There are several degradation phenomena when hydrogen enters into steels, i.e., hydrogen induced cracking, hydrogen embrittlement and hydogen attack, whick are occurred depending on the strength of steel and temperature of environment. The hydrogen monitoring methods proposed by Prof. Yamakawa using an electrochemical hydrogen permeation technique to predict the degredation of steel and actual monitoring results are reported here.
Research on hydrogen embrittlement by electrochemical hydrogen permeation technique is reviewed in this paper. Diffusion coefficient of hydrogen (D) and hydrogen permeation rate (J) can be more easily determined by electrochemical hydrogen permeation technique than other hydrogen measurement methods. Threshold hydrogen content for hydrogen embrittlement can be determined and hydrogen embrittlement mechanism is discussed by focusing effects of metallurgical factors on apparent D. Effects of environmental factors on J have been investigated and hydrogen entering mechanism can be discussed by transition of J. The electrochemical hydrogen permeation technique can be used as a monitoring system in plants. It can be also applied to “in-situ measurement” by combining with other tests of fracture mechanics, ultrasonic wave C-scanning and so on. It will have a potential of new experimental variation to research on hydrogen embrittlement.
OCTG (Oil Country Tubular Goods) and line pipes used for production and transportation of oil and gas are the major steel products which suffer hydrogen embrittlement (HE) because they are often exposed to wet hydrogen sulfide (sour) environments, where a lot of hydrogen enters the steel. Typical damage types of HE are hydrogen induced cracking (HIC) and sulfide stress cracking (SSC). First, hydrogen uptake behavior of the steel in sour environments are explained. Concerning HIC and SSC, their mechanisms and metallurgical controlling factors are outlined. Finally it would be understood that developments of OCTG and line pipes resistant to sour have promoted steel production technology for these several decades.
In order to evaluate the corrosion on the inner part of pipelines in an effective way, the field experiments of a novel coating system have been conducted in the fields in Daqing and Sichuan, China by means of an electrochemical hydrogen permeation method. It was shown from the result of the experiments in Daqing that there was no significant corrosion on the inner walls of the pipes and no degradation of coating films by detecting the much less hydrogen atoms on the outer walls of pipes. Whereas the electrochemical reaction relating to the permeated hydrogen was measured on the outer walls of the pipes in Sichuan oil field, where the hydrogen sulfide content in natural gas is about 50%. The results showed the same manner as the one in the laboratory.
In order to clarify the mechanisms of intergranular stress corrosion cracking of Alloy X750 in high temperature water, the authors have studied the relationship between the distribution of hydrogen and the microstructure of grain boundary precipitates by using electron micro autoradiographic technique. Two types of Alloy X750 with different heat treatment history were prepared. The one is sensitive tothe stress corrosion cracking in high temperature water, which has incoherent precipitates other than M23C6 discretely exist at the grain boundaries. The other has excellent resistance to the stress corrosion cracking and has coherent M23C6 precipitates which semi-continuously exist at the grain boundaries. Tritium and hydrogen were charged into these specimens at room temperature by cathodic charging method. After the charging, specimens attached by the sensitive film (monolayer of fine silver bromide particles film) were kept in a dark box for about 10 day. The distribution of silver particles on the specimen was observed by electron probe microanalyzer and transmission electron microscope. For the specimens which are sensitive to the stress corrosion cracking resistance, the silver particles existed on the grain boundaries, however, for the specimens which have excellent resistance to the stress corrosion cracking, the silver particles were not found at the grain boundaries. From this evidence, it seems that hydrogen trapping behavior at the grain boundaries affects the stress corrosion cracking resistance.
It has been established that mild steels, which undergo the general corrosion in acidic to neutral environments, attain passivity in alkaline environments, thereby becoming liable to localized corrosion such as pitting corrosion and crevice corrosion. In the view of the lack of quantitative data on the transition from general corrosion-to-passivity of mild steel in natural water environments of weak alkalinity, the present authers have determined the empirical E-pH diagram for a mild steel in the simulated ground water of 1mMol/L [HCO3-]+10ppm [Cl-] solutions at 20°C. It has been shown that (1) the transition pH from general corrosion-to passivity (pHd) was determined to be 9.4, and the mild steel was shown to be liable to localized corrosion over large portion of the passivity domain, (2) when bentonite is mixed with the simulated ground water, the pH become as high as 9.6 to 10.3 under low bentonite/solution ratio condition, which is higher than pHd of 9.4, and (3) the localized corrosion domain in bentonite suspension is wider than that in the non-Bentonite-containing solutions.
In order to investigate the influence of weld metals, i.e., LB 26, NB3N and NC 39, on the corrosion rate of the hot-dip aluminized steel in Seawater, the immersion tests of galvanic couples in artificial seawater were made. The galvanic couples consist of three specimens, i.e., carbon steel, Fe-Al alloy and weld metals, which are connected each other by lead wires. It was found that the corrosion rate of carbon steel coupled to NC 39 was less than that of carbon steels coupled to NB 3 N or LB 26. Under galvanic coupling condition with LB 26, LB 26 and carbon steel suffered from corrosion, while Fe-Al alloy did not corrode.
With the aim of studying the precursor and dynamics of delayed fracture of high tension steel, we developed two advanced elastic wave monitoring systems. One is a broad-band digital AE monitoring equipment consisting of a multi-resonant 7MHz AE sensor, a 10MHz pre-amplifier and a 200MHz A/D converter. The overall transfer function was determined as a response to a pulse YAG laser break-down of silicon placed in slit (or crack). The break down was found to simulate the Mode-I fracture with effective source rise time of 0.035μs and successfully utilized to determine the overall transfer function of the system. The waveform simulation of the monitored AE signals revealed the succession of fast and small (<10μm) cleavage cracks with source rise time of 0.07μs to 0.24μs in high hydrostatic stress field. In the 20MHz laser-ultrasonic system developed, a velocity change of spherical longitudinal (P-) wave was continuously monitored to detect the precursor of delayed fracture. Here, the P-wave was excited by pulse YAG laser so that it propagates through the hydrostatic stress field. The velocity decreased by 100m/s at 2.8ks before the first AE signal. The velocity decrease appears to be due to hydrogen induced voids. The method has potential for monitoring the precursor of delayed fracture.
Stress corrosion cracking (SCC) susceptibility of heavy-thick low alloy steels was investigated by slow strain rate tensile (SSRT) tests in high temperature water containing various levels of dissolved oxygen (DO) at temperature of 373K to 593K. Rolled steels with high sulfur content had SCC susceptibility in the wide region of temperature-DO concentration combination, even in DO<0.005ppm at 473K. A forged steel with extremely low sulfur content was almost immune to SCC. SCC maps as functions of temperature and DO concentration were proposed for each steel from the SSRT tests results.