Reinforcement corrosion occurred by chloride ion is one of the main severe deterioration problem in concrete field especially in marine concrete structures. Reinforcement is naturally protected by high alkaline environment available in concrete pore structure; however, the diffusion of chloride ions into concrete reduces the hydroxyl to chloride ion ratio so that the threshold level is exceeded by penetrated chloride ions. Corrosion is a process of metal dissolution at anodic region of the corrosion cell while the cathodic region is connected to the anode to complete the corrosion cell which is necessarily an electrochemical reaction. Depending on how physically separated the corrosion cell is, it is called the macro-cell or micro-cell corrosion. Anodic and cathodic regions are significantly separated to make a macro-cell and this particular separation is occurred mainly by the electrochemical potential imbalance along the steel in concrete. Potential imbalance is generated by many factors; the difference of chloride ions on the steel surface is one of most significant factors. Macro-cell corrosion is severe than the general uniform corrosion because of the small anodic area of the corrosion cell can results a rapid deterioration.

There are several methods to assess reinforcement corrosion or the probability of corrosion. Chloride ion content at steel bar level, moisture content, concrete resistances etc., are used to identify the possibility of corrosion however they are not enough to judge firmly the degree of the ongoing corrosion process. To take the decision about repairing or some other assessment related to corrosion, approximately accurate magnitude and location of active corrosion is necessary. To achieve above, widely used electrochemical measurements; half-cell potential and the polarization resistance data can be obtained. Half-cell potential measurements itself does not provide the corrosion rate but the probability of corrosion. However, the polarization resistance measurements can directly be converted to the corrosion rate based on electrochemical theories.

Though it is explained that electrochemical measurements can be directly used to estimate the corrosion process, the output is highly dependent on the ongoing macro-cell corrosion along the steel bar. Once the macro-cell corrosion is activated, the cathodic reaction of the corrosion cell misleads the directly measured electrochemical measurements so that the reliability of the output is significantly low; this phenomenon is introduced as the cathodic polarization effect. The main objective of this research is to experimentally observe the cathodic polarization effect by direct electrochemical measurements when macro-cell corrosion along the steel bar is significantly active and to prove it by indirect measurements taking separate macro-cell and micro-cell corrosion in to account. Objective is further extended to the identification of net and gross macro-cell current conceptually as well as experimentally as the cathodic polarization effect can be explained then as author explained.

The polarization effect of cathodically active area (cathodic polarization effect) induced by anodic reaction is identified at both major electrochemical measurements; half-cell potential and the polarization resistance. Therefore, an active corrosion process is detected by external measurements even at the locations with zero amount of chloride ion in concrete. However, as expected, the concrete resistance is not affected by this polarizing effect because the concrete resistance does not directly represent the corrosion process. The total corrosion current converted by direct electrochemical measurements (polarization resistance) further clarifies and confirms the effect of cathodic polarization. To confirm the effect and to show its significance, total corrosion currents are evaluated by indirect method where it is the summation of macro-cell and micro-cell current. Macro-cell corrosion current flow from each and every steel element in the steel bar is calculated by models. Micro-cell corrosion currents are determined by the direct polarization measurements obtained for segmented steel bar when steel elements are at disconnected situation. The summation of the anodic currents of macro-cell corrosion calculated by models and the currents of micro-cell corrosion produces the calculated total corrosion. Above calculated total corrosion currents are compared with the total corrosion currents obtained by direct electrochemical measurements to prove the effect of cathodic polarization and its significance on the total corrosion output.

Specially prepared segmented steel bar is used (only for experimental purpose) to measure the macro-cell current flow between elements. Macro-cell currents which are experimentally measured and calculated by models are compared to check the accuracy of each method. The tendency proves that models are compatible up to some extent. However, currents by segmented steel bar are significantly deviates compared to models.

To understand the macro-cell corrosion mechanism and to explain the effect of cathodic polarization, a detailed analysis of corrosion is necessary. For this aim, an effective use of the segmented steel bar is necessary. It is conceptually described that directly measured macro-cell corrosion currents represents the net corrosion current which is a combination of both anodic and cathodic corrosion currents. Hence the concept and the importance of gross corrosion currents are explained in detail. To identify gross corrosion current, the measurement of polarization curves are adopted. However, results proves that even a pure macro-cell cathodic current at cathodic element change its own anodic polarization curves which is totally out of the expectation. The result is enough to express about the complexity of the cathodic polarization phenomenon; also, result brings a new concept about the ability to separate macro-cell and micro-cell corrosion for experimental purpose which is different from real situation.

The clarification of the concept of net and gross macro-cell corrosion current is essential to solve the cathodic polarization phenomenon. To achieve the target, macro-cell corrosion current flow is modeled with respect to the distance between steel elements and the chloride content in concrete. To model, the cathodic to anodic area ratio and the relevant current flow are used and the model is carried out considering different ratios. This model is used to simplify complex macro-cell current flow along the steel bar; an example of solution is provided. Simplified macro-cell current flows are to be used for the evaluation of net and gross corrosion currents.

Discussions and conclusions throughout this study are made based on the segmented steel bar and electrochemical measurements. It is proved that the segmented steel bar does not perfectly reflect the real situation compared to model output of continuous bar assuming that electrochemical processes are equally produced. It is explained, in this study, technological facts about measurements, its accuracy and the process of the segmented steel bar and it is still to be used for the further clarification of the cathodic polarization effect which is considered as the driving factor for future succeeding research studies.

鉄筋コンクリート(RC)構造物中の鉄筋が腐食すると,かぶりコンクリートにひび割れが発生し,発生したひび割れの影響によって腐食速度が加速し,さらに腐食量の増大にともない部材の耐力が低下していく.特に塩化物イオンの侵入にともなう鉄筋腐食(塩害劣化)は,深刻な劣化を引き起こす劣化機構の一つとして,これまでにも多くの研究が行われている.塩害劣化に関する既往研究は,塩化物イオンの侵入に関する研究,鉄筋腐食に関する研究,腐食したRC部材あるいは構造物の耐力低下に関する研究に大別される.ここで,鉄筋腐食に着目すると,腐食機構に基づいた腐食予測モデルの構築や,実構造物の腐食状況を把握するための手法の開発などがあるが,膨大な既存ストックを抱えるわが国においては,実構造物の腐食状況を適切に把握することが望まれている.鉄筋の腐食が電気化学的な現象であることに着目し,自然電位法を用いて腐食の可能性を判定する方法が古くから適用されており,最近では,分極抵抗法を用いて腐食速度を予測する手法も広く適用され始めている.塩害劣化にともなう鉄筋腐食は,ミクロセル腐食とマクロセル腐食が共存しているが,特にマクロセル腐食はアノードとカソードが1m程度離れていても回路を形成する複雑な腐食機構である.このため,マクロセル腐食は,連続的な鉄筋のある限定的な領域をコンクリート表面から計測する電気化学的な計測手法の結果に影響を及ぼし,実際の腐食状況を把握することを困難としている.本論文は,マクロセル腐食が電気化学的な測定結果に及ぼす影響を,様々な計測および予測手法を用いて明らかとするとともに,その影響を排除する可能性に関して検討したものである.

1章は序論であり,研究の背景,目的を述べた後に,本研究の構成を記述している.

2章は既往の研究であり,RC中の鉄筋腐食の機構,各種電気化学的手法,電気化学的計測結果を用いたマクロセル腐食の予測手法,およびマクロセル腐食を直接計測するために提案されている分割鉄筋に関する既往研究を整理し,各々の特徴を明らかとしている.

3章では,マクロセル腐食が電気化学的計測結果に及ぼす影響について検討している.実験では,実構造物レベルを想定し1m程度離れたカソードとアノードの腐食回路が計測結果に及ぼす影響を把握するために,2mのRC試験体を通常鉄筋および分割鉄筋を用い,アノード領域を制御するために塩分の混入領域と量を変化させて作製している.これらの試験体を用いて,自然電位法および分極抵抗法の結果から,非腐食部(カソード)の測定結果が腐食部(アノード)の腐食程度の増加にともない,自然電位の値が卑に,分極抵抗の値が小さく(腐食速度が増加)なる,すなわち,計測結果が実際の腐食状況とかけ離れていることを実験的に明かとしている.さらに,分割鉄筋の試験体を用い,実験的に計測したミクロセル腐食量と3種類の方法(電気化学的測定結果を用いたマクロセル腐食量の予測(2種),分割鉄筋を用いた直接計測)によって求めたマクロセル腐食量の和と,電気化学的計測によって全腐食量を推測した結果の比較から,非腐食部(カソード)におけるマクロセル腐食によるカソード分極現象が,測定結果をミスリードしている可能性があることを指摘している.

4章では,カソード分極現象を明らかとするために分極曲線を計測し考察を加えている.自然電位法,分極抵抗法および分割鉄筋を用いて直接電流を計測する手法では,実際に流れている電流(グロス電流)ではなく,ネット電流しか計測できずカソード分極現象を詳細に理解することができないことを指摘し,分極曲線の計測からグロス電流を把握することを試みたが,分極曲線の計測もグロス電流を把握することはできないことを実験的に明かとしている.

5章では,グロス電流が実測不可能であることを踏まえ,グロス電流を予測可能なマクロセル腐食のモデル化を試みている.分割鉄筋を用いてアノードおよびカソードの各要素間に流れる電流を計測し,アノードとカソードの距離と塩分濃度の関数として,要素間に流れる電流のモデル化を試みたが,分割鉄筋を用いた計測に基づくモデルもグロス電流を把握することが困難であることを明らかとしている.

6章は結論であり,本研究で得られた結果のまとめと今後の展開を記述している.

以上を要約すると,1)マクロセル腐食が発生すると,既往の電気化学的測定手法による計測数値がカソード分極現象の影響を受けるために腐食現象を正確に評価することが困難となること,2)これを解決するためにはグロス電流を把握することが不可欠であること,の二点を明らかにした.さらに,既存の各種手法ではグロス電流を把握することが極めて困難であることを,計測手法の原理・精度,分割鉄筋の特徴などを踏まえて言及したものである.本研究は,既往の腐食計測手法の問題点,理論的な限界点などを指摘し,特定の理想的な条件のもとでの計測と実構造物中の鉄筋腐食との関連を明示したものであり,今後のマクロセル鉄筋腐食の機構解明に向けた道標が与えられている.よって,本論文は博士(工学)の学位請求論文として合格と認められる.