An Eectrochemical Investigation on Additives to the Controllable Electro-Healing Cracks in Nickel

Healing or repairing cracks in materials is highly desired not only from economic considerations but also for their sustainable and reliable applications. For metallic materials, crack healing is challenging due to the limited mobility of metallic atoms in solid state at ambient temperature. Recently, it is reported¹ that an electrochemical process named electro-healing is capable of repairing through-thickness cracks in nickel where metallic ions in electrolytes were introduced as the healing agent. However, in plain healing solutions when there is no additives, the healing crystals grow following a general mode, in which the healing crystals grow oppositely from the two sidewalls of the crack towards the meeting line (plane), where randomly-located micrometer-sized voids or smaller are sometimes observed, see Fig.1 (a). As these voids are detrimental to the load-bearing ability of the healed crack, it is essential to control the growth of the healing crystals to minimize the number of voids that left after electro-healing². 

It is well-established that the addition of inhibitors or accelerators to an electrolyte is capable of suppressing or accelerating the metal deposition rate. In the present work, two inhibitors: polyethyleneimine (PEI) and 2-mercapto-5-benzimidazolesulfonic acid (MBIS), and an accelerator thiourea (TU) were added respectively to the conventional Watts bath to investigate the controllable electro-healing on pure Ni plate with a through thickness crack. Rotating disk electrode (RDE) was introduced to evaluate the electrochemical behavior in crack tips and crack center. Voltammograms and galvanostatic measurements were carried out to evaluate the inhibition effect of PEI, MBIS, synergistic effect of MBIS+TU to the reduction of Ni ions. The cathodic current efficiency was also investigated. SEM observations were used to characterize the morphology of the healed cracks. 

It is demonstrated that the addition of PEI greatly inhibited the reduction of Ni ions. This inhibition is convection dependent. As a result, the growth of healing crystals from the sidewalls was much restricted compared to  that from the crack tips((ν_s<<ν_t) when strong convection was employed to the solution. No meeting line was observed in the healed crack, instead, annual-rings and some large visible defects were found in the healed crack(Fig.1(b)). The former is believed to be the results of fluctuation of PEI diffusion flux, while the latter is attributed to the hydrogen inclusion corresponding the much reduced current efficiency.

 MBIS exhibited a similar inhibit effect to Ni reduction to PEI when added to the Watts bath. The surface adsorption kinetics of MBIS is also controlled by transportation. The difference is that the solution with MBIS demonstrated much higher current efficiencies compared with PEI when similar amount of inhibitor were added(100µM). For example, at a current density of 1 A/dm², the current density of solution with PEI is about 70 %, while that with MBIS is about  97 % when the rotation speed of RDE is 50 rpm. Consequently, except annual rings, no visible defects were spotted in the acquired morphology (Fig.1(c)).   

From the linear sweep voltammetry (LSV), it is observed that as an accelerator, the addition of TU to the MBIS containing solution demonstrated a depolarization effect to the Ni reduction,the growth of healing crystals from sidewalls was much enhanced when TU was presented. A meeting line appeared again(Fig.1(d)). But different from that of plain solution without additives, there is no voids or pores left along the meeting line after electro-healing. Instead of that perpendicular to the meeting line, the healing crystals grew at an acute angle to the meeting line, which is believed to be the consequence of simultaneous growth from crack sidewalls and the crack tip. It is expected that a satisfactory recovery of load bearing ability will be accomplished after the controllable electro-healing with the co-addition of both MBIS and TU. 

Fig. 1 SEM morphologies of cracks after electro-healing  in different solutions: (a) Plain Watts bath without additives(general growth mode); (b) 35µM PEI, (c) 50µM MBIS and (d) combination of 50µM MIBS and 20µM TU. Black arrows indicate the growth direction of the healing crystals, white arrows in (a) indicate pores and voids. White dot-dashed  line in (a) and (d) designates the meeting line. Watts bath contains: 1M NiSO₄•6H₂O, 0.2 M NiCl₂•6H₂O and 0.5 M H₃BO₃.

Ref.

1. X.G. Zheng, Y.N. Shi and K. Lu, Materials Science & Engineering A, 561, 52 (2013).
2. X.G. Zheng, Y.N. Shi and K. Lu, J. Electrochem. Soc.,160, D289(2013)