The present work involves an investigation of aqueous corrosion of pure aluminium (Al) and its alloys using electrochemical impedance spectroscopy and electrochemical noise (EN) measurement.
Chapter III-1 is concerned with the analysis of electrochemical impedance spectra of the anodised pure Al foil used for Al electrolytic capacitors. The capacitance of an Al electrolytic capacitor is mainly dependent on the surface area of Al foil. In this regard, several articles have been devoted to the study of increasing the surface area by the formation of etch tunnels. Since the ions in the electrolyte diffuse through the etch tunnel for charge saturation of the electrolytic capacitor, the electrochemical behaviour of the Al electrolytic capacitor will be affected by the tunnel length distribution. In this chapter, for the numerical calculation of the total impedance of the electrode, the transmission line model was modified to reflect the etch tunnel length distribution representing the contributions of the etch tunnel length to the total impedance. From the agreement of the numerically calculated impedance spectra with the experimentally measured spectra, it was concluded that the deviation of the impedance spectra from the ideal capacitive behaviour in the high frequency region originates from the characteristic frequency dispersion which itself it dependent upon the etch tunnel length distribution.
In chapter III-2, the effects of sulphate, thiosulphate and hydrogensulphate ion additives on the pitting corrosion of pure Al in 1 M NaCl solution were investigated as a function of solution temperature using double current step experiment. It is generally accepted that these ion additives have an essential role in increasing the surface area of Al foil used for Al electrolytic capacitors. From the analysis of the galvanostatic potential transients obtained from the double current step experiment, it was suggested that both sulphate and thiosulphate ions retard the initiation of the etch pits, and at the same time, inhibit the passivation of the etch pits during the current interruption to promote the subsequent re-activation of the etch pits over the whole solution temperature range. In contrast, it was found that hydrogensulphate ions enhance the passivation of the etch pits during current interruption with rising solution temperature as well as the initiation of the etch pits.
Chapter IV-1 looks at the effects of sulphate and molybdate ion additives on the pitting corrosion of pure Al using EN measurement. The cumulative probability determined from the shot-noise theory was quantitatively analysed using the Weibull distribution function based upon a stochastic theory. From the statistical variation in the frequency of events, it was found that the width of the distribution of the frequency of events was broader in the presence of sulphate ions. On the other hand, the distribution of the frequency of events shifted to a lower frequency region in the presence of molybdate ions. These results revealed that both sulphate and molybdate ions retards pit initiation. Moreover, from the two linear regions in one Weibull probability plot, it was concluded that pitting corrosion was clearly distinguished from uniform corrosion in the stochastic analysis in an engineering sense.
In chapter IV-2, the electrochemical noise obtained from anodised Al in 0.1 M NaCl solution was analysed to evaluate the resistance to pitting corrosion as a function of the anodising solution and formation potential. The application of the stochastic theory to electrochemical noise provides additional information on the corrosion process than provided by the shot-noise theory alone. The occurrence of two linear regions on one Weibull probability plot indicated that this method of analysis allowed us to distinguish between pitting corrosion and uniform corrosion in an engineering sense. Moreover, from the lower pit embryo formation rate of Al anodised in boric-ammonium phosphate solution than that rate of Al anodised in boric-borate solution, it was concluded that the oxide film formed in boric-ammonium phosphate solution provides good pitting corrosion resistance compared with the film formed in boric-borate solution.
Chapter IV-3 is concerned with investigating the effects of sulphate, nitrate and sodium sulphide on the corrosion of pure Al in an alkaline solution using EN measurement. A stochastic theory was applied to the experimentally obtained electrochemical noises based upon the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, the present analysis method permitted us to distinguish between the dissolution of Al and the stochastic process concerning the series of nucleation, growth, and detachment of hydrogen bubbles in an engineering sense. Moreover, the conditional event generation rate and the average charge in each event were quantitatively determined for the stochastic process of hydrogen bubbles as well as the dissolution of Al. The effects of sulphate, nitrate and sodium sulphide additives on the corrosion of pure Al in an alkaline solution are discussed in terms of the conditional event generation rate and the average charge in each event.
In chapter IV-4, the effects of sulphate, nitrate and phosphate ion additives on the pit initiation of pure Al were investigated in a hydrochloric acid solution as a function of anion concentration using EN measurement. From the quantitative analysis of the electrochemical noise using the Weibull distribution function based upon a stochastic theory, it was found that the pit embryo formation rate increased with the addition of phosphate ions. On the other hand, the pit embryo formation rate increased with sulphate or nitrate ion concentration up to around 0.05 M and then decreased gradually with increasing sulphate or nitrate ion concentration. Moreover, from the three linear regions in one Weibull probability plot, it was concluded that pit initiation coupled with hydrogen evolution could be clearly distinguished from uniform corrosion coupled with hydrogen evolution and the stochastic process concerning the series of nucleation, growth and detachment of hydrogen bubbles in the stochastic analysis.
Chapter V-1 is devoted to the investigation of corrosion behaviour of AA2024-T4, AA7075-T651 and AA7475-T761 Al alloys in aqueous neutral chloride solution using EN measurement. This chapter serves as a valuable comparison. From the quantitative analysis of the electrochemical noise using the Weibull distribution function based upon the combined stochastic theory and shot-noise theory, it was found that pitting corrosion could be distinguished from uniform corrosion in the stochastic analysis in an engineering sense. The susceptibility to pitting corrosion was appropriately evaluated by determining the pit embryo formation rate in the stochastic analysis. The susceptibility decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition). Moreover, effect of ageing time on the pitting corrosion of AA7475 was discussed in terms of the pit embryo formation rate based upon the combined stochastic theory and shot-noise theory.
In chapter V-II, the effects of $TiO_2$, PbO and NiB additives on stress corrosion cracking (SCC) of alloy 600 were investigated using EN measurement. First, the induction time for SCC was quantitatively determined based upon the shot-noise theory. Furthermore, by employing the stochastic theory, SCC was successfully distinguished from uniform corrosion in terms of the frequency of events in an engineering sense. The results revealed that the crack propagation rate increased with increasing conditional event generation rate and the susceptibility to SCC decreased in the following order: PbO > absence of impurities ? NiB > $TiO_2$.
In the appendix, the effects of chloride ion addition to fluoboric acid solution on the pickling of the high strength hot-rolled steel were investigated using open-circuit potential transient technique and ac impedance spectroscopy. The occurrence of an open-circuit potential plateau indicates the dominant dissolution of one oxide phase in the scale. In contrast, the open-circuit potential decay results from the simultaneous dissolution of two oxide phases in the scale. Thus, the shape of the open-circuit potential transient is crucially determined by the oxide scale structure. Time to reach a steady-state value of the open-circuit potential decreased with increasing chloride ion concentration, indicating accelerated descaling. The decrease in the electrical resistance of the oxide scale estimated from ac impedance spectra also suggested that dissolved chloride ions accelerate descaling. From the experimental findings, it is concluded that the addition of chloride ions to fluoboric acid solution accelerates descaling and simultaneously impedes the subsequent dissolution of the bare steel in the pickling process.
알루미늄은 밀도가 낮고, 고강도이면서 비교적 높은 내부식성을 가지는 재료로 그 응용범위가 넓다. 이러한 세가지 주요 특성 중에서 내부식성은 주로 알루미늄이 대기 중에 노출될 때 형성되는 산화피막의 특성에 기인한다. 그러나 염화이온 같은 할로겐 이온이 다량으로 존재하는 환경에서는 산화피막이 부분적으로 파괴가 되어 pitting corrosion이라는 국부부식이 일어나게 된다.
제III장에서는 오히려 이러한 국부 부식을 이용하여 표면적을 넓힘으로써 알루미늄 전해 커패시터의 정전 용량을 늘리는 것에 관한 내용을 다루었다. 우선 제III-1장에서는 에치 터널의 길이 분포가 정전용량에 미치는 영향을 알아보았으며 제III-2장에서는 표면적을 늘리기 위하여 실제로 사용되는 $SO_4^{2-}$, $S_2O_3^{2-}$, $HSO_4^-$ 이온이 어떠한 기구에 의하여 표면적을 늘리는지를 알아보았다.
제IV장에서는 전기화학 노이즈 법을 이용하여 여러가지 환경에서의 순수 알루미늄 부식을 연구하였다. 노이즈를 분석하기 위하여 확률론적인 방법(stochastical method)을 도입하였는데 우선 shot-noise theory를 이용하여 frequency of events을 $\f_n$ 구하고 이를 누적확률분포(cumulative probability plot)로 나타내었다. 그러나 수학에서 사용되는 확률밀도함수(probability density function)는 시간 domain에서 정의되므로 $f_n$ 을 1/$f_n$로 단순변환하여 누적확률분포를 주파수 domain에서 시간 domain으로 변환하였다. 그 다음 누적확률밀도가 수명 예측에 가장 널리 사용되는Weibull 분포를 따른다는 가정 하에 Weibull 분포 함수의 모양 변수와 크기 변수 값을 실험적으로 구하기 위하여 Weibull probability plot을 구하였다. 최종적으로는 조건부 이벤트 발생 속도(conditional event generation rate)를 구함으로써 정량적으로 부식성을 평가하였다.
제IV-1장에서는 $SO_4^{2-}$ and $MoO_4^{2-}$ 이온이 중성용액에서의 pitting corrosion에 미치는 영향을 전기화학 노이즈를 이용하여 분석하였다. 분석 결과 $SO_4^{2-}$ 과 $MoO_4^{2-}$ 이온 모두 pit 의 생성을 억제하고 있음을 알 수 있었다. 또한 Weibull probability plot상에서 2개의 뚜렷한 선형 구간을 보여줌으로써 확률론적인 방법을 이용하여 pitting corrosion을 uniform corrosion으로부터 분리해낼 수 있었다.
제 IV-2장에서는 아노다이징 용액에 따른 pitting corrosion에 대한 저항성을 평가하였다. boric-borate와 boric-ammonium phosphate 의 두 가지 아노다이징 용액에서 같은 두께의 산화피막을 인위적으로 만들고 각각에서 측정된 노이즈를 확률론적인 방법을 이용하여 분석하였다. 그 결과 제IV-1장에서와 마찬가지로 Weibull probability plot상에서 2개의 뚜렷한 선형 구간을 보여줌으로써 전체 부식이 pitting corrosion과 uniform corrosion으로 나누어짐을 알 수 있었다. 또한 boric-borate보다 boric-ammonium phosphate 용액이 pitting corrosion에 더 저항성이 있는 산화피막을 형성한다는 사실을 알아내었다.
제 IV-3장에서는 전기화학 노이즈법을 이용하여 순수한 알루미늄의 알칼리 부식을 연구하였다. 알루미늄의 경우 다른 금속과는 달리 알칼리 용액에서 화학적으로 용해되고 이에 수반되는 반응으로 수소 기체가 발생하게 된다. 본 장에서는 $SO_4^{2-}$, $NO_3^-$, $Na_2S$ 가 바로 이 알루미늄의 알칼리 부식에 미치는 영향을 알아보았다. 확률론적인 분석결과 Weibull probability plot상에서 두개의 선형구간이 나타남을 확인할 수 있었는데 이는 uniform corrosion과 수소 기체 발생 반응으로 나뉘어 생각할 수 있었다.
제 IV-4장에서는 산성용액에서 $SO_4^{2-}$, $NO_3^-$, $PO_4^{3-}$ 의 첨가가 pitting corrosion에 미치는 영향을 알아보았다. 염화 이온이 들어있는 염산의 경우 Weibull probability plot상에서 3개의 선형 구간을 보여주었는데 이는 전체 부식 반응이 uniform corrosion, pitting corrosion, 수소 기체 발생반응의 3가지 반응으로 구성되어 있음을 의미한다.
제 V-1장에서는 중성의 염화 이온 함유 수용액에서 항공 구조용 재료로 널리 쓰이는 AA2024-T4, AA7075-T651, AA7475-T761 알루미늄 합금들에 대한 부식성을 평가하였다. 제IV-1, IV-2장과 유사하게Weibull probability plot상에서 2개의 선형 구간을 보여줌으로써 전체 부식 반응이 pitting corrosion과 uniform corrosion으로 이루어져 있음을 알 수가 있었다. pitting corrosion에 대한 susceptibility는 AA2024-T4 (the naturally aged condition) > AA7475-T761 (the overaged condition) > AA7075-T651 (the near-peak-aged condition) 순으로 감소하였다.
제 V-2장에서는 NiB, PbO, $TiO_2$ 불순물이 니켈계 합금인 alloy 600의 응력부식파괴에 미치는 영향을 알아보았다. Alloy 600은 높은 내식성 때문에 원자력 발전소의 증기발생기 재료로 주로 사용되고 있다. 그러나 실제 현장에서 응력부식파괴 현상이 빈번히 발생되어 원전의 원만한 가동을 방해하는 요인이 된다. 본 연구에서는 우선 shot-noise theory를 이용하여 응력부식파괴가 시작되는 시간(induction time for stress corrosion crack initiation)을 정량적으로 결정할 수 있었다. 또한 확률론적인 분석 방법을 통해 Weibull probability plot상에서 uniform corrosion과 stress corrosion crack propagation의 2개의 다른 부식 기구가 존재한다는 것을 확인할 수가 있었다. 또한 조건부 이벤트 발생 속도에 근거한 crack 전파 속도를 정량적으로 구하여 각 불순물의 영향을 알아보았다. 실험 결과 NiB는 별다른 효과가 없었고 PbO는 응력부식파괴를 촉진하였으며 $TiO_2$ 는 응력부식 파괴를 억제하였다.
