Cathodic delamination accelerated life test method

Abstract

A method for conducting an accelerated life test of a polymer coated metallic sample includes placing the sample below the water surface in a test tank containing water and an oxygen containing gas. Cathodic polarization of the metallic portion of the sample is increased. This can be by using a voltage source or a sacrificial anode. Dissolved oxygen in the test tank water is also increased. Dissolved oxygen can be increased by providing oxygen under pressure to the tank or through an aerator under the water surface. Temperature can also be regulated to accelerate the test speed. Delamination of the sample is periodically tested through a peel test or by other means. The invention also provides a reaction model independent method for calculating activation energy.

1 . A method for conducting an accelerated life test of a polymer coated metallic sample comprising the steps of:
 providing a test tank;   providing water and gas in said test tank where said test tank has a layer of gas over said water;   providing the polymer coated metallic sample to be tested under said water in said test tank;   increasing cathodic polarization of said metallic portion of said sample;   increasing the amount of oxygen dissolved in said water in said test tank; and   measuring the delamination of said coating from said sample with respect to time.   
   
   
       2 . The method of  claim 1  wherein said step of increasing cathodic polarization comprises:
 attaching an electrode to said metallic portion of said sample; and   applying a cathodic voltage to said electrode.   
   
   
       3 . The method of  claim 2  wherein said cathodic voltage is less than 0.6 volts. 
   
   
       4 . The method of  claim 2  further comprising monitoring an electrical current through said attached electrode. 
   
   
       5 . The method of  claim 1  wherein said step of increasing the amount of oxygen dissolved in said water comprises increasing a portion of oxygen in said gas. 
   
   
       6 . The method of  claim 5  wherein said step of increasing the amount of oxygen dissolved in said water further comprises providing said gas in said test tank under pressure. 
   
   
       7 . The method of  claim 6  wherein said step of providing said gas comprises providing oxygen through an aerator under the surface of said water. 
   
   
       8 . The method of  claim 1  further comprising raising the temperature in said test tank. 
   
   
       9 . The method of  claim 8  wherein said step of raising the temperature in said test tank comprises providing a temperature control device in communication with said water in said test tank. 
   
   
       10 . A method for conducting a peel test of a polymer coated metallic sample comprising the steps of:
 making a peel test specimen having a polymer joined to a substrate wherein an attached portion of said polymer is joined to said substrate and a free portion of said polymer is not attached to said substrate;   supporting said peel test specimen in an environment for testing in such a manner that said polymer is allowed to peel downward from said substrate;   joining a weight to said free portion of said polymer;   recording a start time when said weight is joined to said polymer;   periodically measuring said attached portion of said polymer and said elapsed time from said recorded start time; and   calculating a peel rate from said attached portion measurement and said elapsed time.   
   
   
       11 . The method of  claim 10  wherein said peel test specimen is an ASTM peel test specimen. 
   
   
       12 . The method of  claim 10  wherein said peel test specimen is supported by a frame. 
   
   
       13 . The method of  claim 10  wherein said substrate is metal. 
   
   
       14 . The method of  claim 13  further comprising the step of cathodically polarizing said substrate. 
   
   
       15 . The method of  claim 14  further comprising increasing the oxygen content of said environment for testing. 
   
   
       16 . The method of  claim 15  further comprising raising the temperature of said environment for testing. 
   
   
       17 . A method for computing the activation energy for cathodic delamination comprising:
 conducting a first peel rate test at first temperature to obtain a first peel test time;   conducting a second peel rate test at a second temperature to obtain a second peel test time;   computing a rate acceleration factor from the ratio of said first peel test time to said second peel test time; and   calculating an activation energy from said rate acceleration factor, said first temperature and said second temperature.   
   
   
       18 . The method of  claim 17  wherein said step of conducting a peel rate test at a given temperature comprises:
 making a peel test specimen having a polymer joined to a substrate wherein an attached portion of said polymer is joined to said substrate and a free portion of said polymer is not attached to said substrate;   supporting said peel test specimen in an environment for testing in such a manner that said polymer is allowed to peel downward from said substrate;   joining a weight to said free portion of said polymer;   recording a start time when said weight is joined to said polymer;   periodically measuring said attached portion of said polymer and said elapsed time from said recorded start time; and   calculating a peel rate from said attached portion measurement and said elapsed time.   
   
   
       19 . The method of  claim 17  wherein the step of computing an activation energy utilizes the gas constant and the Arrhenius equation.