ASTM D6208 Standard Test Method for Repassivation Potential of Aluminum and Its Alloys by Galvanostatic Measurement

Get Certified ASTM D6208 Testing for Reliable Repassivation Potential Evaluation 

The electrochemical behavior of aluminum when subjected to controlled galvanostatic polarization is determined by ASTM D6208. The technicians place an aluminum coupon into a coolant and corrosive-water solution with a constant current density. Technicians record the potential vs. time response and find out the range of max. and min. potential needed for the method. To obtain reliable and reproducible ASTM D6208 results, it is important that the specimen is prepared appropriately, the current is controlled, the position of the electrodes is correct, and the potential is recorded accurately.

This standard includes:

• Material type: Aluminum alloy 3003-H14 (UNS A93003) as the standard material, with guidance for similar testing of other aluminum alloys and metals. 
• Purpose: Determine repassivation behavior and evaluate relative susceptibility to pitting corrosion. 
• Property measured: Break potential, E_B, and protection or repassivation potential, E-G. 
• Method: Galvanostatic polarization for 20 min at constant current density in corrosive water solution containing a coolant and chloride. 
• Result: Maximum potential during initial polarization, minimum potential after reaching that maximum, and potential-time behavior. 
• Test Limitations: The measurements are not intended to provide a quantitative correlation with other test methods or with localized corrosion susceptibility in service. The method is mainly used to assess the relative behavior in the given laboratory conditions. 
• Environmental conditions: The controlled test solution is composed of coolant and corrosive water with chloride, sulfate, and bicarbonate, and the electrochemical configuration is maintained. 
• Applications: Pitting-corrosion evaluation, inhibitor effectiveness studies, aluminum-alloy comparison, corrosion research, and electrochemical testing.

What are the uses of ASTM D6208 testing? 

ASTM D6208 testing allows laboratories to understand the behavior of aluminum alloys under conditions that can promote pitting corrosion. The method is especially useful for the study of corrosion inhibitors, as the protection potential is responsive to inhibitors’ performance. This standard-

• Determines the break and protection potentials of aluminum. 
• Evaluates relative susceptibility to pitting corrosion. 
• Helps assess the effectiveness of corrosion inhibitors.
• Facilitates comparison of corrosion behavior of aluminum alloys. 
• Provides potential-versus-time data for electrochemical analysis. 
• Supports coolant and corrosion research. 
• Assists in visual examination of specimens to detect pitting and repassivation behavior.

Which Materials Can Be Tested Under ASTM D6208? 

ASTM D6208 refers to the AA3003-H14 (UNS A93003) standard aluminum alloy for precision and bias work. The standard also guides similar measurements on other aluminum alloys and metals. The working electrode is an aluminum coupon of 2 to 6 mm thick cut from sheet material.

Why is ASTM D6208 important? 

ASTM D6208 is a controlled electrochemical method for determining the susceptibility of the passive layer to breakdown and the ability of the aluminum surface to repassivate after localized corrosion. The maximum potential, E_B, refers to resistance to oxide-film breakdown, whereas the minimum potential after the maximum, E-G, gives information about protection against continued pitting.

ASTM D6208 Testing Process and Data Collection 

Technicians prepare the aluminum working electrode and the coolant/corrosive-water test solution. Analysts assemble the three-electrode cell and position the reference and auxiliary electrodes correctly. Skilled personnel apply a constant current of 872 μA, equivalent to approximately 100 μA/cm², for 20 min. The system preserves the potential as a function of time. From the potential-time curve, the maximum potential E_B and the next minimum potential E_G are identified. Technicians can then test the aluminum surface for pits, deposits, and discoloration as a supporting check for interpretation.

Common Challenges and Troubleshooting 

The correct specimen preparation, positioning of electrodes, current control and potential recording are essential for obtaining accurate results. The technicians should clean the aluminum coupon in a consistent way without any unnecessary surface preparation that could have an impact on the passive film. The analyst should defoam the Luggin and assure the correct spacing of electrodes and correct current output prior to the test. Unusual potential-time curves should also be examined by skilled people and the pitting behavior should be distinguished from general corrosion or from unstable electrochemical conditions.

• The laboratory reports E-B and E-G in V versus the standard hydrogen electrode (SHE) and records the applied current density in A/m².
• Technicians document the highest voltage achieved during initial polarization, the lowest voltage after reaching the peak, polarization time, and test conditions.
• Analysts record the specimen identification, alloy condition, electrode configuration, potential-time curve type, pit number, pit depth, deposits, or surface discoloration. 
• Engineers use the potential and corrosion observed to compare against applicable inhibitor and material specifications to assess pitting initiation and protection.

Link to ASTM D6208