Changes in the SSRT Plasticity Parameters of Low Carbon Steel with Polarization Potential in Carbonate Bicarbonate Solution
This paper reports on the influence of the electrochemical polarization potential on ductility at fracture of 1020 steel immersed in carbonate bicarbonate solution at 52 degC during slowstrain rate testing (SSRT) under straining rate 10 6 s 1 All the plasticity parameters showed the same dependency on the applied potential, but reduction of area exhibited the highest sensitivity and smallest scatter. Area reduction ps remains almost unchanged ~53% at potentials above 600. 550 mV (on the standard hydrogen scale) while at lower potentials the apparent ductility of steel decreases to ps 1030 mV) 24 The changes in area reduction at increasing cathodic potentials was attributed to the role of hydrogen, though the presented data could not explain why the embrittling effect of hydrogen is manifested even at potentials anodic to free corrosion potential, where transgranular SCC is observed. The insensitivity of plasticity parameters to the ranges of potential where a network of intergranular stress corrosion cracks is formed brings attention to the usefulness of these parameters for characterization of SCC. Introduction The SCC studies performed during the past few decades have exposed a wide spectrum of metalenvironment systems that exhibit crack initiation and propagation under specific combinations of surface conditions and mechanical straining, while without the surrounding environment and specific surface conditions no failure of the material could be detected [1,2] SCC phenomena are rooted in a synergistic effect of the influence of mechanical and chemical fields on the material [3] so it would be incorrect to consider only only one of a fracture mechanical or chemical approach to the problem. These two are quite different, however, mostly from the scale point of view, since chemistry operates with interatomic interactions and electron structure of matter, while fracture mechanics looks at the material as a continuum, usually with homogeneous properties. Continuum mechanics could not possibly explain phenomena at the atomistic or even electronic level since this is where there is no more continuity and all the formalism has to collapse. A chemical approach to SCC, at first sight, seems to be in a superior position since it can incorporate mechanical properties through the interatomic bonding energy, which together with crystallographic parameters defines the strength of the material. This chemical approach to the materials strength is indeed very good, but only for the materials of highest purity, preferably in a monocrystalline condition. Such materials, however, are usually not susceptible to SCC. Moreover, they exist only in small quantities and are used for special applications where SCC is not an issue of concern. Real structural materials, which do exhibit SCC failures, could easily contain half of the periodic table of elements and huge number of 0D 1D 2Dand 3D defects/imperfections with several different phases dispersed in a polycrystalline material. Such compositions render rigorous chemical analysis of mechanical behavior practically impossible. Since the explanation of SCC phenomenon is somewhere in between the two approaches, there is no possibility for advancing, except to move in the direction of severely increasing the number of variables in the chemical description of the process, or to move towards finite element analysis of the system, substituting atoms for nodes in the grid that simulates atomic arrangement in real material. Both of these approaches, eventually, should meet in a chemomechanical model of the material. The number of studies in chemomechanics mechanochemistry with application of computational facilities is growing quickly and the time is probably near when SCC will be modeled and analyzed by use of super computers. Until then, however, we are destined to labor in the labs and exercise our knowledge and fantasy in every attempt to bridge the gap between these two approaches. We have to do it in order to accumulate sufficient databanks of relative information and to generate enough plausible ideas, hypotheses and theories concerning the origins of SCC. Returning from the dreams about the future to the contemporary realities, let us look at some results related to the area of carbonate SCC of low carbon and low alloy steels a type of SCC discovered half a century ago on high pressure transmission pipelines. A descriptive term carbonate is used here to emphasize a specific role of a certain chemical species carbonate ions and, thus, to place this type of SCC into a class with sulfide SCC and caustic SCC other distinct types of SCC where the role of chemical species defines the process of crack initiation and propagation. Although numerous studies have been dedicated to this type of failure so far [4 6] with much parameterization of both mechanical and chemical constants and variables there is still much need for data, with the hope that future analyses will provide a comprehensive explanation for this type of failure. We studied SCC of low carbon steel immersed into an electrolyte containing a concentrated solution of carbonate and bicarbonate (a.k.a. hydrocarbonate) anions, the former being well known from beverages and the latter from baking soda, both easily found in many households. It has been discovered that such an electrolyte could be formed between the disbonded coating and the metal surface of buried transmission pipelines under the influence of cathodic protection. The presence of such an electrolyte and tensile stress in the upper elastic range can cause SCC of most carbon and low alloyed steels with one additional requirement: the electrochemical potential of the surface has to be in the narrow range of potentials responsible for the formation of specific films with certain mechanical properties and chemical compositions [6] Among the key factors that define the mechanism of stress corrosion damage is the electrochemical potential a thermodynamic parameter that together with temperature and the activities of chemical species defines the electrochemical equilibria and the most stable (under the circumstance) chemical compounds on a metal surface [6,7] Among the other effects, the electrochemical potential can intensify the effect of hydrogen, or stimulate the localized anodic dissolution in the crack tip (a chemical mechanism for crack advancement) We are not going to elaborate here on the electrochemical mechanisms of the formation of surface films, or other electrochemical processes, but to show how a purely chemical parameter, which is a surface electrochemical potential, can affect the otherwise identical combination of material environment stress conditions, reflected by the changes in the plasticity parameters of the failed specimens. For those who approach SCC from the fracture mechanics view point, this work might serve as an illustration not only of the importance of chemical parameters in such types of failures, but also of the validity of utilization of the plasticity parameters in evaluation of SCC damage. Experimental Approach Material. We studied cylindrical samples machined from cold drawn rods 7.9 mm in diameter made of a typical low carbon steel AISI type 1020 with the following chemical composition of the most important components: C 0.17% Mn 0.77% Si 0.016% P+S 0.03% This material consisted mainly of two phases ferrite (a Fe) and pearlite (a mixture of a Fe and iron carbide Fe3C) Microstructural analysis of a transverse section of the sample, conducted with the help of Leco 300 Metallograph revealed a lamellar morphology of pearlite illustrated on Fig. 1 (pearlite is a darker phase) with the ASTM grain size number 11. The measured hardness of the material was HRB 90.5 92.5, suggesting percent of cold work of about 50 Fig. 1. Transverse section of the cold drawn rod made of 1020 steel. Slow strain rate technique (SSRT) SSRT together with optical microscopy was used to provide information on a synergistic effect of mechanical and chemical forces on the material. SSRT has proven to be a useful and probably the most effective laboratory method for studying SCC, allowing one to accumulate stress corrosion data in a relatively short period of time [8] This is very important in studies of SCC since this type of failure is often associated with very long incubation times required for crack initiation, a fact that is especially relevant to carbonate SCC of pipeline steels, where crack initiation time could be as long as decades. Parkins and coworkers have proven that the application of SSRT in studying carbonate SCC can quickly give valuable information needed for parameterization of this type of failure [9 11] Similarly to a conventional tensile tester, SSRT employs dynamic straining of the specimen monotonically increasing strain to failure, however, with considerably slower straining rates [12] The SCC data produced by this method is very dependent on the strain rate. If the specimen is strained too quickly, SCC is not manifested because the rate of stress enhanced electrochemical reactions involved in the crack growth is not sufficient and metal fails in a ductile manner, as if there is no effect of surrounding environment. On the other hand, if the straining is too slow, the kinetics of the passive film growth are high enough to sustain the crack tip under a stable passive condition, which is not influenced by strain sufficiently to fracture the passive film and produce any crack growth [8] As reported by Parkins and Zhou [11] a 2 10 6 s 1 strain rate is able to generate SCC in carbon steel in carbonate bicarbonate solutions of different compositions and temperatures. Based on this information, we constructed an SSRT machine that was able to generate strain rates from 0.7 10 6 s 1 and greater in samples having 50 mm gage length. The results reported here are related only to the set of the S