International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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The ec-pen in quality control: Determining the corrosion resistance of stainless steel on-siteM. Büchler, C.-H. Voûte, D. Bindschedler, and F. Stalder
SGK, Swiss Society for Corrosion Protection, Technoparkstr. 1, CH-8005 Zürich,
In numerous technical applications the knowledge of the local electrochemical behavior is necessary for the understanding of the mechanisms involved. This information helps to understand the corrosion behavior of materials, to evaluate the corrosion risk of specific inhomogeneity, and to design mitigation measures for the improvement of the corrosion resistance. Hence, an electrochemical sensor and a handheld potentiostat were developed that allow fast and easy access to local electrochemical information. Easy handling, short sample preparation time and the possibility to run experiments on virtually any size object with various surface geometries opens a vast field of applications. Moreover the possibility to run non-destructive corrosion tests make it a powerful tool in quality control and process development. Typical applications of the set-up are presented.
Stainless steel is increasingly used in civil engineering. In most applications its high corrosion resistance makes it the material of choice. Nevertheless, this property can be easily compromised by manufacturing processes, such as welding and cold work or environmental influences such as scratching and pollution with iron dust to name a few. These effects can severely decrease the corrosion resistance of the stainless steel. Therefore, the high expectations in service life could not be met in numerous cases. This fact demonstrates the need for a quality control. However, the quality control of the corrosion resistance of stainless steel in civil engineering is mostly limited to visual inspection, which only allows the determination of indirect values and depends to a high degree on the experience of the controlling engineer.
In contrast, electrochemical techniques have been developed in corrosion research that allow the reliable determination of the corrosion resistance of stainless steel under laboratory conditions. These techniques have never been used on a routine basis in civil engineering, as they involve cutting out samples and measurements in the lab, making it an expensive and time-consuming procedure. With the development of a new electrochemical sensor [1, 2], the so called ec-pen, and a hand held control unit it became possible to run electrochemical tests even in rough environment (Fig. 1 and 2). The ec-pen allows the evaluation of the corrosion resistance of stainless steel on-site and determination of the effectiveness of mitigation measures. Hence, the quality control of stainless steel manufacturing is possible based on the determination of a direct value, resulting in a high reliability. The results obtained in quality control of stainless steels in civil engineering and the scientific background of the test are presented.
The electrochemical sensor
A tube of 1.4432 was welded under nitrogen. The welding additive was DIN 1.4430. No post treatment of the weld was performed.
A screw of 1.4404 was subjected to cold work resulting in a surface pollution with iron. For increasing the corrosion resistance it was degreased, subjected to oxidizing cleaning and passivation. The corrosion resistance of the component was characterized electrochemically after each process step.
Results and discussion
Comparative quality control of welds
Up to now the quality control of the corrosion resistance of welded components was generally limited to visual inspection. The look of the weld, the presence of defects and heat tint were the essential criteria used. The correct interpretation of the visual inspection depends on the experience of the expert. Additionally, there is a high degree in uncertainty, as the visual inspection is not a direct measure for the corrosion resistance of the material. The ec-pen in combination with the handheld control device offers the possibility of determining the corrosion resistance directly. Its application is demonstrated on the weld on a DIN 1.4301. At first the instrument is calibrated on the base material not affected by the welding process. The instrument determines the pitting potential (Fig. 3). As this value is subject to a certain scatter caused by the statistical distribution of the corrosion initiation sites in the metal, it is useful to run several calibrations. Of those pitting potentials the instrument calculates the average, which allows compensating for the scatter. Once the calibration is finished the instrument decreases the testing potential for 140 mV. By performing potentiostatic tests, it is readily determined whether the investigated spot exhibits comparable corrosion resistance as the base material or whether the corrosion resistance is significantly decreased (Fig. 4). Hence, uncertainties in the visual inspection can easily be excluded, resulting in an increased reliability of the quality control in field applications or in the testing of sample weldings. In the present example, it was clearly shown that the corrosion resistances of the discolored area in the heat affected zone strongly affects the corrosion resistance (Fig. 4 and 5). The cleaning with the Clinox pro (Nitty Gritty GmbH) resulted in an increase of the corrosion resistance in the same range.
Process optimization by determining the pitting potential
Based on a few tests with the ec-pen the evaluation and optimization of the post treatment of the weld was possible, while the visual inspection used so far would have resulted in an erroneous evaluation of the corrosion resistance.
In order to obtain more detailed information about the corrosion behavior of the weld, additional investigations with a potentiostat that allowed the measurement of the current were performed. The result is shown in Fig. 8. The zone with the yellow oxides does not exhibit a passive behavior. The treatment in water results in a certain improvement, but the passivation with HNO3 is required to obtain comparable pitting potentials as the bulk material (Fig. 9). It is interesting to note that even this strong passivation treatment was not sufficient to obtain comparable passivity as on the bulk, as the passive current density is still significantly higher in the HAZ.
In the present example it was found that the essential conclusions obtained with a standard electrochemical set-up was possible to obtain with the handheld instrument on site.
Quality control of stainless steel with standard calibration
Nevertheless, there is a large field of problems related to quality control in surface treatment of stainless steel components of smaller size that can easily be handled in the laboratory. Under these well-controlled temperature conditions it is possible to use the ec-pen with a standard calibration. This is illustrated with the surface treatment of a screw for application in corrosive environment.
After the cold work process a screw is polluted with iron particles. Subsequent degreasing, oxidizing cleaning, and passivation the corrosion behavior is increased, as can be concluded from the increasing pitting potential found in the polarization scans (Fig. 10). Additionally, the passive current density is decreased demonstrating the improvement of the passive layer. Determining the pitting potential with the Clinox Tester shows a clear increase of the pitting potential with the passivation procedure. Based on the obtained results it is clear that the components exhibit a pitting potential above 0.46 V SCE after a successful surface treatment. The potential can be set at this value by running the calibration with an external calibration box. Once the value is set a large number of tests can run in a very efficient and straightforward way as is shown in Table 1. The easy way of running the test and the short testing time offers a fast and reliable way of keeping track of the efficiency of the surface treatment.
The possibilities offered by the new electrochemical sensor are illustrated with various practical applications. The robust design, the lack of any maintenance, and the fast applicability allow for the evaluation of the corrosion resistance of stainless steel components of any size and virtually any surface geometry on site. Cutting out samples for laboratory characterization is therefore obsolete, making the technique virtually nondestructive. During the test the metal surface is polluted with chlorides, which have to be washed off afterwards to avoid possible corrosion. In case of highly aggressive applications it is recommended to passivate the surface after the test. Contrary to visual inspection it is possible to determine the corrosion resistance directly, making the quality control independent on the experience of the executing engineer. On the other hand it offers a good alternative to the time consuming salt spray test. Three different ways of applications are possible:
This work was possible thanks to the generous support of Nitty Gritty GmbH. A special thank is directed to Thomas Suter from ETH Zürich for the valuable discussions.