COMPARISON OF HALF-CELL POTENTIALS AND CORROSION RATE MEASUREMENTS – A FIELD EXPERIENCE WITH EVALUATION OF REINFORCEMENT CORROSION
半電池電位與腐蝕速率測(cè)量方法的比較-現(xiàn)場(chǎng)評(píng)估鋼筋腐蝕狀況的經(jīng)驗(yàn)
Thomas Frølund, COWI A/S, Parallelvej 2, DK-2800 Lyngby, TDF@cowi.dk
Oskar Klinghoffer, FORCE Technology, Park Allé 345, DK-2605 Broendby,
osk@force.dk
KEYWORDS: Half cell potential (HCP), Corrosion rate (Icorr), Galvanostatic Pulse Measurements (GPM), concrete structures
關(guān)鍵詞：半電池電位（HCP）,腐蝕速率（Icorr），Galvanostatic脈沖測(cè)量（GPM），混凝土結(jié)構(gòu)
1. BACKGROUND
1、背景
Since 1978 Half Cell Potential (HCP) mapping [1] has been used for detecting corroding areas on concrete structures in Denmark. In the beginning (after balcony gangway has totally collapsed) this method was mainly used on carbonated structures and balconies exposed to de-icing salts. Later the method was used for all kind of structures and the experiences were discussed in Newsletters published by the Danish Corrosion Centre [2]. It was early recognised that the interpretation of the HCP results were difficult or misleading in wet and semi-wet structures where lack of oxygen as well as corrosion would lead to potential gradients.
早在1978年，丹麥就開始使用半電池電位圖法來確定混凝土結(jié)構(gòu)的被侵蝕范圍。一開始，這種方法主要用于碳化混凝土結(jié)構(gòu)和容易受到除冰鹽侵蝕的陽臺(tái)檢測(cè)使用，后來，這種方法推廣用于所有的結(jié)構(gòu)，而且使用經(jīng)驗(yàn)在丹麥腐蝕中心的刊物上做了討論。對(duì)于缺少氧氣而且潮濕或者半潮濕的混凝土結(jié)構(gòu)，腐蝕會(huì)導(dǎo)致產(chǎn)生電勢(shì)差，半電池電位結(jié)構(gòu)的解釋很難，半電池電位的結(jié)果很難解釋或者很容易引起誤解。
A typical potential map of a highway bridge pillar is shown in fig.1. The pillar is exposed to de-icing salts splashed from the passing cars up to a level of 2 meters, but also has a high humidity at the ground level caused by capillary suction. The water filled pore system in the concrete makes the potential drop because the oxygen necessary to maintain the passive film will not be able to diffuse into the concrete fast enough.
圖1顯示了一個(gè)高速公路橋梁柱子的典型電位圖，2米高的柱子暴露于容易受到過往車輛飛濺冰鹽的環(huán)境，而且由于虹吸作用，地表處的濕度很大，混凝土孔隙充滿了水，使得保養(yǎng)必須的鈍化薄膜不能夠很快的彌散充滿到混凝土結(jié)構(gòu)里，導(dǎo)致混凝土結(jié)構(gòu)的電勢(shì)下降了。
Fig.1. Typical potential map of a high way concrete pillar exposed to de-icing
salts
The ongoing corrosion process is described by the chemical reaction:
2Fe+O2+2H2O↔2Fe+++4OH-
The Fe++ forms at the high pH in concrete a complex protective film with oxygen. This film can be broken down by the chloride ions from de-icing salts or by neutralising of the high pH e.g. by carbonation.
The interpretation problem is that the potential will drop either because the loss of passive film due to lack of oxygen (the corrosion process will not be able to proceed without oxygen) or because chlorides break down the passive film and start corrosion.
正在進(jìn)行的腐蝕進(jìn)程可以用下面的化學(xué)反應(yīng)公式來表述：
2Fe+O2+2H2O↔2Fe+++4OH-
高PH值的鐵質(zhì)氧化后行成復(fù)合鈍化膜，這鈍化膜會(huì)由于除冰鹽的氯離子或者碳酸根離子中和作用而被破壞掉。
很難解釋的是由于缺少氧氣導(dǎo)致的鈍化膜損壞同樣會(huì)使得電勢(shì)降低，而氯離子同樣會(huì)破壞掉鈍化膜導(dǎo)致鋼筋開始腐蝕。
In 1994 it was decided to develop an equipment for concrete structures based on well know techniques for determination of corrosion rates to be able to distinguish between active corrosion and the lack of oxygen situation.
1994年，決定開發(fā)一種成熟先進(jìn)的設(shè)備用于確定混凝土結(jié)構(gòu)的腐蝕速率，而且能夠區(qū)分開是活躍的腐蝕狀態(tài)還是缺少氧氣的情況。
The results presented in this paper are all based on Galvanostatic Pulse Measurements (GPM). This polarisation technique makes it possible within a short time (typically 10 seconds) to calculate the corrosion rate [3,4]. The equipment gives both the corrosion rate and the half-cell potential as well as the resistance between the hand-held electrode placed on the examined concrete surface and the reinforcement. Four different examples from on-site investigations are described below, one where there is a good correlation between the HCP method and the GPM method and 3 where the HCP measurements are misleading.
這篇論文所呈現(xiàn)的結(jié)果都是用Galvanostatic脈沖法測(cè)量得到的。極化技術(shù)使得能夠在10秒的短時(shí)間內(nèi)計(jì)算腐蝕速率。這儀器除了可以測(cè)試腐蝕速率外，還能測(cè)試半電池電位，還有放置于所測(cè)混凝土表面電極與鋼筋間的電阻率。4個(gè)不同的現(xiàn)場(chǎng)測(cè)試案例將會(huì)在下文進(jìn)行闡述，其中一個(gè)案例半電池電位(HCP)結(jié)果與脈沖電流測(cè)試（GMP）結(jié)果具有很有的相關(guān)性，其他三個(gè)案例半電池電位測(cè)量結(jié)果則容易讓人誤解。
2. EXAMPLES OF ON-SITE INVESTIGATIONS
2、現(xiàn)場(chǎng)測(cè)試案例
2.1 Example no. 1
2.1 案例一
Two parallel bridges were built in 1965-67 in the Copenhagen area, where the highway crosses over a railway line, a parking lot and two minor roads. The eastern bridge was rehabilitated extensively at a very high price in 1978, after which the western bridge have only received much less rehabilitation, but substantial inspection, test- loadings, probabilistic assessment etc., which essentially have kept the bridge in function at a much less cost that the eastern part.
1965-1967年，哥本哈根區(qū)建造了兩座平行的橋，公路線跨越了鐵路線，有一個(gè)停車場(chǎng)和兩條支線公路。1978年，東面的橋梁進(jìn)行了費(fèi)用昂貴的大修，而西面的橋梁經(jīng)過檢查測(cè)試評(píng)估后還比較牢固，所以只采用了費(fèi)用較低的小幅簡(jiǎn)單修復(fù)。
Initial inspections, core investigations and chloride profiling in 1999 (fig. 2) pointed out column No.S303 to be suitable for corrosion rate measurements.
1999年，進(jìn)行了取芯和氯離子含量測(cè)試（圖2），說明了No.S303圓柱體適合進(jìn)行腐蝕速率測(cè)試。
Fig 2. Chloride profiles at level 0.3m and level 1 m
圖2.在0.3米和1米處的氯離子含量情況
Electrical continuity in the reinforcement was checked and a permanent connection was welded to the reinforcement. The vertical reinforcement (Ø35 mm) is typically in 60 mm depth and the horizontal (Ø14 mm) in 40 mm depth. Already in 1999 the chloride content in level 0.3 m is so high that active corrosion can be expected. In September 2000 and in April
2001 corrosion rates were determined together with the half- cell potentials and resistance measurements [5] (fig 3).
設(shè)備其中一端固定連接在鋼筋上，并施加連續(xù)的脈沖電流。垂直的鋼筋（Ø35 mm）在60mm深處，水平鋼筋（Ø14mm）在40mm深處。在1999年，0.3米深處的氯離子含量已經(jīng)非常高了，應(yīng)該會(huì)存在積極的腐蝕情況。2000年的9月份和2001年的4月份，利用半電池電位和電阻率測(cè)試方法確定了腐蝕速率。（圖3）
In this case there is a rather good correlation between resistance, half-cell potential and corrosion rate mapping.
在這個(gè)案例里，腐蝕速率圖、電阻率、半電池電位存在著很好的相關(guān)關(guān)系。
To verify the corrosion state a break-up was made at the position 90 degrees south near ground level, see fig. 4.
為了驗(yàn)證腐蝕情況，在南面90度的方向，打開了靠近地面位置的結(jié)構(gòu)。
Fig. 4 Corroding reinforcement. Cross section loss: 1-2 mm
圖4.被腐蝕的鋼筋，截面損失約為1-2mm
As the constructions have been examined close during the last 20 years it is possible to make a good estimate of the initiation of corrosion. Calculation of the average corrosion rate from the cross section loss of app. 2 mm and assuming the corrosion was initiated after app. 10 years gives an average corrosion rate of 9µA/cm22, which is with in the range of corrosion rates determined at this position by the GPM. The very low half-cell potentials agree with the high corrosion activity.
在過去的20年時(shí)間里，由于建筑物進(jìn)行了非常精密的檢查測(cè)試，因此有可能對(duì)剛開始的腐蝕進(jìn)行比較好的評(píng)估。計(jì)算得出的截面平均腐蝕速率接近2mm，假定差不多10年后平均腐蝕速率達(dá)到9µA/cm2，此時(shí)將開始腐蝕。該區(qū)域的腐蝕速率范圍是由GPM方式測(cè)試得出的。非常低的半電池電位適合于高活躍的腐蝕狀態(tài)測(cè)試。
2.2 Example 2
2.2 案例2
The next two examples are from a bridge foundation and a bridge deck in Greenland. The foundation was investigated in the tidal zone as shown in fig. 5 [6].
接下來的兩個(gè)案例是在格陵蘭島的一座橋的橋墩和橋面板。調(diào)研的橋墩處于潮間帶，見圖5.
Fig. 5. The investigated area and the location of the chloride profile.
圖5. 調(diào)研區(qū)域與氯化物剖面位置
The chloride concentration in the depth of the reinforcement is in the range between 0,3% and 0,7% of the concrete weight. As indicated by the half-cell potential measurements corrosion should therefore be expected. However the measured corrosion rates are low and the verification by visual inspection (fig. 7) shows no damage to the reinforcement.
鋼筋處的混凝土氯含量大概是0.3%~0.7%之間。根據(jù)半電池電位測(cè)量，應(yīng)該存在腐蝕情況，但是，測(cè)量得出的腐蝕速率非常地，而且通過直觀觀察鋼筋，沒有發(fā)生銹蝕或者破損。
Fig. 6 HCP and Icorr of bridge foundation 橋墩的半電池電位與腐蝕電流
Fig. 7 Photo of break-up
圖7 打開結(jié)構(gòu)后的圖片
2.3 Example 3
2.3 案例3
The bridge deck from the same bridge in Greenland showed very different results as shown on fig. 8 and fig. 9.
圖8與圖9顯示了格陵蘭島同一座橋的橋面板完全不同的結(jié)果。
Fig.8. The investigated area and the location of the chloride profile.
The typical dept of the reinforcement is here minimum 40-50 mm and the chloride concentration in this depth is near 0, 3% of the concrete weight. At this high chloride concentrations the half-cell potentials are expected to be low but the most negative values measured are all above -100 mV vs. Ag/AgCl (fig. 9).
當(dāng)鋼筋處于保護(hù)層內(nèi)至少40-50mm深處，而且氯離子含量接近0.3%時(shí)，半電池電位應(yīng)該是比較低的，但是用Ag/AgCl參比電極測(cè)出來的電位值都在-100 mV以上(圖9)。
Fig.9. The read circle indicates the location of the chloride profile and the break-up shown at fig. 11.
圖9.圓形顯示了氯化物輪廓面 圖11是打開結(jié)構(gòu)的圖像
The corrosion rate map fig.10 shows a completely different picture and indicates active corrosion at several locations.
圖10的腐蝕速率圖顯示了完全不同的圖片和指明了幾處腐蝕活躍的區(qū)域。
Fig. 10. Corrosion rate of the bridge deck. The read circle indicates the location of the
chloride profile and the break-up shown at fig. 11
圖10 橋面板的腐蝕速率，圓形指出了氯化物輪廓面 圖11則是打開結(jié)構(gòu)后的圖片
Fig.11. The first picture shows the corroded reinforcement and some water from cooling
the diamond-cutting blade. The second picture shows the damage and the mortar repairs.
圖11.第一張照片顯示了銹蝕的鋼筋和一些冷卻金剛石切割鋸片的水。第二章照片顯示了橋面板的破壞情況和砂漿修補(bǔ)情況。
General comments to examples 2 and 3
案例2與案例3的概要注解
This bridge is located in a very cold environment. During the measurements described above the temperature was 15 °C at midday and this explains the rather high corrosion rates at the bridge deck. Further there were some damages to the concrete surface due to the traffic directly on the concrete surface and a lot of mortar repairs.
這座橋處于非常寒冷的環(huán)境里，測(cè)量過程中，正午的溫度是15 °C。在橋面板上會(huì)有更高的腐蝕速率，由于交通荷載直接作用在混凝土表面上，所以混凝土表面受到有更多的損害和存在很多的砂漿修補(bǔ)處。
2.4 Example 4
2.4 案例4
In this example of swimming pool wall the conditions for performing corrosion rate measurements were not ideal. Tiles covered the inside of the swimming pool but preliminary performed GalvaPulse measurements showed that the joint filler was porous. Due to this fact it was possible to conduct the corrosion rate measurements by means of GalvaPulse equipment. The outside reinforcement was corroding at the casting joint between the pool floor and the pool walls and it was found necessary to investigate the inside reinforcement although no rust stains were visible.
案例里的游泳池池壁的腐蝕速率測(cè)試環(huán)境不理想。游泳池內(nèi)壁都采用瓷磚覆蓋，但是最初使用GalvaPulse的測(cè)試結(jié)果顯示接縫料多孔滲水。基于這樣的事實(shí)，使用GalvaPulse測(cè)試腐蝕速率是可行的。游泳池池底與池壁接縫處的外部鋼筋已經(jīng)被腐蝕了，即使看不見銹斑，內(nèi)部鋼筋也很有必要進(jìn)行量測(cè)。
The results were projected to a plane plot where the cast joint is in the centre of the plot fig. 12.
測(cè)試結(jié)果反映在一張平面圖上，圖12的中央是結(jié)合處位置。
圖12 沿著池底經(jīng)過結(jié)合處再到池壁的腐蝕速率圖
Even at these un-ideal measuring conditions the GalvaPulse pointed out the reinforcement corrosion points which was confirmed by visual inspections in breakups fig. 13.
即使在不理想的測(cè)試環(huán)境里，GalvaPulse也能準(zhǔn)確指出鋼筋腐蝕地點(diǎn)，圖13的實(shí)際照片證明了這點(diǎn)。
3. CONCLUSIONS:
3、結(jié)論：
1. Two techniques for evaluation of reinforcement corrosion, half-cell potential (HCP) and galvanostatic pulse measurements (GPM) are presented and discussed.
1、目前有兩種評(píng)價(jià)鋼筋腐蝕的技術(shù)，半電池電位（HCP）和Galvanostatic脈沖電流測(cè)試（GPM）。
2. The evaluation of corrosion by means of the traditional half-cell potential technique using the existing standards may lead to mistakes in cases where the concrete is water saturated, carbonated and also exposed to the very low temperature.
2、在低溫、碳酸化作用和飽水環(huán)境下的混凝土結(jié)構(gòu)，如果只用目前標(biāo)準(zhǔn)的半電池電位方法進(jìn)行腐蝕評(píng)估的話，容易導(dǎo)致錯(cuò)誤的結(jié)果。
3. Complimentary measurements by means of galvanostatic pulse technique determining the corrosion rate contribute to the unambiguous evaluation of reinforcement corrosion also under conditions where the results obtained by the HCP technique could be misleading.
用Galvanostatic脈沖電流測(cè)試（GPM）測(cè)試腐蝕速率是一種令人贊賞的測(cè)試方法，即使在用HCP半電池電位方法容易獲得不準(zhǔn)確結(jié)果的情況下，也能很明確的評(píng)估鋼筋的腐蝕情況。
4. Four examples from on-site measurements are presented. Three of them show the need of using corrosion rate measurements together with half-cell potential for reliable evaluation of the actual corrosion state.
文中介紹的四種現(xiàn)場(chǎng)測(cè)試的案例，其中三個(gè)案例說明了要正確評(píng)估腐蝕狀況，除了使用HCP半電池電位法外，同時(shí)進(jìn)行腐蝕速率測(cè)試是非常必要的。
5. Passive areas are defined by galvanostatic pulse measurements as areas where the potential curve on the instruments computer screen has not reached a steady-state after pulsing over 5-10 seconds. In areas with active corrosion, areas where the potential curve exhibit a steady-state potential after 5-10 seconds, the corrosion current is measured as accurate as it can be expected from an on-site measurement taking into account the variation of the area of the reinforcement polarized over, the actual corroding area of the reinforcement and the inherent variations in moisture condition of the concrete and the temperature.
使用GPM方法對(duì)一個(gè)區(qū)域脈沖時(shí)間超過5-10秒后，儀器電腦屏幕上顯示的電壓曲線到達(dá)不了一個(gè)穩(wěn)定不變的水平時(shí)，說明了該區(qū)域是腐蝕不活躍區(qū)域。在腐蝕活躍區(qū)域，施加脈沖時(shí)間5-10秒后，電壓曲線將會(huì)達(dá)到穩(wěn)定，此時(shí)測(cè)量的腐蝕電流會(huì)被精確測(cè)量�，F(xiàn)場(chǎng)測(cè)量時(shí)應(yīng)該重視鋼筋極化區(qū)域的變化，實(shí)際的鋼筋腐蝕區(qū)域和潮濕混凝土環(huán)境下固有的溫度變化。
6. It is not possible to estimate the actual loss of cross sectional area of the reinforcement from a single GPM measurement. If multiple GPM measurements are taken over a period of time, an average value can be estimated. Alternatively the reinforcement must be exposed in the most corrosion active areas as done in these 4 examples.
在鋼筋交叉組合區(qū)域僅僅使用一次GMP測(cè)量來評(píng)估實(shí)際損失是不可能和不足夠的。如果在一段時(shí)間內(nèi)進(jìn)行多次GPM測(cè)量，利用平均值就可以進(jìn)行評(píng)估了。測(cè)量過程中，就像在4個(gè)案例中，鋼筋必須暴露于腐蝕活躍區(qū)域。
4. REFERENCES:
[1] American Society of Testing and Materials: “Standard Test Method for Half-Cell Potentials of uncoated Reinforcing Steel in Concrete” ASTM C876, 1987.
[2] H. Arup: "Potential Mapping of Reinforced Concrete Structures", The Danish Corrosion Centre Report, January 1984
[3] B. Elsener, O. Klinghoffer, T. Frølund, E. Rislund, Y. Schiegg and H. Böhni: "Assessment of Reinforcement Corrosion by Galvanostatic Pulse Technique, Proc. Int. Conf. on Repair of Concrete Strictures, Svolvaer, Norway, pp 391 - 400,1997.
[4] J. Mietz and B. Isecke: "Electrochemical Potential Monitoring on Reinforced Concrete Structures using Anodic Pulse Technique", in "Non destructive Testing in Civil Engineering" ed. Bungey, H., The British Institute of NDT, 2, 567,1993.
[5] T. Frølund. F.M. Jensen and R. Bässler: "Determination of corrosion rate by means of the galvanostatic pulse technique”, First International Conference on Bridge Maintenance, Safety and Management, IABMAS 2002, Barcelona 2002.
[6] H. E. Sørensen and T. Frølund: "Monitoring of Reinforcement Corrosion in Marine Concrete Structures by the Galvanostatic Pulse Method", Proceedings of International Conference on Concrete in Marine Environments, Hanoi-Vietnam, October 2002.
