Scratch repassivation

The critical pitting potential cpr lies between the breakdown potential and the protection potential and can be determined by the scratch repassivation method. In the scratch repassivation method for localized corrosion, the alloy surface is scratched and exposed to a constant potential. The current change is monitored as a function of time and this will show the influence of potential on the induction time and the repassivation time. A careful choice of the level of potential between the breakdown potential and the critical pitting potential can give the critical pitting potential for a chosen material in given conditions.42 (Scully)14... 

The scratch repassivation technique discussed above may be more useful for studying SCC than pitting [107]. This is because natural pitting is often initiated at defect sites in the passive film that are electronic or chemical in nature. Such sites are not necessarily probed by a mech inical scratch. Scratching can simulate SCC where mechanical film rupture processes are operative and anodic dissolution controls the SCC process. In general, once the passive film is mechanically disturbed at some potential, an electrochemical current can be measured that will decay back to a low level when repassivation has occurred. A measure of the crack growth rate, V, (cm/s) can be obtained from these data... 

Mechanical scratching is favored by some as a means by which a bare surface can be created. In this technique, one is ascribing no importance to (or taking any engineering credit for) initiation time. The surface is held at a constant potential and then a portion of it is scratched, usually with a diamond-tipped point. The current is monitored with time. For potentials below En the surface will repassivate rather rapidly, as shown in Fig. 46. Just above E,p, the surface... 

Figure 46 Schematic of applied current density vs. time observed for mechanical scratching of a surface exposed to a solution above and below its repassivation potential.

From an engineering perspective, the repassivation potential is a more important parameter than the potential for pit nucleation. We want to know the potential below which pits will not grow. This is analogous in theory to measuring KiC or Kiscc in mechanical and SCC testing. One way to test this is to produce a completely bare surface that is dissolving rapidly, and determine at what potential it can repassivate. An easy way to do this is what may be termed an electrochemical scratch. ... 

These tests focused on the determination of a materials resistance to localized (pitting) corrosion. To accomplish this goal, three types of electrochemical experiments were conducted (cyclic polarization, electrochemical scratch, and potenti-ostatic holds) to measure several key parameters associated with pitting corrosion. These parameters were the breakdown potential, EM, the repassivation potential, Etp, and the passive current density, tpass. 

Repassivation processes have become an important subject in stress corrosion studies and also in other forms of corrosion, e.g., pitting corrosion and corrosion fatigue. A range of scratching and straining electrode techniques have been employed. While it is not possible to go into detail, the results have to be examined in relation to the techniques employed, e.g., has repassivation started before the scratching or straining has stopped It is important also to know whether the current measured under potentiostatic conditions is a complete anode current or the difference between an anode current and a cathode current (most commonly due to H" " ion reduction). Typical repassivation rates correspond to an equation (2 1 ) of the type ... 

Atomic force microscopy has been combined with nano-indentation and nano-scratching studies [95]. The hardness (and, to a similar extent, the friction coefficient) of passivated titanium was three to four times higher under in situ conditions, this was assigned to a much faster repassivation process in the presence of the passivating electrolyte solution. Nanotribology, particularly surface friction forces mea-... 

Many different techniques have been used to depassivate or bare small areas of metal to follow the repassivation kinetics, including, scratching, breaking, guillotining, impinging particles, and incident laser irradiation. Burstein and coworkers have developed and extensively used the scratching and guOlotining approaches [42-44,... 

One approach to using the AFM to study localized corrosion is to press hard vnth the tip or scratch the surface to stimulate passive film breakdown. Scratching with large tips has been used with success to study the repassivation process [120, 121]. By scratching in a controlled fashion over a small area with an AFM tip, it is possible to study the conditions under which the freshly bared surface will repassivate or propagate into localized corrosion [120, 121]. The effects of potential and environment, including inhibitors can be probed. 

S. Ahila, B. Reynders, H.J. Grabke, The evaluation of the repassivation tendency of Cr-Mn and Cr-Ni steels using scratch technique, Corros. Sci. 38 (1996) 1991—2005. 

The oxide film grown in this process has a thickness of 0.5 pm. It is a film of a zinc oxide/hydroxide containing different chromium oxides (CrO(OH), Cr20j, Cr205, CrOj) and zinc chromate (ZnCrO ). The chromate ions are rather mobile. If a zinc layer on a steel substrate gets a scratch somewhere on the zinc, the chromate will diffuse to this scratch and repassivate the surface (the self-healing mechanism). ... 

Contain elements that enable fast repassivation rate at scratches and flaws on multiple occasions. 

The electrochemical behaviour of SAE 304 (Mat.-No. 1.4301) in demineralised water (< 1 mg/1 chloride) at 25 °C was investigated and compared with the behaviour in a 3% NaCl solution [85]. Corrosion potential-time curves after mechanical activation of the surface by scratching show repassivation in the 3% NaCl solution as well as in high purity water the corrosion potentials increase from negative values (< -200 mVscE) to the positive values of around +40 mVscE- However, in high purity water this process occurs faster. 

ASTM F746-81 test (mV) Scratch test (mV) Potentiodynamic test Current 10 p,A/cm (mV) Repassivation (mV)... 

---