Introduction
Stainless steels can be prone to various different types of corrosion, dependent on the combination of alloy composition and operating environment. In service situations the localized corrosion mechanism most commonly encountered is almost certainly pitting corrosion. It can be extremely difficult to predict the incidence of pitting corrosion, particularly as the complexity of the corrosive environment increases, and even more so when fluctuations occur in the environmental parameters, as often arises in process plant. It is clearly not possible to accumulate data by evaluating the performance of the wide range of stainless steels in the vast number of different and often variable corrosive environments found in industry.
Various guides relating to the pitting performance stainless steels have been published over the years, perhaps the most common being the pitting index formula, ie
PI = %Cr + (3.3 x %MO) + (X%N)
Where X = 11 to 30
This provides a good qualitative indication of the comparative pitting resistance of different alloys, but cannot be easily used to give a quantitative prediction of performance in relation to a particular corrosive environment. More useful information is provided by laboratory immersion testing or by critical pitting temperature data, but these are generally limited to a few standard test solutions, such as 10% FeCl3 which has been shown to be of very limited relevance to most service environments. In contrast, testing in more representative solutions either requires unacceptably long exposure periods, or provides insufficient information to form the basis of an accurate prediction.
Hence, performance must be predicted from an understanding of corrosion behavior in a restricted range of environments which form the basis of most service conditions. The effect of other significant environmental parameters can then be superimposed onto the base data, using a combination of available information relating to the effect of such parameters, and service experience.
The most significant environmental conditions which influence the pitting corrosion behavior of stainless steels are the chloride ion concentration, temperature and pH level. This article describes a program of work in which the pitting corrosion susceptibility of a range of stainless steels has been determined by a simple electrochemical method within a matrix of these key environmental parameters. The resultant data have been processed to produce pitting corrosion engineering diagrams which are designed to provide corrosion engineers with a more systematic aid to materials selection than currently available data. The translation of the raw electrochemical data directly into useful engineering diagrams has been simplified and considerably accelerated by the development of a computer program.
Experimental Procedure
The materials selected for examination were representative of the range of standard and high alloy stainless steels. The test solutions were prepared freshly for each test, and the pH and chloride ion concentration was adjusted using the sodium chloride and hydrochloric acid, except at low chloride ion concentrations where monochloroacetic acid was used. Four chloride ion levels of 0.03, 0.1, 1.9 and 10% were used, each at four pH levels of 7, 4.5, 3 and 1.5. Slight acidity of the distilled water was neutralized by sodium hydroxide for test solutions at pH = 7.
Test samples were suspended in the electrochemical cell using the attached wire and were connected to the potentiostat, along with the reference electrode and a platinum counter electrode. The test solution within the cell covered each electrode and was deaerated with nitrogen, and solution temperatures of 25, 35, 45 or 55 C were used, controlled by immersion of the cell in regulated water baths and measured by a thermometer in the actual test cell. The test sample was initially polarized to -400 mV (v SCE) for 1 minute, after which the potential raised at 1 mV/sec. Current flowing in the cell was monitored, and the test was continued until the anodic current density at the specimen exceeded 500 uA/cm2 resulting from either pitting, crevicing at the metal/paint interface or transpassivity. The potential at which this occurred was noted as the pitting potential unless pitting, or any other form of corrosion, was not evident ton the test surface, in which case transpassivity was assumed. All samples were examined visually after testing, to ensure that a pit had developed when indicated by the test result. If crevicing was apparent, the result was discarded and the test repeated with a repeated surface.
Tests were performed in order of decreasing severity of the test solution. Where it was evident that particular alloy was not susceptible to pitting corrosion, no further testing under less severe conditions was carried out.
Experimental Results
The anodic polarization determination produced a characteristic relationship between potential and the measured current density, the most important feature being a rapid rise in current density above a certain critical potential, as shown in Figure 1. Where this potential exceeded 1000 mV transpassivity was assumed. For lower potential values the specimen test surface was examined carefully for evidence of pitting. If crevice corrosion was observed at the pain/metal interface, the result was discarded and the surface reprepared and retested. The onset of crevice corrosion, which occurred only occasionally, was generally apparent from the unexpectedly low break potentials on the anodic polarization curve. In general, pit initiation, as revealed by the first increase in current density from the passive level, was followed by sustained and rapid propagation and, hence, rapid current rise. It was found, in particular with the more resistant alloys, the early pit propagation could be slow, with particular repassivation occurring and a correspondingly slow rise in current with increasing potential.
When comparing a wide range of alloy compositions, differences in pitting behavior are not adequately taken into account if the pitting potential was taken as the potential of the first current increase. Experience shows that, once a current density of 500 uA/cm2 was reached, propagation would be sustained. The potential at which this current density was reached was, thus, taken as the pitting potential and represents the potential at which and equal level of irreversible pitting corrosion was occurring in a given environment rather than the first, perhaps transient, evidence of passive film breakdown.
Relationships between pitting potential and the major environmental test parameters was represented graphically, as shown in Fig. 1. The effect of pH is shown in Fig. 1 a), and was observed to be small at levels above pH = 5, particularly at the higher chloride levels where the pitting potential was generally of a low order. A progressive reduction in pH below this level resulted in a more significant decrease of the pitting potential, with a minimum at about pH = 3. At lower pH levels the onset of general acid corrosion was evident and resulted in an apparent increase in the pitting potential. This behavior was particularly evident at the lower chlorine ion concentrations.
The effect of increasing temperatures was clearly to reduce the pitting potential, as shown in Fig. 1b), and the relationship between pitting potential and temperature was approximately logarithmic. It was notable that the difference in pitting potential between the different alloys decreased with increasing temperature as shown in Fig. 1c).
Overall the most significant parameter with respect to the pitting potentials of the alloys was the chloride ion concentration, and a characteristic relationship was apparent (See Figure 2a). This took the form of an initially high rate of decrease in pitting potential with increasing chloride levels in the range 300 to 1000 ppm, but with the rate reducing progressively on further increases to 19000 and 10000 ppm chloride ion concentrations. Of particular importance was the fact that the relationship was close to logarithmic such that a plot of pitting potential against the logarithm of the chloride ion concentration produced essentially a straight line for each of the pH, temperature and alloy combinations examined, as illustrated by the example in Fig. 2b).
A feature noted with all the materials examined, regardless of alloy content, was the marked change in the other wise consisted relationship between pitting potential and the environmental conditions which was apparent at pH levels below pH = 3. This was particularly evident at the lower chloride ion concentrations as shown in Fig. 1a), and was attributed to the development of general acid corrosion. It was concluded that these results should be excluded from the data base to be used in the construction of the engineering diagrams.
To process the results into a form suitable for constructing the engineering diagrams, the consistent and reproducible linear relationship between the pitting potential and the logarithm of the chloride ion concentration was used. To interrelate all the environmental parameters, isopotential curves were produced of the logarithm of the chloride ion concentration against temperature at constant pH levels using calculated best straight line relationships.
The curves indicate the limiting chloride ion concentrations, pH and temperature for the development of pitting corrosion for a given material in an environment at a given potential. The main requirement in progressing to useful engineering diagrams is that the potential of the system must be known.
By: J W Fielder and D R Johns
British Steel Technical
Swinden Laboratories
Moorgate Rotherham
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