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How to evaluate the corrosion-resistance of electroplating layer on the metal?

Source：Corrtest Time：2021-12-02 View：0

1. Layer bonding strength testing

   Put the test sample into the 90℃ 5% NaOH solution(1.054g/mL) for current(10/dm2) treatment for 2min, there will be some bubbles in the places of poor bonding strength. If after 15min treatment, there is still no bubbles on the surface of the layer, it’s deemed to be good in bonding strength. This can also be conducted in 60℃ 5%(mass percentage)H2SO4 solution under 10A/dm2 current treatment. The bubble will appear at places of poor bonding strength. 
   This test is only applicable to layer where the hydrogen at cathode can diffuse. For instance, this test is workable for Ni or Ni-Cr layer but not workable for Pb, Zn, Sn, Cu coating.
   The precipitated hydrogen will diffuse through the coating, and accumulates at any discrete portion between the layer and the base metal. The generated pressure will make bubbles appear.

2. Potentiodynamic scanning

a. cathodic polarization curve
   Measuring the cathodic polarization curve, i.e., from scanning from stable potential to the negative. It can be used to study the influence of various additives in electroplating liquid on cathodic polarization to choose the best additive.(normally to choose the additive which is most significantly enhance the cathodic polarization.) It can also used to choose the decide the most suitable content of a specific additive by the cathodic polarization curves reflecting the influence of different contents of a single additive. In addition, it can also be used to evaluate the performance of the electroplating liquid by measuring the cathodic polarization curve of different electroplating liquids.
   During the measurements, please pay attention to the potential scanning range. The ideal range is from stable potential to the correspondent potential value vs. maximum current density that the normal technique can give. It is meaningful to study the cathodic polarization curve within the current range.

b. anodic polarization curve

   There is mainly two applications to measure the anodic polarization curve:
   Scanning from stable potential to the positive is to study the dissolution behavior of the anode(anodic material as WE) . According to anodic polarization curve, the user can analyze if the passivation appears at the anode during the electroplating process. If it does appear, then it can be chosen as an activator.
  Another application is to study the anodic behavior of the electroplating(or conversion film). Normally the user can choose 3％ NaCl solution as the electrolyte and the plating layer as the WE to do the anodic polarization and to study the corrosion resistance of the plating layer by analyzing the curve. Under the same anodic potential, the lower the anodic current is, the better the corrosion resistance is. Therefore, we can compare the corrosion resistance of different metal plating layers(or conversion films) or plating layers (of a same metal) generated by different techniques. We can also evaluate the pitting corrosion resistance of the passive electroplating layer by analyzing the flyback curve. That is, through measuring the pitting corrosion potential and the protection potential, and the Scy to judge the pitting corrosion tendency.

 3. Tafel plot

  By measuring Tafel plot the researcher can study the corrosion resistance of the plating layer (or conversion coatings). If the measured corrosion current density is low, or corrosion resistance is big, then it means the layer (or conversion film) is of good corrosion resistance. The user can compare the corrosion resistance of different layers (or conversion films) generated by different techniques.



 


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