Potentiostatic tests

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

Test methods available are the free-running test (galvanic cell), galvano-static test (constant current) and potentiostatic test (constant potential). These are always run in conjunction with visual examinations with particular emphasis on dissolution pattern. 

The critical information required from testing may include one or all of the following tendency to passivation, anode operating potential and capacity. The tests, whilst all capable of producing information on the above, tend to be particularly suited to certain applications. For example potentiostatic testing is useful for evaluating passivation tendencies but not generally appropriate to anode capacity determination. 

Streicher , however, considered this approach to be unsound and pointed out that the short duration of the potentiostatic studies carried out by France and Greene cannot be used to predict long-term behaviour in service. The prolonged dialogue between these workers was well summarised in the review article by Cowan and Tedmon who concluded that these particular potentiostatic tests cannot be regarded as accelerated tests for service environments and that predicting future industrial service for periods longer than the test is not advisable. 

Potentiostatic tests " have been used and Wilde and Williams in potentiokinetic studies of the critical breakdown potential of stainless steels (Types 430 and 304) in 1 -0 mol dm" NaCl, showed that the nature of the gas used to purge the solution has a pronounced effect on the value of... 

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

Budd and Booth found the potentiostatic test best for investigating the intergranular and layer corrosion of aluminium alloys. 

Experimental CFTs were determined by Brigham and Tozer in three different ways based on chloride solutions two electrochemical and one chemical. In electrochemical testing in sodium chloride (NaCl) solution, the methods involved recording the current while either increasing the potential at suitable selected temperatures (apotentiodynamic test) or increasing the temperature at suitable fixed potentials (a potentiostatic test). Alternatively, a potential was established by the redox couple (Fe(in)/Fe(n) in a simple immersion test in ferric chloride solution. 

The potential-independent CPT can be determined by two different electrochemical methods a potentiostatic test method using a sufficiently high potential, and a potentiodynamic test method. The potential-independent CPTs are well-defined, experimental results having a reproducibility of approximately l°Cby potentiodynamic testing and approximately 2°C by potentiostatic testing. 

The potential-dependent CPT is determined by potentiostatic testing using a relatively low potential the scatter of results is about 5°C or higher. 

In 1980 Bemhardsson et introduced an automated electrochemical method for CPT determination. The specimen is mounted as described in Section IV.2 (ii) using a stream of argon to avoid crevice corrosion and 0.02-5% sodium chloride as electrolyte. The CPT is determined by a potentiostatic test method using an instrument called the Santron CDT 400 for potential control, temperature control, and current measurements. 

Figure 17. Measurement of the potential-dependent CPT using a potentiostatic test developed by Bemhardsson el...

Figure 21. Determination of the potential-independent CPT by the potentiostatic test according to Amvig and Bisgird. Applied potential 700 mV/SCE rate of temperature rise 1 °Cymin. (Reprinted from Ref. 69 by permission of NACE International, Houston, Texas.)...

Two other aspects of electrochemical phenomenology associated with localized corrosion should be appreciated before we discuss individual test techniques common observations during potentiostatic testing and common observations during open circuit testing. Careful interpretation of these tests can provide useful information on the processes that control localized corrosion. 

While long-term potentiostatic tests can be useful, they are extremely time-consuming and expensive. Potentiostaircase tests can be performed in lieu of potenti-odynamic tests, though at equivalent potential scan rates, the results should be identical. In addition, the time frames involved are usually still very short compared to the projected life. Thus a number of approaches have been developed for accelerating the process of initiation in order to determine the potential below which initiated sites will repassivate (i.e., Ew). 

Potentiodynamic polarization determines Eap for positive scans, whereas negative scans yield E, . If they are different, the polarization curve shows a hysteresis. In many cases, this difference gets smaller with decreasing scan rates, indicating that the critical potentials are influenced by the composition of the pit electrolyte as well as kinetic factors like pit nucleation and pit growth. The ASTM standard G61 applies 10mVmin [11]. Potentiostatic tests depend less on the experimental conditions and thus are more reliable but time consuming. Usually a potential is applied and the current density is followed for some time. If the current decreases continuously, E < np will hold, whereas it increases when E exceeds np-If pits are formed at > np and then the potential is stepped to less positive values, the current density will drop continuously when E < rp is reached. For some systems, both critical potentials are... 

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