Effect of SAW fluxes on electrochemical corrosion & microstructural behavior of API X70 weldments:

The influence of hydrogen sulfide on corrosion of iron under different conditions

Abstract: Hydrogen sulfide (H 2 S) can either accelerate or inhibit corrosion of iron under different experimental conditions. What H 2 S has done to both the anodic iron dissolution and cathodic hydrogen evolution, in most cases, is to have a strong acceleration effect, causing iron to be seriously corroded in acidic medium, but H 2 S can also have a strong inhibition on the iron corrosion under certain special conditions where H 2 S concentration is below 0.04 mmol dm −3 , pH value of electrolyte solution is within 3–5 and the immersion time of the clectrode is over 2 h. The inhibition effect of H 2 S on the iron corrosion is attributed to formation of ferrous sulfide (FeS) protective film on the electrode surface. Moreover, the structure and composition of the protective film is closely related to H 2 S concentration, pH of solutions and the immersion time of iron. Accordingly, the influence of the three factors on the inhibition effect is investigated in this paper by means of AC impedance technology together with the potentiostatic steady-state polarization. A probable reaction mechanism is proposed to interpret theoretically how H 2 S inhibits the corrosion of iron.

Effect of microstructure on the hydrogen trapping efficiency and hydrogen induced cracking of linepipe steel

Abstract: The hydrogen trapping efficiency in different microstructures is compared, and the critical hydrogen flux for hydrogen induced cracking (HIC) is determined for API X65 grade linepipe steel. By controlling the start cooling temperature (SCT) and the finish cooling temperature (FCT) in thermomechanically controlled process (TMCP), three different kinds of microstructure such as ferrite/degenerated pearlite (F/DP), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) are obtained. A modified ISO17081(2004) standard method is used to evaluate the hydrogen trapping by measuring the permeability (JssL) and the apparent diffusivity (Dapp). Microstructures affecting both hydrogen trapping and hydrogen diffusion are found to be DP, AF, BF and martensite/austenite (M/A) constituents. The hydrogen trapping efficiency is increased in the order of DP, BF and AF, with AF being the most efficient. HIC is initiated at the local M/A concentrated region when the steel has such microstructures as F/AF or F/B. Although the trapping efficiency of bainite is lower than that of AF, bainite is more sensitive microstructure to HIC than to AF.

The Formation of Ferrous Monosulfide Polymorphs during the Corrosion of Iron by Aqueous Hydrogen Sulfide at 21°C

Abstract: The initial stages of corrosion of iron by unstirred saturated aqueous H/sub 2/S solutions at 21/sup 0/C and atmospheric pressure have been examined as a function of time, pH (from 2 to 7, adjusted by addition of H/sub 2/SO/sub 4/ or NaOH), and applied current. Detailed examination of the morphology and phase identity of the corrosion products has led to a qualitative mechanistic understanding of the corrosion reactions. Mackinawite (tetragonal FeS/sub 1-x/) is formed by both solid-state and precipitation processes. Cubic ferrous sulfide and troilite occur as precipitates between pH = 3 and pH = 5, subsequent to metal dissolution upon cracking of a mackinawite base layer formed by a solid-state mechanism. The corrosion rate, and the relative amounts of these phases produced, are controlled by pH, applied current, and the degree of convection. The corrosion rate increases with decreasing pH; the quantity of precipitated material peaks near pH = 4, below which dissolution becomes the dominant process as the solubilities of the sulfide solids increase. Significant passivation was observed only at pH = 7, when the initial mackinawite base layer remained virtually intact. The solid-state conversion of cubic ferrous sulfide to mackinawite at 21/sup 0/C was monitored by x-raymore » diffractometry. The resulting kinetics are consistent with the Avrami equation for a nucleation and growth process with a time exponent of 3.« less

Effect of H2S on the CO2 corrosion of carbon steel in acidic solutions

Abstract: The objective of this study is to evaluate the effect of low-level hydrogen sulfide (H 2 S) on carbon dioxide (CO 2 ) corrosion of carbon steel in acidic solutions, and to investigate the mechanism of iron sulfide scale formation in CO 2 /H 2 S environments. Corrosion tests were conducted using 1018 carbon steel in 1 wt.% NaCl solution (25 °C) at pH of 3 and 4, and under atmospheric pressure. The test solution was saturated with flowing gases that change with increasing time from CO 2 (stage 1) to CO 2 /100 ppm H 2 S (stage 2) and back to CO 2 (stage 3). Corrosion rate and behavior were investigated using linear polarization resistance (LPR) technique. Electrochemical impedance spectroscopy (EIS) and potentiodynamic tests were performed at the end of each stage. The morphology and compositions of surface corrosion products were analyzed using scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the addition of 100 ppm H 2 S to CO 2 induced rapid reduction in the corrosion rate at both pHs 3 and 4. This H 2 S inhibition effect is attributed to the formation of thin FeS film (tarnish) on the steel surface that suppressed the anodic dissolution reaction. The study results suggested that the precipitation of iron sulfide as well as iron carbonate film is possible in the acidic solutions due to the local supersaturation in regions immediately above the steel surface, and these films provide corrosion protection in the acidic solutions.

Characterization of inclusions of X80 pipeline steel and its correlation with hydrogen-induced cracking

Abstract: In this work, the microstructures of an X80 pipeline steel were characterized, and their susceptibilities to hydrogen-induced cracking (HIC) were investigated by hydrogen-charging, electrochemical hydrogen permeation and surface characterization. It is found that the microstructure of X80 pipeline steel consists of a polygonal ferrite and bainitic ferrite matrix, with martensite/austenite (M/A) constituents distributing along grain boundaries. The inclusions existing in the steel include those enriched with Si, Al oxide, Si–ferric carbide and Al–Mg–Ca–O mixture, respectively. The majority of inclusions are Si-enriched. Upon hydrogen-charging, cracks could be initiated in the steel in the absence of external stress. The cracks are primarily associated with the Si- and Al oxide-enriched inclusions. The diffusivity of hydrogen in X80 steel at room temperature is 2.0 × 10−11 m2/s, and the estimated hydrogen trapping density in the steel is as high as 3.33 × 1027 m−3.