Evaluating the Performance of 2-Mercapto-5-Benzimidazolesulfonic Acid in Controllable Electro-Healing Cracks in Nickel

Healing or repairing cracks in materials is of significant importance for sustainable development of the world. As the mobility of metal atoms is quite limited at service temperature, it remains a challenge to realize crack healing in metallic materials. Recently, an electrochemical process named electro-healing was introduced to heal cracks in nickel.  Meanwhile，it is also recognized that to get a satisfactory recovery of strength, it is critical to minimize pores or voids in the healed crack by controlling the growth mode of healing crystals, which might be realized by the addition of additives to the healing solution. In our recent work, we found that controllable electro-healing cracks was achieved by the addition of polyethyleneimine (PEI) and saccharin to the Watts bath under free convection. Nevertheless, some defects were found along the interface between the substrate and the healing crystals when PEI and saccharin were added to the solution, which might be attributed to the lowered current efficiency. As with the presence of inhibitors, hydrogen evolution, the secondary reaction on the cathode, becomes less ignorable, the definite result of which is the reduction of current efficiency. Hence, additives with relatively higher cathode current efficiency has to be introduced to the controllable electro-healing of cracks. 

2-mercapto-5-benzimidazolesulfonic acid (MBIS) has been found to an effective inhibitor in the reduction of nickel. In the present work, different concentration of MBIS was added to conventional Watts bath to systematically evaluate the performance of MBIS in the controllable electro-healing on thin pure Ni plate with a through thickness crack. Voltammograms under different concentration and convection condition were carried out to evaluate the inhibition ability of MBIS to the reduction of nickel. Rotating disk electrode (RDE) with rotation speeds of 50 rpm, 300 rpm and 2000 rpm was employed to assess the convection dependent performance of MBIS. The cathodic current efficiency and surface coverage were examined. These characterizations together with the SEM observation of the healed crack were analyzed to get the correlation between the addition of MBIS and the electro-healing process of crack. 

It is demonstrated that with the addition of MBIS, a hysteresis loop forms in the positive-scan and negative-scan voltammetry. With the increment of MBIS concentration, the loop becomes larger, the onset of the inhibition breakdown moves to a more negative value, illustrating that the inhibition of MBIS is concentration dependent. This is in accordance with the reported result. The inhibition is also found to be convection dependent, with a similar behavior to concentration when the rotation speed of RDE increases. One interesting phenomenon in the voltammetry of MBIS-added solution is the transition between the breakdown of inhibition (active) and the rebuilt of inhibition (passive). The phenomenon only occurs when the MBIS concentration or the rotation speed of RDE is higher than a certain value. The phenomenon has not been observed with the presence PEI at a given concentration. If a minimal flux of inhibitor is required to maintain the inhibition blocking layer in the passive state, this transition from passive to active may imply a fast mass transfer of MBIS at higher concentration or higher intensity of convection. Additionally, it is found that at a given concentration the current efficiency of MBIS is much higher than PEI but with a similar surface coverage. 

The morphology of the healed crack reveals that the healing crystals grows in a controllable manner. The reduction of Ni on the sidewalls has been consistently inhibited, while the growth of healing crystals has been much less restricted from the crack tips. No defects were found along the interface between the substrate and the healing crystals which is believed to be the result of the much higher current efficiency. Although, annul-ring like defects still exist, the average distance between the annual-ring of MBIS is much larger that that of PEI, indicating a longer active growth period of healing crystals from crack tips. Further in-depth analysis is still underway to understand the phenomenon.