影响腐蚀的结构因素.ppt

第二章影响腐蚀的结构因素
第三节异种金属组合因素
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Dissimilar metal corrosion results from the coupling of two different metals in a corrosive electrolyte. It is also called bimetallic corrosion or galvanic corrosion.
When a galvanic couple forms, one of the metals in the couple es the anode and corrodes faster than it would all by itself, while the other es the cathode and corrodes slower than it would alone.
异种金属彼此接触或通过其他导体连通,处于同一个介质中,会造成接触部位的局部腐蚀。其中电位较低的金属,溶解速度增大,电位较高的金属,溶解速度反而减小,这种腐蚀称为电偶腐蚀,或称接触腐蚀、双金属腐蚀。
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This is one of the mon forms of corrosion as well as one of the most destructive. Here’s a classic example of galvanic corrosion; a stainless screw in contact with a cadmium plated steel washer.
电偶腐蚀是最常见的腐蚀形式之一,也是最具破坏性的腐蚀之一。图为不锈钢螺丝与镀镉垫片接触,发生电偶腐蚀。
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一、电偶腐蚀原理
从混合电位的角度分析电偶腐蚀电池形成原理,可以清楚地理解电偶腐蚀过程。
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lgi
+E
icorr1
ic
Ec
icorr1’
icorr2’
icorr2
总氧化反应
总还原反应
偶接后:M2为阳极,M1为阴极,电偶电流从M1流向M2。
M2发生阳极极化,腐蚀电流从icorr2增加到icorr2’;M1发生阴极极化。腐蚀电流从icorr1降到icorr1’。
阳极体金属腐蚀速度增加的效应,称为接触腐蚀效应;阴极体金属腐蚀速度减小的效应,称为阴极保护效应。
电偶腐蚀电池形成原理
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一、电偶腐蚀原理
The figure shows the rate determination when a third electrode process is added at a potential between the first two electrode reactions. The rule that must be applied is that the TOTAL OXIDATION RATE MUST EQUAL THE TOTAL REDUCTION RATE. The dashed lines represent the total rates.
下图表示当电极电位处于原两个电极的电极电位之间的第三个电极加入系统中时的腐蚀速率的确定。所应用的规则是总氧化反应速率必然等于总还原反应速率。
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Corrosion rate determination for a three electrode system
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The corrosion rate for electrode 2 has increased from icorr to icorr 1+2 as it is the only anodic reaction. This is the case when the corrosion potential for three electrodes is above the two electrode potential. The resulting corrosion potential is more negative than the third electrode reverse potential.
电极2的腐蚀速率从icorr增加到icorr 1+2 ,因为它是唯一的阳极。这是三电极的混合电位正于双电极的腐蚀电位的情况。最终系统的腐蚀电位负于外加的第三个电极的腐蚀电位。
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As such it can only contribute to the cathodic reaction rate. The third electrode is therefore protected from corrosion. The second electrode dissolu