Tesis

Austenitic stainless steels are widely used in food, pharmaceutical, chemical, petrochemical and nuclear industries, among others, due to their good combination of properties such as workability, mechanical strength and corrosion resistance. However, they can show problems which can lead to failures in service. One of the most important problems are the susceptibility to intergranular corrosion and stress corrosion cracking (SCC) with intergranular (IGSCC) and transgranular cracks (TGSCC).

To reduce the failure occurrence probability, it is necessary to maintain the passivity of these steels, ensuring a minimum of approximately 10.5 % by weight of chromium on the entire stainless steel surface. Therefore, these materials are usually subjected to thermal solubilization treatments, between 1050 °C and 1150 °C, to dissolve all the chromium and carbon in the austenitic matrix, followed by a rapid cooling to ambient temperature, to prevent the precipitation of carbides. This treatment allows to obtain a microstructure with maximum resistance to intergranular corrosion and SCC in a wide variety of environments. But, if these materials are subjected to temperatures in the range from 500 °C to 800 °C, for example by excursions in process variables or during welding, the precipitation of Cr₂₃C₆, a chromium-rich carbide, can occur, preferentially at grain boundaries (GB). The precipitation of carbides at GB causes chromium-depleted zones in areas adjacent to GB. When Cr concentration falls below 10.5 %, the formation of an adequate passivating layer is impeded and corrosion resistance decreases. This phenomenon is known as sensitization.

A sensitized microstructure has lower resistance to chloride stress corrosion cracking. Moreover, some SCC processes such as SCC in polythionic acids, which affects components in petroleum refineries, and SCC in enviroments with fluorides, species present in thermal insulators and in welding electrode coatings, until now, only have been observed in sensitized materials.

In industrial practice it is important to know whether an austenitic stainless steel is sensitized. The early problem detection allows taking actions that can prevent and avoid unwanted outages or accidents, reducing risks and even allowing to increase the useful life of an equipment or facility.

There are different tests to evaluate the sensitization of austenitic stainless steels. Among them, the practices established in the ASTM A 262 standard and the electrochemical tests called electrochemical potentiokinetic reactivation (EPR), in the single-loop (SL-EPR) and double-loop (DL-EPR) variants. Electrochemical measurements provide more information, because they allow quantifying the material sensitization condition and, in certain cases, allow the determination of an index called degree of sensitization (DOS).

The SL-EPR and DL-EPR methods are widely used for the characterization of austenitic

stainless steel, acceptance/rejection of batches, selection of welding parameters that do not affect the alloy, among others. Despite the SL-EPR and DL-EPR methods were developed for non-destructive field measurements, in most cases they are performed in a laboratory on small samples, so, techniques are usually applied destructively.

In some industrial applications of these steels, it is necessary to determine, non-destructively, service aptitude of facilities and equipment that have been operating for a certain time, have experienced excursions in process variables that favor the sensitization process or have had repairs and/or modifications by welding or similar processes. Therefore, it is desirable to have a technique able to detect sensitized areas in stainless steel components non-destructively in situ.

The aim of this work is to return to the original idea of in situ application of an electrochemical technique, for determining non-destructively the sensitization condition of austenitic stainless steels in equipment and/or facilities components.

It is based on the DL-EPR test, and the necessary modifications, to the method and experimental cell, are analyzed, to transfer its usual application in the laboratory to in situ measurements. In this method, a potential sweep is carried out in the noble direction from the corrosion potential (E_corr) to a predetermined potential in the passive region. Then the direction of the potential sweep is reversed, in the active direction. The sweep ends upon reaching the initial value of E_corr. The ratio of the reactivation peak current (I_r), from the reverse scan, and the activation peak current (I_a), from the forward scan, is used as a parameter to quantify sensitization.

The solution used for DL-EPR measurement in austenitic stainless steels contains sulfuric acid and thiocyanate ions and is generally prepared immediately before use. Additionally, laboratory determination is usually made in nitrogen deaerated solutions. Since it is impractical when making a field measurement, it would be convenient to use a naturally aerated solution prepared in advance. Therefore, the influence of oxygen content in the solution and the effect of the solution aging on the ratio I_r/I_a was studied.

Tests were made in austenitic stainless steel AISI 304 (UNS S30400) and AISI 303 (UNS S30300). Both alloys have similar Cr contents, but 303 has higher P and S contents, to improve machinability. These steels were subjected to different heat treatments, to obtain different material sensitization conditions. The percentage of cold work on the 304 stainless steel was varied, to determine its influence on the electrochemical measurement. The 303 stainless steel has a higher inclusions content, which made possible to study the influence of these second phases on the test.

Finally, a simple and low-cost electrochemical cell was developed, which can be fixed on flat and curved surfaces, horizontal or vertical.

The conclusions of this thesis are as follows:

- It is possible to discriminate between sensitized and unsensitized stainless steels by DL-EPR measurements under natural aeration conditions.

- The solution can be stored for up to four weeks without significantly affecting the results.

- The inclusion content modifies the value of the reactivation peak in solubilized specimens in DL-EPR curves, directly affecting the I_r/I_a. This effect can lead to false positives. However, if there are control samples of a component, or if a measurement is made prior to start up, it is possible to apply the technique in a comparative way to determine if the component was sensitized in service. If previous metallurgical condition of the stainless Steel is unknown or there are no control samples, the false positive alerts about a susceptibility in the material and justifies further analyzes, to have a more detailed characterization of the situation.

- With the cold work percentages used, a decrease in the reactivation peak of DL-EPR curves was detected, affecting I_r/I_a. The effect was not enough to modify the categorization according to the sensitization condition prior to cold work.

- Measurement cells were fabricated with readily available materials and 3D-printed parts, providing the required functionality, at low cost and with enough versatility to adapt to the different geometries of the components.

These conclusions encourage the use of DL-EPR measurements in situ to determine non-destructively the sensitization condition of type 304 austenitic stainless steel and stimulate further work to extend this application to other materials.

Keywords: Intergranular corrosion, DL-EPR, sensitization, stainless steel.