Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance

Definitions

• Electrically insulating layers are often used as corrosion protection layers.
• the corrosion process of the metal is essentially not caused by a chemical change in the lacquer layer in the liquid corrosive medium, but by pores in the protective layer or defects during the coating process.
• the corrosion process of the metal is essentially not caused by a chemical change in the lacquer layer in the liquid corrosive medium, but by pores in the protective layer or defects during the coating process.
• the smallest pores during a relatively short storage period can mean that the contents of the can become unusable.
• the so-called double-layer capacity is determined in various methods and devices.
• the measurement of the double layer capacity only allows the detection of comparatively large coating defects (see, for example, DE-OS 33 39 151), but not of pores in the coating.
• studies have been carried out to measure corrosion on food and beverage cans, the basis of which is the measurement of the short-circuit current density in conjunction with potential measurements (O. Maerks, HK Ziegler, can corrosion and its measurement, Neuemaschine 7, 936, 1974). Further investigations are based on the measurement of the rest potential changes of tin cans as a function of the storage time (O. Maerks, measurement of the anodic tin dissolution at defects in the coating of tinplate, Packing Review 25, No.
• the invention has for its object to provide a method and an apparatus for testing electrically insulating layers on metal parts, by means of which the quality of the coating can be assessed.
• the method according to the invention and the device according to the invention are intended to enable rapid working, so that a quality check of the manufactured parts is possible even during a coating process.
• the voltage response to a current pulse of short duration or the current response to a voltage pulse of short duration is significantly influenced by coating errors, such as pores, etc.:
• coating errors such as pores, etc.:
• the coating contains defects, for example pores, holes, cracks, etc., an additional current flows through the defects, which significantly reduces the voltage build-up per unit of time in the "initial phase of the capacitive behavior", as was recognized according to the invention.
• a constant current or constant voltage pulse of short duration is therefore applied for quantitative testing of the protective layer, and the change over time in the voltage applied to the electrolytic cell or the current flowing through the cell is detected during and shortly after the duration of the pulse.
• the present invention carried out analysis of the potential-time behavior of the standing with an electrolyte in contact be iste ⁇ th metal surface according to a galvanostatic current or potentiostatic voltage pulse allows a quantita ⁇ tive evaluation of the quality of the protective layer, that is, de * r layer thickness and the density and Size of the errors in time periods which are at most of the order of seconds, but generally less than one second.
• the method according to the invention thus also allows a coating process to be controlled, for example in such a way that deficiently coated parts can be sorted out and re-coated.
• the inventive method and the device have the unexpected advantage that almost any electrolyte can be used; in particular, when testing coke cans, for example, it is possible to work with cola as an electrolyte; the influence of the filled beverage on the coating during the storage period can thus be examined.
• FIG. 1 shows a block diagram of a first exemplary embodiment of the invention
• FIG. 2 shows a block diagram of a second exemplary embodiment of the invention
• FIG. 1 and 2 show two embodiments of the invention, which differ in that in the embodiment shown in FIG. 1, constant current pulses are used, while in the circuit shown in FIG. 2 constant voltage pulses are applied.
• a coated metal part M to be tested is located in a trough 2 filled with an electrolyte 1, in which is also arranged a counter electrode G.
• a pulse generator 12 connected to the constant current generator 11 is provided, which can be triggered either manually (component 13) or automatically (component 14).
• Meta M applied voltage U is amplified by a preamplifier 15 °.
• the time course of the voltage is recorded and the maximum value of the voltage present during a pulse is stored with a peak value memory 16, which is connected to the pulse generator 12 for this purpose, and is displayed by means of a millivoltmeter 17.
• the circuit shown in FIG. 2 has, instead of the constant current transmitter, a constant voltage transmitter 11b and a reference electrode BE, which can be, for example, a Haber ' Luggin capillary.
• a constant voltage transmitter 11b By means of the counterelectrode G, the reference electrode BE and the metal part to be tested, a three-point circuit is implemented with which line influences etc. can be largely eliminated.
• the time behavior of the voltage drop across a resistor R is detected again and the peak value occurring during a pulse is stored with the peak value memory 16 and displayed with the millivolt meter 17.
• the remaining components correspond to those shown in Fig. 1, so that a description can be omitted.
• the time behavior of the variables to be measured is converted into a digital value by means of a fast analog / digital converter 18 and applied to a computer 19 which carries out the quantitative check of the change in the measured variable over time.
• the electrolyte can simply be filled into the beverage cans to be tested.
• the outer wall of the metal can 31 can be grounded, for example, by placing it on an electrically conductive base 32.
• the counter electrode 'G is introduced into the beverage cans and the test is carried out using one of the circuits shown in FIGS. 1 to 4.
• the method according to the invention has the particular advantage that the test can be carried out with the beverage to be filled in later as an electrolyte.
• Cola in particular is readily suitable as an electrolyte, so that the method according to the invention not only allows the testing of the coating process during the manufacture of the beverage cans, but also the creep behavior of filled beverage cans under real conditions.
• FIG. 6a and 6b show a possibility of using the method according to the invention or the device according to the invention for testing coated surfaces.
• An electrolyte container 2 is used, the Cross-sectional area is significantly smaller than that of the sheet to be tested.
• the electrolyte container 2 is placed on the sheet and sealed with a seal 33 so that no electrolyte can escape even when the sheet is placed vertically.
• the electrolyte container 2 is then, for example, continuously moved over the testing sheet.
• suction cups are provided with which the electrolyte container 2 is fastened to the area of the coated sheet to be tested.
• Fig. 7 shows a further embodiment for ⁇ examination of coated wires.
• the coated wire is drawn through an electrolyte container 2, in which the counter electrode G and an electrolyte 1 are located in a manner known per se.
• curves a and b show approximately capacitive behavior of the cover layer and the potential building up reaches a relatively high final value
• curves c and d show a clear flattening of the potential increase between 1, 5 and 2 volts, which indicates the beginning of metal dissolution and the development of oxygen in small pores of the cover layer.
• the final value of the potential for measurements c and d is approx. 30% below the final value for measurements a and b.
• the potential-time curve of curve e has a rest potential that differs from curves a to d, hardly any capacitive (linear) behavior and a final potential value that reaches less than 10% of the final values of measurements a and b.
• the curves a and b can be assigned to a good paint finish, the curves c and d to a coating which is just tolerable.
• the curve of the potential of curve e can be assigned to a defective paint finish that does not provide the metal with adequate protection against corrosion during a limited storage period.
• Fig. 9 again shows the curve a of a "good beschich ⁇ tet" to be designated 'beverage can.
• Feehhlleerrffflä vom to be recognized, which amount to about 10 -6 of the total area.
• the size d (dU / dt) / dU (with current pulses) plotted against the voltage U over several volts is a linear function for perfect coatings. The deviations of this size from the linear course represent a measure of the defects in the coating and allow even the smallest defects to be identified. Coating defects can be detected, the size of which is up to 10 "of the total area.

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Citations (5)

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• 1985
  • 1985-03-29 DE DE19853511706 patent/DE3511706A1/de active Granted
• 1986
  • 1986-03-29 JP JP50209686A patent/JPS62502986A/ja active Pending
  • 1986-03-29 EP EP19860902307 patent/EP0217881A1/de not_active Withdrawn
  • 1986-03-29 WO PCT/DE1986/000134 patent/WO1986005881A1/de not_active Application Discontinuation

Patent Citations (5)

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