next, with the valve 6 kept open, the valve 9 is opened and the first air pump 14 is actuated for sucking and evacuating the air from the metering column 2. Then the etchant in the reaction column 1 is passed to the metering column 2 through the feed line 10. When the level sensor 13 detects the surface of the etchant in the metering column 2, the first air pump 14 is interrupted and the valve 9 is closed. Then the metering column 2 is full of the etchant to the height b1 corresponding to the detecting end of the sensor 13. Since this operation is carried out with the valve 6 opened, the reaction column 1 is continuously replenished with the etchant from the tank 4 in an amount corresponding to the amount that has transferred from the reaction column 1 to the metering column 2. Next, with the valve 9 closed, the valve 12 is opened, the first air pump 14 is actuated to pump air into the metering column 2 to pressurize the interior thereof. Then the etchant in the metering column 2 is forced back to the tank 4 through the return line 11 until the surface of the etchant in the metering column comes down to the height b2 corresponding to the distal end (detecting end) of the return line 11. The arrangement is now ready for measurement. Next, the valve 6 is closed and the valve 9 and the air vent valve 27 associated with the first air pump 14 (see FIG. 2) are opened for starting measurement. Then the specimen 8 is etched with the etchant in the reaction column 1, where hydrogen gas generates during etching so that the reaction column 1 increases its interior pressure. In response to the increasing interior pressure, that is, the quantity of hydrogen gas generated, the etchant in the reaction column 1 is purged to the metering column 2 through the feed line 10. A timer (not shown) is used to measure the time taken until the surface of the etchant in the metering column 2 rises from level b2 to level b1. The point of time when the etchant reaches level b1 can be detected by the liquid level sensor 13. By measuring the time taken until the surface of the etchant in the metering column 2 rises from level b2 to level b1 in this way, the time taken for releasing a quantity (unit quantity) of hydrogen gas equal to the volume of the region of the metering column 2 between levels b2 and b1 can be readily and accurately obtained. In the above - mentioned arrangement, measurement is done with the reaction column 1 partially immersed in the etchant 5 in the tank 4 so that the etching for measurement is effected under the same temperature condition as the actual etching, ensuring very accurate measurement. Automatic measurement is possible by using a programmed computer control for controlling the relevant operations including valve operation, air pump actuation, and time measurement and the calculation to be described later may also be carried out by the same computer, resulting in fully automatic measurement. Of course, these operations may be carried out manually if desired. According to the etching rate determining method of the invention, the etching rate is computed from the unit quantity hydrogen release time obtained by the above - mentioned procedure or the like. The following computing factors are necessary other than the time. P: atmospheric pressure (atm) V: unit volume (liter) (corresponding to the volume between b2 and b1 in metering column 2 in the above - illustrated procedure) T: temperature (K) S: surface area of specimen (cm 2 ) For example, the etching rate v ( µm/hr) for aluminum may be computed from these computing factors and the unit quantity hydrogen release time t (sec) in accordance with the following formula. The hydrogen gas generation associated with etching of aluminum with acid is represented by the reaction formula: Al + 3H + → Al 3+ + 3/2H 2 Aluminum molecular weight: 26.98154 g/mol Aluminum density: 2.6988 g/cm 3 Aluminum volume per mol: 99966.6 cm 23 · µm Provided that hydrogen gas follows PV = nRT wherein n is moles and R is a constant equal to 0.08211, then the mole number m of aluminum is given by m = (2/3)PV/RT. The quantity d of aluminum etched is d = 99966.6 m/S wherein S is the surface area of the specimen. Then, the etching rate v ( µm/hr) is v = d × 3600/t = 2.922 × 10 9 × PV/TSt wherein t is a measured time. In this regard, the etching rate computed by the inventive method may contain errors which are caused by various factors including (1) a time lag associated with liquid surface detection by the level sensor, (2) dissolution of hydrogen gas in the etchant, (3) pressure variations (including atmospheric pressure variation, tank liquid surface variation, and column height variation), (4) specimen thickness reduction, (5) concentration variation in the reaction column, (6) temperature changes and the like. The error factors may be accommodated as follows. (1) Time lag associated with liquid surface detection by the level sensor The time lag can be made negligible by carrying out measurement on a minute basis. (2) Dissolution of hydrogen gas in the etchant It need not be taken into account where the etching tank is operating in practice because hydrogen is in saturated state. If the etching tank is quiescent, a correction may be made in accordance with the solubility and dissolving rate of hydrogen, but such a correction is not necessary in most cases. (3) Pressure variations (including atmospheric pressure variation, tank liquid surface variation, and column height variation) The major cause is an atmospheric pressure variation which is approximate to 1% at 10 millibar. The error caused by column height variation is about 1% at a height variation of 10 cm, but no error is introduced if the pressure within the reaction column is measured and used for computation. (4) Specimen thickness reduction For a specimen dimensioned 10 × 10 × 1 mm and an etching rate of 25 µm/hr, 10 hours etching results in a specimen thickness reduction of 0.5 mm or an area reduction of about 8.3%. This factor can be eliminated by forming the specimen in plate shape and coating the edges of the specimen, thereby reducing the surface area reduction by etching, or alternatively, computing a surface area change from the measured etching rate and making a correction using it. (5) Concentration variation in the reaction column The concentration varies over 10% when the ratio of S/V (surface area/volume) of the specimen is 1 dm 2 /liter. This can be eliminated by increasing the S/V ratio. Alternatively, the average rate may be determined by setting the S/V ratio equal to that of a metallic part to be etched in practice. (6) Temperature changes A temperature change of 3% leads to an error of 1%. Such an error can be eliminated by measuring the temperature in the reaction column and using the measured temperature for computation. In the practice of the inventive method, the measurement of the time taken until the predetermined unit quantity of hydrogen gas is generated is not limited to the above - mentioned arrangement and measurement may be made by any suitable method. Also changes may be made to the elements of the measuring arrangement. For example, the metering column 2 may be a measuring cylinder as in the Example described below whereupon an increase of liquid volume in the metering column can be visually determined. The mechanism for channeling the etchant into the reaction column 1 need not use the priming column 3 or an arrangement may be made such that the etchant is channeled directly into the reaction column 1. EXAMPLE An example is given below by way of illustration and not by way of limitation. Using the arrangement shown in FIG. 1, the etching rate of an etchant with respect to aluminum was measured. The etchant used was "Mill Etch" (trade name). The specimen 8 was an aluminum plate of 10 × 10 × 1 mm (surface area 2.4 cm 2 ). The reaction column 1 had a volume of 25.5 ml. The metering column 2 was a measuring cylinder having a measurable volume of 10 ml. The time corresponding to a liquid increment of 1 ml in the measuring cylinder was measured. The etching rate was computed in accordance with the following formula. The results are shown in Table 1. v = 2.922 × 10 9 × PV/(T.S.t) P: atmospheric pressure (atm) V: unit volume (liter) T: temperature (K) S: surface area of specimen (cm 2 ) t: measured time (sec) An aluminum part of 10 × 10 × 1 mm was etched with the same etchant before the etching rate was measured by the same procedure as above. This etching procedure was repeated 4 times in total. The results at the end of each procedure are shown in Tables 2 to 4. It is to be noted that in Tables 2 to 4, the outlet height refers to the difference in height between the reaction column 1 and the metering column 2. An error caused by a differential pressure resulting from the outlet height is eliminated by measuring the pressure within the reaction column 1 and using this pressure for computation of the etching rate. Time (sec) Total (sec) Rate ( µm/hr) 1 ml 173 173 28.43 2 ml 183 356 26.88 3 ml 189 545 26.03 4 ml 191 736 25.75 5 ml 197 933 24.97 6 ml 197 1130 24.97 1 - 6 ml 1130 - 24.97 2 - 6 ml 957 - 24.32 2nd, specimen exchanged 2.38 cm 2 , outlet height 72 mm Time (sec) Total (sec) Rate ( µm/hr) 1 ml 153 153 27.02 2 ml 165 318 25.05 3 ml 167 485 24.75 4 ml 165 650 25.05 5 ml 168 818 24.61 1 - 5 ml 818 - 25.26 2 - 5 ml 665 - 24.86 3rd, outlet height 155 mm Time (sec) Total (sec) Rate ( µm/hr) 1ml 155 155 26.67 2 ml 169 324 24.46 3 ml 170 494 24.32 4 ml 169 663 24.46 5 ml 172 835 24.03 1 - 5 ml 835 - 24.75 2 - 5 ml 680 - 23.89 4th, outlet height 69 mm Time (sec) Total (sec) Rate ( µm/hr) 1 ml 161 161 25.68 2 ml 170 331 24.32 3 ml 170 501 24.32 4 ml 175 676 23.62 5 ml 177 853 23.35 1 - 5 ml 853 - 24.23 2 - 5 ml 692 - 23.89 It is evident from Tables 1 - 4 that as every etching procedure is carried out, the etching rate becomes slower, indicating that the etching ability becomes lower. Since the etching rate can be easily and accurately determined by the etching rate measuring method of the present invention, the invention facilitates the management of an etchant in an etching process. As described above, the method and apparatus for determining the etching rate according to the present invention permits accurate determination of the etching rate with a relatively simple arrangement, enabling secure and low - cost management of an etchant in an etching process. There has been described herein what is considered to be a preferred embodiment of the present invention.

Claims: A method for determining the etching rate at which a metal part is etched with an acid or alkali etchant in an etching tank, the metal part generating hydrogen gas when etched with said etchant, said method comprising: (A) providing an apparatus comprising an etching tank (4) containing etchant (5), a reaction column (1), a metering column (2), and a priming column (3), wherein said reaction column (1) is in fluid communication with said etching tank (4), said priming column (3) and said metering column (2) are in fluid communication with said reaction column (1), and said metering column (2) is in fluid communication with said etching tank (4), (B) determining the unit quantity hydrogen release time, by (B1) evacuating air from said priming column (3), thereby channelling etchant into said reaction column (1) from said etching tank (4), and channelling etchant from said reaction column (1) into said priming column (3), until said reaction column (1) is filled with etchant and said priming column contains a predetermined level (bl) of etchant, (B2) evacuating air from said metering column (2), thereby channelling etchant from said reaction column (1) into said metering column (2) until said metering column (2) contains a predetermined initial level of etchant and said reaction column (1) is full of etchant, (B3) pressurising said metering column (2), thereby forcing etchant into said etching tank (4), until said metering column (2) contains a predetermined final level (b2) of etchant which is lower than said predetermined initial level (b1), (B4) etching a specimen of the same material to be etched with the etchant in said reaction column (1), thereby generating hydrogen gas to pressurise said reaction column (1) and purge etchant to said metering column (2) thereby increasing the level of etchant in said metering column (2), (B5) measuring the time beginning with said etching and ending when the level of etchant in said metering column (2) again reaches said predetermined initial level (b1) and (C) computing the etching rate from said unit quantity hydrogen release time. A method according to Claim 1 wherein the metal part comprises aluminium or an alloy thereof. A method according to Claim 1 wherein the metal part comprises magnesium or an alloy thereof. Apparatus for determining the etching rate at which a metal part is etched with an acid or alkali etchant in an etching tank, wherein said metal generates hydrogen gas when etched with said etchant, the apparatus comprising: (A) an etching tank (4); (B) a closed reaction column (1) to accommodate a specimen of the same material as a metal part to be etched in said etching tank, the reaction column (1) having an etchant inlet line (7) in fluid communication with the etching tank (4), (C) a metering column (2) having a feed line (10) in fluid communication with the reaction column (1), a return line (11) in fluid communication with the etching tank (4), a liquid level sensor (13, 15) for sensing the level of liquid in the metering column, a first pump (14) for evacuating air from the metering column and for pressurising the interior of the metering column, and (D) a priming column (3) having a line (17) in fluid communication with the reaction column (1), a liquid level sensor (18, 19) comprising a sensing end located at a level inside said priming column(3) and a second pump (20) coupled to said priming column (3) for evacuating air from the priming column (3). Apparatus according to Claim 4 wherein the etchant inlet line (7) communicates with the bottom of the reaction column (1), the feed line (10) communicates with the bottom of the reaction column (1) and the bottom of the metering column (2), the return line (11) has a proximal end disposed at a level inside the metering column (2) and a distal end disposed above the etching tank (4) and the line (17) communicates with the top of the reaction column (1) and the bottom of the priming column (3). Apparatus according to Claim 4 or Claim 5 wherein valves (6,9,12,16) are provided in each of the etchant inlet line (7) for the reaction column (1), the feed line (10) and return line (11) for the metering column (2) and the line (17) for the priming column (3). Apparatus according to any one of Claims 4 to 6 wherein said first and second pumps (14, 20) for the metering column (2) and the priming column (3) respectively each comprise: (a) a pump body(22)having a suction end and a discharge end, (b) a loop line (23) connecting said suction end to said discharge end, (c) a suction end valve (24) and a discharge end valve (25) disposed in said loop line (23)near said suction end and said discharge end, respectively, (d) a coupling line (26) having a proximal end connected to said loop line (23) between said suction end valve and said discharge end valve, and a distal end connected to said metering column, (e) an air venting valve (27) connected to said coupling line, such that when said air venting valve (27) is closed, said discharge end valve (25) is closed, and said suction end valve (24) is open, said air pump (20) functions as a suction pump, and when said air venting valve (27) is closed, said discharge end valve (25) is open, and said suction end valve (24) is closed, said air pump (20) functions as a discharge pump. Apparatus according to any one of Claims 4 to 7 wherein the reaction chamber (1) is at least partially immersed in etchant in tank (4).

Inventors: HASHIMOTO SHIGEO, KAWASAKI SHOGO, HASHIMOTO, SHIGEO, KAWASAKI, SHOGO, Hashimoto, Shigeo, C. Uyemura &amp; Co., Ltd., Kawasaki, Shogo, C. Uyemura &amp; Co., Ltd.
Applicants: UYEMURA C & CO LTD, C. UYEMURA & CO, LTD
Application year: 1992
Main classification: G01N7
IPC codes: C23F1, G01N7