An analysis of composition of the layers formed under CVD of tantalum on stainless steel substrates

Interaction of the reagents and the substrate stainless steel was analyzed during the deposition of tantalum coatings by reduction of tantalum bromide with cadmium. It was found that at deposition temperatures above 1000 K, the iron and chromium contents in stainless steel will reduce tantalum pentabromide passing into the gas phase in the form of bromides. As a result, not only tantalum bromide is reduced on the substrate, but also iron bromide which in the initial stages of deposition leads to the formation of iron containing sublayer in the coating. An increase in the contact surface of stainless steel interacting with reagents leads to an increase in the content of iron bromide, in which case a coating is formed from a Ta + Fe2Ta mixture. In addition, chromium bromide is formed on the substrate, which is removed along with the rest of the reaction products. It causes chromium depletion in the surface layers of steel.

On the Role of Grain Size and Carbon Content on the Sensitization and Desensitization Behavior of 301 Austenitic Stainless Steel

The effect of grain size in the range 72 to 190 μm and carbon content in the range 0.105–0.073 wt.% on the intergranular corrosion of the austenitic stainless steel 301 has been investigated. Grain boundary chromium depletion has been studied directly using energy dispersive X-ray spectroscopy combined with scanning transmission electron microscopy and indirectly using double loop electrochemical potentiokinetic reactivation tests. In addition, chromium depletion has been modelled using the CALPHAD Thermo-Calc software TC-DICTRA. It is shown that the degree of sensitization measured using the double loop electrochemical potentiokinetic reactivation tests can be successfully predicted with the aid of a depletion parameter based on the modelled chromium depletion profiles for heat treatment times covering both the sensitization and de-sensitization or self-healing. Additionally, along with intergranular M23C6 carbides, intragranular M23C6 and Cr2N nitrides that affect the available Cr for grain boundary carbide precipitation were also observed.

Corrosion fatigue properties of narrow gap laser welded 301L stainless steel

In this paper, the corrosion-fatigue behavior of 301L stainless steel (SS) welded joints using narrow gap laser wire welding under the different stress ratio and concentrations of Cl[Formula: see text] has been investigated. Corrosion fatigue life curve (S-N) under different conditions was tested and fitted to obtain the fatigue limit. The microstructures and fracture surfaces of specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM). EBSD results have shown that chromium depletion in the weld heat-affected zone (HAZ) contributes to the decrease of corrosion fatigue properties in the HAZ.

Optimization of the Double Loop Electrochemical Potentiokinetic Reactivation Method for Detecting Sensitization of Nickel Alloy 690

Nickel alloy 690 (UNS N06690) is one of the current choices for nuclear power plant steam generator tubing. The severity of certain stress corrosion cracking submodes in Ni-Cr-Fe alloys, such as Alloy 690, may be a function of the alloy sensitization condition. Sensitization results from chromium carbide precipitation at or near grain boundaries when the alloy is exposed to temperatures from 500°C to 800°C, which causes local chromium depletion in the adjacent matrix. The objective of this work was to study the parameters that control the double loop electrochemical potentiokinetic reactivation (DL-EPR) method and to develop an optimized test routine for Alloy 690. Alloy 690 specimens were tested under different thermal treatments, tailored to obtain a variety of concentrations of chromium in solid solution near the grain boundary (GB) region. The DL-EPR method was applied to Alloy 690 in solutions with different concentrations of sulfuric acid and potassium thiocyanate, at 30°C and 50°C. The optimal condition for detecting sensitization of Alloy 690 was 0.5 M H2SO4 + 0.001 M KSCN, at 30°C. Optical micrographs confirmed an intergranular type of attack under thermally aged (sensitized) conditions. The ratio of the reactivation to the activation peak intensities (Ir/Ia) obtained in the DL-EPR test agreed well with mass loss obtained in boiling nitric acid solution with Cr(VI) additions (modified Huey test). Chromium depletion profiles near the GB of thermally treated specimens under different conditions were measured with the energy dispersive x-ray spectroscopy technique, performed on a transmission electron microscope. A model available in the literature was fitted to those results. The Ir/Ia ratio for Alloy 690, obtained when using the optimal DL-EPR testing conditions, correlated well with the Cr depletion zone width and depth near the GB.

Prevention of Corrosion in Austenitic Stainless Steel through a Predictive Numerical Model Simulating Grain Boundary Chromium Depletion

In this chapter, a mathematical model for rate of formation of chromium carbides near the grain boundary, which is a pre-cursor to chromium depletion and corresponding sensitization behavior in stainless steels, is presented. This model along with the diffusion equation for chromium in the grain has been used to obtain chromium depletion profiles at various time and temperature conditions. Finite difference method has been used to solve the above equations in the spherical co-ordinate system and the results of time-temperature-sensitization diagrams of four different types of alloys have been compared with those of experiment from literature. For the problem of low temperature sensitization and corresponding inter-granular corrosion in austenitic stainless steel, it is very difficult to carry out experiment at higher temperatures and justify its validity at lower operating temperatures by extrapolation. The development of predictive models is highly useful in order to design the structures for prevention of corrosion of the material in aggressive environments.

Electrochemical techniques for estimating the degree of sensitization in austenitic stainless steels

The thermal exposure of austenitic stainless steels in the temperature range of 500°C–800°C results in the precipitation of chromium-rich M23C6 carbides along the grain boundaries and concomitant chromium depletion. When chromium level falls below 12% at such depleted zones, the material is said to be sensitized. When the sensitized material is exposed to corrosive media, intergranular corrosion (IGC) and intergranular stress corrosion cracking take place. ASTM standard A262-13 is available for detecting the susceptibility to IGC. However, the extent of chromium depletion of shop-fabricated or field-constructed components is required to determine the presence or absence of sensitization for critical applications. In this review, various electrochemical techniques that can be employed to assess the degree of sensitization (DOS) in austenitic stainless steels are critically reviewed. Techniques such as electrochemical potentiokinetic and potentiostatic reactivation with recent modifications for the accurate estimation of the DOS are analyzed. The possibility of using AC impedance technique and electrochemical noise technique for assessing the DOS is also presented. The merits and demerits of each technique and areas where further research should be focused are presented in this review.

Intergranular Corrosion Susceptibility of Alloy 600 after Autogenous Tungsten Inert Gas and Laser Beam Welding using Electrochemical Technique

Intergranular corrosion and intergranular stress corrosion cracking is influenced by precipitation of chromium carbides at grain boundaries and formation of chromium depletion regions. The present study focuses on understanding the carbide precipitation and subsequent sensitization in the weldments of Alloy 600 using two different welding techniques. The effect of heat input on microstructure and IGC susceptibility was measured using electrochemical reactivation test. The SEM studies were done to evaluate the presence of chromium depleted regions. The carbide was indentified to be Cr7C3 using TEM. The laser beam weldments showed an increased resistance to IGC as compared to TIG weldments.

Macro and Micro Electrochemical Techniques to Study Influence of Sigma Phase Formation in a Duplex Stainless Steel

In the present study macro electrochemical (anodic polarization) and micro electrochemical (scanning electrochemical microscopy (SECM) area scan measurements at passive potential) techniques have been used to study the influence of sigma phase and/or the resultant chromium depletion regions on localized corrosion behavior of aged type 2205 duplex stainless steel (DSS) in neutral chloride ion solution. DSS type 2205 was subjected to aging at 750 °C for 30 min, 10 h and 48 h. The formation and growth of the sigma phase with heat treatments was assessed by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. The influence of formation of sub-microscopic and bulky sigma phase on intergranular corrosion (IGC) and pitting corrosion was investigated by various electrochemical techniques including electrochemical potentiokinetic reactivation (EPR), potentiodynamic polarization and SECM. Apart from EPR tests, ASTM A 262 Practice B test was carried out to evaluate the presence of chromium depletion regions with heat treatments. The results showed that with increasing aging duration, the degree of sensitization and IGC rates initially increased and then decreased with heat treatment. The pitting potentials decreased continuously with increase in aging duration up to 10 h as assessed by potentiodynamic polarization tests. The SECM area scan measurements showed more metastable pitting corrosion events for 30 min and 10 h aged specimens compared to the 48 h aged specimen at passive potential in 0.1M neutral chloride ion solution.