The conventional a.c.impedance technique only provides polarization resistance inversely proportional to the corrosion rate. The proportionality constant has to be determined in an independent experiment. However, the harmonic components appearing in a.c. impedance measurments reflect nonlinear polarization characteristics of the corrosion system under study. By means of barmonic analysis, corrosion current could be obtained directly and Tafel slopes could be determined simultaneously. In the present paper, the basic formulas based on power series expansion for manipulation of harmonic data are derired. Mild steel corroding in 0.5N H_2SO_4 was chosen for experimental verification. The first three harmonic components are measured by a frequency response analyzer with harmonic selector. The measured data have been corrected for IR drop due to solution resistance. The shunting effect of the double layer capacitance has also been eliminated by subtracting the capacitive current from the first harmonic component. The effect of frequency of a. c. signal is discussed. The results show that corrosion rate determined by harmonic analysis correlates well with those from a.c. impedance and weight loss measurements. Tafel slopes obtained are in good agreement with those measured from polarization curves.

As an expansion of our previous work, harmonic data manipulation with Fourier series expansion has been carried out for the simultaneous determination of instantaneous corrosion rate and Tafel slopes of corrosion system. The Fourier series coefficients are calculated by computer numerical computation. The proposed method does not imply any restriction for the amplitude of the stimulating a. c. signal. Close examination of relation between the amplitude of a. c. signal and corresponding harmonic components shows that with increasing a. e. signal, all harmonic components increase while the second and third harmonic components increase much more quickly than the first. The advantage of Fourier series expansion method over power series expansion method is that under larger a. c. signal accurate measurement of the corresponding second and third harmonic components become easy. Using an appropriate computer program which combines two subroutines for the Fourier series expansion and for the solution of simultaneous equations, three unknown parameters of corrosion system—corrosion current, anodic and cathodic Tafel slopes—are obtained from the first three harmonic current components. Experimentl verification of the theoretical considerations is shown with mild steel in 0.5N H_2SO~4.

Electrochemical method was used to study the effects of the passive film, polarization potential and the pH value of the solution on the permeation behavior of hydrogen in a stable austenitic stainless steel 310. The diffusion coefficient of hydrogen at 25℃ in this steel was determined to be(1.2+0.1)×10~-8cm~2/sec,and that of the passive film was estimated to be about 10~(-17)cm~2/sec. The saturated hydrogen permeation current increased with the decrease of the pH value of the solution; it also increased not only with the increase of cathodic polarization potential, but also with the increase of anodic polarization potential if it is above the breakdown potential of the passive film. The anodic polarization curves of the hydrogen charged specimens in 0.1N H_2SO_4 solution were shifted to the right, i. e., hydrogen increases the anodic reactivity of the austenitic stainless steel. Furthermore, hydrogen charged solution treated specimens exhibited a tendency toward intergranular corrosion.

The stress corrosion cracking(SCC)process of 18-8Ti and 17-12Mo stainless steel in boiling 42％ MgCl_2 solution was studied by using constant load tests. The endurance creep behavior of these steels was examined. Simulated galvanic cell tests and stress corrosion tests using pre-creeping specimens were conducted. The results suggested that SCC of austenitic stainless steels in boiling 42％ MgCl_2 solution results from synergism and competition between deformation-enhanced activation and repassivation of surface films. It is rather difficult to divide the cracking process into essentially different periods, The induction periods obtained with various parameters tend to be inconsistant At the early stage of crack propagation period(tp), it is the number of cracks that increases mainly, after which cracks propagate along the length and depth. The tp-σ(applied stress)relation seems to follow an exponential law. The SCC of both steels is transgranular(TC)-intergranular(IC); TC is predominantly observed in 18-8Ti steel, while IC prevails in 17-12Mo steel.

The simultaneous oxidation and sulfidation of chromium and nickel-chromium alloys was studied at 800 C in the atmospheres of ps_2=1×10~(-6) atm. and po_2=5×10~(-21)-5×10~(-20) atm., which are within the Cr_2O_3 stability field and just beside the Cr_3S_4-Cr_2O_3 equilibrium boundary of Ellingham diagram for Cr-O-S. It was found that, at the initial reaction stage, Cr_3S_4 was formed along with Cr_2O_3 and scales were in the form of texture strueture. Aninereasein the oxygen partial pressure or a decrease in chromium content in the alloys restrained the development of the texture structure. The reaction mechanism is discussed.

Electrochemical modulation spectroscopy(ECMS)is a relatively new tool for the study of electrode surface. A model DTG-1 multi-purpose ECMS instrument has been designed and constructed in our laboratory. A discriptiot of the instrument systems and performance characteristics of this instrumenn is given.

Hydrogen embrittlement of stable austenitic stainless steel Cr20Ni25 has been investigated. Crack formation and phase transformation on the surface of the specimens have been observed. It is shown that electrolytic hydrogen charging promotes the transformation of e-martensite on the surface of specimens, the formation of surface crack and the formation of new fcc phase. Transgranular cleavage fracture is shown for the charged layer in tensile specimen. The crack propagates along slip face.The present study suggests that stable austenitic stainless steel is also subject to hydrogen embrittlemt. The transformation of α′-martensite may be not necessary for hydrogen embrittlement of austenitie stainless steel.

Influence of laserglaze on the microstructure and corrosion resistance of a cast nickel-base superalloy has been studied, which contains very high contents of refractory metals(Mo 33 Wt％, W 17 Wt％). After surface treatment by laserglaze the alloy structure changes extraordinarily. This change of microstruc-ture has beneficial influence on some of the alloy Properties. Hardness of the laser melted region is evidentiy increased, and corrosion resistance is also improved.

A study of corrosion of Zn-plating and Cd-plating layers caused by organic chemical atmosphere is summarized in this paper. The main corrosive ingredients volatilized from organic materials and the corrosion products have beenidentified. Effects of some factors, such as ingredients and density of the organic atmosphere, humidity and temperature of the environment etc. on corrosion have been investigated.