US3293155A

US3293155A - Method for determining the corrosion resistance of anodized aluminum parts

Info

Publication number: US3293155A
Application number: US471283A
Authority: US
Inventors: Stone Jack
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1965-07-12
Filing date: 1965-07-12
Publication date: 1966-12-20
Anticipated expiration: 1983-12-20

Description

Dec. 20, 1966 J. STONE 3,293,155

METHOD FOR DETERMINING THE CORROSION RESISTANCE OF ANODIZED ALUMINUM PARTS Filed July 12; 1965 2 Sheets-Sheet 1 7'/MER INTEGRA 7' I0 U/V/ T RECORWER F/G. Z

JACK S TO/VE INVENTOR.

4 7' TOR/VEYS' Dec. 20, 1966 J. STONE 3,293,155

METHOD FOR DETERMINING THE CORROSION RESISTANCE OF ANODIZED ALUMINUM PARTS 2 Sheets-Sheet 2 Filed July 12, 1965 5 mwgk K h? m 0 d United States Patent 3,293,155 METHOD FOR DETERMINING THE COR- ROSION RESISTANCE OF AN ODIZED ALUMINUM PARTS Jack Stone, Detroit, Mich., assignor to Ford Motor Company, Dear-born, Mich, a corporation of Delaware Filed July 12, 1965, Ser. No. 471,283 19 Claims. (Cl. 204-1) This application is a continuation-in-part of my copending application Serial No. 67,110 filed November 3, 1960, now abandoned.

This invention relates to a method for evaluating metal oxide coatings upon a metal base or substrate. More specifically, this invention relates to a method for evaluating the corrosion resistance of such coatings electrochemically by contacting the coating opposite the metal base with an aqueous electrolyte, and evaluating the electrical resistance of such coating with respect to time over a predetermined period of time under impressed voltage between the electrolyte and the metal base.

In a preferred embodiment of this invention, the coating is placed within an electrical circuit wherein the metal base serves as a cathode, said circuit comprising said base, an anode, an aqueous electrolyte in contact with said coating and said anode, and a significant electrical resistance in series electrical connection with said coating exterior to said electrolyte, impressing an electrical potential upon said circuit causing a first voltage drop across the exterior resistance and a second voltage drop across the coating, and integrating the voltage across said coating with time for a predetermined period of time. In another embodiment, current through said coating is integrated with time for a predetermined period of time.

While this test method has general application for evaluating the corrosion resistance of metal oxide coatings upon a metal base, to avoid duplication of description this invention is hereinafter explained in detail with reference to anodized aluminum, a widely used industrial product comprising an aluminum base and an aluminum oxide coating upon and formed from such base.

Anodized aluminum is now widely used for automobile trim and in a wide variety of other metal products. The oxide coating may be polished to a mirror-like finish but also finds use with other finishes.

In the anodizing process, an aluminum surface is electrochemically converted to a hydrated form of aluminum oxide and conventionally this conversion is carried out by passing current through an electrolytic cell in which the aluminum object is installed as the anode with a sulfuric acid electrolyte and a lead cathode. The relatively porous aluminum oxide coating is conventionally sealed in boiling Water to decrease its porosity and to increase its corrosion resistance.

The corrosion resistance of the oxide coating on an anodized aluminum surface may vary widely with differences in coating thickness and coating continuity. The latter may be viewed as including both the uniformity of the original conversion and the quality of the post conversion seal. Minimum thickness may vary considerably from the average thickness. Such variance can result from impurities in the aluminum surface subjected to the anodizing process which resist the anodizing process and/or prevent conversion of the aluminum below at a rate commensurate with the conversion of other portions of the surface. Except for an almost complete absence of coating in a localized area, these variances in coating depth are not discernable by visual inspection prior to use exposure. Electrochemical examination of oxide coatings formed by anodizing aluminum, by way of measuring the electric current through the coating after a measured period of electrochemical attack, has been disclosed by Dr. Roy C. Spooner in an article entitled The Sealing of Sulfuric Acid Anodic Films on Aluminum, published in the Technical Proceedings of the 44th Annual Convention of the American Electroplaters Society.

It now has been discovered that one may rapidly and objectively evaluate the corrosion resistance of an anodized aluminum substrate by installing such substrate as the cathode of an electrolyte cell, applying a voltage across the coating for a predetermined and fixed period of time and integrating the voltage across said coating through said period of time, i.e. measuring the area beneath the resultant voltage-time curve. v

It is therefore one object of this invention to provide an accelerated corrosion test for metal oxide coated metal parts.

A further object is to provide a simple test which can be efliciently applied in controlling the production of anodized aluminum.

A further object is to provide a rapid and quantitative corrosion test which correlates closely with field exposure results.

Other objects and advantages of this invention will be made more apparent as this description proceeds, particularly when considered in connection with the accompanying drawings in which:

FIGURE 1 is a schematic sectional view of a portion of an electrolytic cell used in determining the corrosion resistance of anodized coatings on aluminum parts;

FIGURE 2 is a schematic diagram illustrating one embodiment of electrical circuitry that may be used in carrying out the test method of this invention with the electrolytic cell therein shown in section; and

FIGURE 3 is a graphic illustration of recorded voltages across anodized coatings in a fixed period of time in which two voltage-time curves are representative of a difference in the corrosion resistance of the anodized coatings on two aluminum sheets.

The oxide coating formed by the anodizing process has a high electrical resistance in comparison With its aluminum substrate which is an excellent conductor of electricity. By establishing an electrolytic cell on the surface of the anodized coating, one can measure the corrosion resistance of the coating, the corrosion resistance being inversely proportional to the electrical resistance exhibited with time by the coating under electrochemical attack. The test is accelerated by using an aqueous electrolyte solution that becomes slightly corrosive to the coating in the conduct of the test, e.g. sodium chloride, sodium chloride containing minor amounts of cupric chloride and acetic acid, acetic acid, etc.

In FIGURE 1 is seen an electrolytic cell 11 which comprises a glass tube 12 having a rubber grommet 13 attached to one end. The rubber grommet 13, which has an opening approximately one-eighth of an inch in diameter, is placed in firm contact with an anodized surface 14 on an aluminum base 15.

An electrode 16, which may be platinum due to its inertness to corrosive liquids, is placed in the glass tube 12. An electrolyte solution 17 is then introduced into the glass tube 12 so that the solution 17 will makecontact with the portion of the surface 14 defined by the opening in the rubber grommet 13.

An electrolyte solution 17, is prepared by dissolving 190.0 grams of sodium chloride and 1.0 gram of cupric chloride dihydrate in 4 liters of distilled water. The pH of this solution is then adjusted to between 2.8 and 3.0 by the addition of acetic acid.

As seen in FIGURE 2, the aluminum base 15 is connected to the negative terminal of a direct current electric power source 18 to become the cathode of the electrolytic -3 cell. The electrode 16 is connected to the positive terminal of the power source 18 to become the anode of the electrolytic cell 11. A resistance 19 and optionally a milliammeter 20 are placed in series with the electrode 16 and the power source 18.

The resistance 19 should be of sufiicient magnitude so that a significant and measurable change in the voltage drop across the coating 14 occurs with a corresponding change in current through the coating. In one preferred embodiment, a 53,000 ohm resistor has been used with a constant applied voltage from power source 18 of 46 volts. To avoid the use of unnecessarily sensitive and expensive instrumentation, the resistance 19 should not be below about 100 ohms and preferably not below about 10,000 ohms. With most commercial depth anodized aluminum coatings a potential of at least about volts across the coating is advisable. This voltage should not be in excess of that at which the electrochemical deterioration is effected in a reasonably gradual manner so that the electrical resistance of the coating is dissipated over a significant time period. In choosing the amplitude of resistance 19, consideration must be taken of the amplitude of the applied voltage and vice versa.

With coatings having a corrosion resistance significantly better than the required minimum for a given use, the applied voltage at power source 18 should be high enough to eifect an electrochemical breakdown of the electrical resistance of that portion of the coating in contact with the electrolyte within the predetermined test period taking into account the intervening resistance. Electrochemical breakdown of the coating under test permits relatively unrestricted fiow of electrical energy through the coating and is evidenced by a marked increase in current flow through the coating, observable on milliammeter 2t}, and a corresponding decrease in the electrical resistance of the coating.

The test is initiated by closing a self-releasing, starting switch 21 which starts a conventional timing device in timer 22 and via suitable conductors closes

contact switches

23 and 24 and causes the applied voltage of power source 18 to be impressed upon the circuit. The applied voltage results in a first voltage drop across resistance 19 and a second voltage drop across the coating 14. The resistance of the circuit conductors is insignificant. When the voltage is applied the initial current flow is dependent on the conductivity of the coating 14. As the current begins to flow, hydrogen is discharged at the cathode and the electrolyte becomes more alkaline, particularly adjacent to the cathode. As the test proceeds, the insulating properties of the coating deteriorate and the eifective electrical resistance of the coating decreases. The applied voltage is maintained at a constant level and when a decrease occurs in the electrical resistance of the coating 14 the difference of potential across the coating decreases and a proportionally greater voltage drop occurs across the resistance 19. The voltage across the coating 14, and hence the changes therein, is recorded throughout the test period by a voltage recorder 25, a conventional electronic measuring and tracing device, providing a voltage-time tracing such as shown in FIGURE 3. The use of recorder 25 is optional and is here illustrated and described to facilitate understanding of the function and purpose for integration unit 26 hereinafter discussed.

To obtain an objective evaluation of the corrosion resistance of the oxide coating 14, the voltage drop across the coating is integrated with time throughout the test period. In practice, an interval of three minutes has been found short enough to be acceptable in commercial production and long enough to give a meaningful test. Obviously, the time may be extended as an essentially constant current through the coating results after localized breakdown of the coating is'completed although no purpose is achieved by continuing test after a major proportion of the original resistance has been dissipated. Functionally, the acceptable minimum is a time sufficient to obtain from a coating of given depth an initial measure ment of electrical resistance and a meaningful measurement of its resistance to electrochemical attack. Preferably, this time is at least one minute. This evaluation may be done manually and/or by routine calculus measurement of the area below the voltage-time tracing on the recording chart. As a practical matter this measurement is made by conventional electronic integra tion instruments indicated in FIGURE 2 by an integration unit 26. This unit includes a conventional voltage to frequency converter and counter which measures the voltage-time integral of the input signal over the time interval selected. The voltage to frequency converter generates a continuous pulse train whose rate is proportional to the magnitude of the input voltage integrated between each output pulse. One such converter that has been satisfactorily employed in conducting such tests had a range of 0 to 100,000 cycles per second. The measurement provided by the counter is a function of volt-seconds integrated over the preselected time interval and is automatically ecorded. This reading or a selected fraction or multiple thereof provides an objective figure of merit for the coating under test. In a plurality of tests the figures of merit are proportional to the areas under the corresponding voltage-time tracings in recorder 25 When the selected time interval has expired, a converi= tional switching mechanism is actuated by timer 22 open= ing switches 23 and 24 and terminating the test.

By way of example, two anodized aluminum sheets were evaluated with a test unit schematically illustrated in FIGURE 2 using an applied voltage of 46 volts and a resistance at 19 of 53,000 ohms in series with the coatings being tested. The tracings obtained for each ofthe two tests are superimposed on a single graph and illustrated by lines A and B in FIGURE 3. The corresponding figures of merit obtained from the integration unit 26 for such tests were 1690 and 860 volt-seconds respectively. These figures are obtained by dividing the total number of pulses counted within the predetermined test period by the number of pulses the given voltage-frequency con verter provides per volt-second input. This divisor will depend upon the rating of the converter used. Thus,- if a converter has an output of pulses per volt-second input, the total number of pulses for the test period is divided by 100.

The test method of this invention is repeated by moving the electrolyte cell to several locations on the surface of the anodized aluminum part. The figures of merit for all tests are averaged it relatively consistent and compared with a predetermined standard expressed in the same figure of merit units. Such standard is obtained by exposing tested articles to use exposure tests and correlating test values with the results of the exposure tests.

The invention as hereinbefore discussed has contemplated a resistance 19 of fixed amplitude. In a second embodiment resistance 19 is increased either manually or automatically by conventional electrical control means so as to maintain the current registered at milliamrn'etef 20 at an essentially even level. With the coating 14 undergoing electrochemical deterioration, there is a tendency for an increase in current in the circuit with the applied voltage remaining constant. With an increase in the value of re sistance 19 sufiicient to balance the resistance loss by coating 14, the current flow is stabilized and the voltage drop across coating 14 decreases even more than when a fixed resistance 19 is employed. The optional recording and the integration of voltage across the coating with time are carried out in the same manner as before.

Anodized coatings may be evaluated by maintaining a constant voltage and integrating the current through the coating with time. This test eliminates the need for resistance 19 of FIGURE 2. This method although suggested by the voltage integration test aforedescribed is not equivalent to such test. In the voltage integration method heretofore described, the voltage across the coating de-= cfeases pfoportionally with a decrease in the electrical resistance of the coating as the coating deteriorates thereby providing a more restrained electrochemical attack upon the coating and causing a more controlled deterioration of the coating. In the current integration method, the voltage across the coating remains essentially constant with increasing current since there is no other significant resistance in the circuit and the deteriorating coating is subjected to relatively more severe conditions in the later stages of the test, or automatic means must be incorporated in the circuitry to reduce the applied voltage at predetermined current levels and allowance made for such changes in the recording and integration units to automatically compensate for the resultant distortion of the current integral with time. If operated without change in the applied voltage, this test may be carried out with the circuitry illustrated in FIGURE 2 modified by replacing integration unit 26 with a unit providing a figure of merit equal or proportional to the ampere-seconds associated with the current through the coating during the test period. The optional voltage recorder 25 may be replaced with a current recorder.

I claim:

1. The process for evaluating a metal oxide coating upon a metal base which comprises placing said coating within an electrical circuit including integration means wherein said metal base serves as a cathode of an electrolytic cell, said circuit comprising said base, an anode, an aqueous electrolyte in contact with said coating and said anode, impressing a constant difference of electrical potential upon said circuit which provides a resultant difference of electrical potential across said coating and measuring the electrical resistance of said coating with respect to time by integrating with said integration means a function of said electrical resistance selected from the group consisting of electric current through said coating and difference of electrical potential across said coating over a predetermined period of time.

2. The process for evaluating a metal oxide coating upon a metal base which comprises placing said coating within an electrical circuit including integration means wherein said metal base serves as a cathode of an electrolytic cell, said circuit comprising said base, an anode, an aqueous electrolyte in contact with said coating and said anode, impressing a constant difference of electrical potential upon said circuit which provides a resultant difference of electrical potential across said coating and evaluating the corrosion resistance of said coating by integrating with respect to time, with said integration means, for a predetermined period of time a function of the electrical resistance of said coating that changes with time during said predetermined period of time selected from the group consisting of electric current through said coating and difference of electrical potential across said coating.

3. The process of claim 2 wherein said function that changes with respect to time is the difference of electrical potential across said coating.

4. The process of claim 2 wherein said function that changes with respect to time is the electric current through said coating and between said anode and said base.

5. The process for evaluating a metal oxide coating upon a metal base which comprises placing said coating within an electrical circuit wherein said metal base serves as a cathode of an electrolytic cell, said circuit comprising integration means, said base, an anode, an aqueous electrolyte in contact with said coating and said anode, said base being in electrical connection with a first conductor terminal, said anode being in electrical connection with a second conductor terminal, said terminals being in electrical connection With an electrical energy generation source adapted to apply a constant voltage across said terminals, applying a constant voltage across said terminals resulting in application of at least a portion of said voltage across said coating and evaluating the corrosion resistance of said coating by integrating with respect to time. with said 6 integration means, for a predetermined period of time a function of the electrical resistance of said coating that changes with time during said predetermined period of time selected from the group consisting of electric current through said coating and difference of electrical potential across said coating.

6. The process of claim 5 wherein said circuit includes a resistance in excess of about ohms in series with the electrical resistance of said coating and the function of said electrical resistance that changes with time is the difference of electrical potential across said coating.

7. The process of claim 6 wherein said resistance is of fixed amplitude.

8. The process of claim 6 wherein said resistance is a variable resistance adjustable to maintain the resultant electric current through said circuit at an essentially constant level.

9. The process of claim 5 wherein said circuit includes a resistance in excess of about 10,000 ohms in series with the electrical resistance-between said anode and said base.

10. The process of claim 5 wherein said period of time is at least one minute.

11. The process for evaluating a metal oxide coating upon a metal base which comprises placing said coating Within an electrical circuit wherein said metal base serves as a cathode of an electrolytic cell, said circuit comprising integration means, said base, an anode, an aqueous electrolyte in contact with said coating and said anode, and a significant electrical resistance in series with the electrical resistance of said coating exterior to said electrolyte and evaluating the corrosion resistance of said coating by applying a constant difference of electrical potential to said circuit providing a first difference of electrical potential across said significant resistance and a second difference of electrical potential across said coating, and integrating the difference of electrical potential across said coating with time for a predetermined period of time with said integration means.

12. The process for evaluating an oxide coating upon and formed from an aluminum base which comprises placing said coating within an electrical circuit wherein said aluminum base serves as a cathode of an electrolytic cell, said circuit comprising integration means, said aluminum base, an anode, an aqueous electrolyte in contact with said coating and said anode, and a resistor providing a resistance in excess of about 100 ohms exterior to said electrolyte and in series with the electrical resistance of said coating, evaluating the corrosion resistance of said coating by applying a constant difference of electrical potential across said circuit thereby providing a first difference of electrical potential across said resistor and a second difference of electrical potential across said coating which decreases with a decrease in the electrical resistance of said coating with resultant increase in electric current through said circuit, and integrating the difference of electrical potential across said coating with time for a predetermined period of time with said integration means.

13. The process of claim 12 wherein said predetermined time is at least one minute.

14. The process of claim 12 wherein said resistor provides a resistance in excess of about 10,000 ohms.

15. The process of claim 12 wherein said coating is a coating produced electrochemically in the presence of sulfuric acid.

16. The process of claim 12 wherein said integration is effected by converting said second difference of electrical potential to difference of electrical potential pulses, the frequency of which is a function of the difference of electrical potential across said coating with time, and impressing said pulses upon a counter.

17. The process for evaluating a metal oxide coating upon a metal base which comprises placing said coating Within an electrical circuit including integration means wherein said metal base serves as a cathode of an electrolytic cell, said circuit comprising said base, an anode, and

7 in aqueous electrolyte in contact with said coating and ;aid anode, and evaluating the corrosion resistance of said :oating by impressing a constant difference of electrical aotential upon said circuit which provides a resultant dif- Eerence of electrical potential across said coating and integrating the electric current through said coating with time for a predetermined period of time with said integration means.

18. The process for evaluating an oxide coating upon and formed from an aluminum base which comprises placing said coating Within an electrical circuit including integration means wherein said metal base serves as a cathode of an electrolytic cell, said circuit comprising said base, an anode, and an aqueous electrolyte in contact with said coating and said anode, and evaluating the corrosion resistance of said coating by impressing a constant difference of electrical potential upon said circuit which provides a resultant diiference of electrical potential across said coating and integrating the electric current through said coating with time for a predetermined period of time with said integration means.

19. The process of claim 18 wherein said period of time is in excess of one minute.

References Cited by the Examiner UNITED STATES PATENTS 2,786,021 3/1957 Marsh 204l 2,894,882 7/1959 Strodtz' 204-195 2,960,455 11/1960 Frankenthal 204195 OTHER REFERENCES Brennert, Joun, Iron & Steel Institute, volume (1937), pages l0lp-1l1p.

Campbell et al., Trans. of the Electrochemical Soc., volume 76 (1939), pages 303328.

May, Jour. of the Institute of Metals, No. 2 (1928), volume XL, pages 147-175.

Miley, Carnegie Schlorship Memoirs, Iron and Steel Institute, volume 25, pages 201-208 (1936).

Price et al., Trans. of the Electrochemical 800., volume 76 (1939), pages 329-340.

Spooner, Technical Proc. of the 44th Annual Convention of the Am. Electroplaters Soc., June 20, 1957, pages 132-142.

JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.