1、外文原文 II: Some questions on the corrosion of steel in concrete. Part : Corrosion mechanism and monitoring, service life prediction and protection methods J.A. Gonzdlez , S. Felifd, P. Rodffguez , W. Lfpez , E. Ramlrez , C. Alonso , C. Andrade ABSTRACT This second part addresses some important issues
2、that remain controversial despite the vast amounts of work devoted to investigating corrosion in concrete-embedded steel. Specifically,these refer to: 1) the relative significance of galvanic macrocouples and corrosion microcells in reinforced concrete structures； 2) the mechanism by which reinforce
3、ments corrode in an active state； 3) the best protective methods for preventing or stopping reinforcement corrosion； 4) the possibility of a reliable prediction of the service life of a reinforced concrete structure ； and 5) the best corrosion measurement and control methods. The responses provided
4、are supported by experimental results, most of which were obtained by the authors themselves. 1. INTRODUCTION Concrete-embedded steel is known to remain in apassive state under normal conditions as a result of thehighly alkaline pH of concrete. The passivity of reinforcementsensures unlimited durabi
5、lity of reinforcedconcrete (1KC) structures. However, there are someexceptional conditions that disrupt steel passivity and causereinforcements to be corroded in an active state. This hasraised controversial interpretations, some of which werediscussed in Part I of this series 1. This Part II analys
6、esthough far from exhaustively, other - to the authors mindsat least - equally interesting issues on which no generalconsensus has been reached. 2. MATERIALS AND METHODS The reader is referred to Part I for a detailed descriptionof the materials and methods used in this work.Most of the experimental
7、 results discussed herein wereobtained with the same types of specimens and slabs.Galvanic couples were determined on speciallydesignedspecimens, such as those shown in Figs. 1 and 2.Near-real conditions were simulated by using a beam thatwas 160cm long and 7 x 10 cm in cross-section. The beamwas ma
8、de from 350 kg cement/m 3, half of which contained no additives, while the other half included 3%CaC12 by cement weight 2, (Fig. 1). In order to study theeffect of the Sanod/Scathoa ratio on galvanic macrocouples,they were modelled by surrounding a small carbon steelanode with a stainless steel (AIS
9、I 304) cathode and vice versa (Fig. 2). In this way, the ratios consistensy was assured. Inaddition, the potential and icorr of stainless steal and thoseof the passive structures were very similar. Fig. 1 - Beam used to measure icoTr and Ecorr inFig. 2 - Scheme of galvanic macrocouples embedded conc
10、rete with andwithout chlorides and toin chloride- containing mortar used to study the illustrate the significance of passivesteel/activeeffect of the Sanod/Scathod ratio and their relative steel macrocouples.significance to corrosion microcells. 3. RESULTS AND DISCUSSION 3.1 What is the relative sig
11、nificanceof galvanic macrocouples and corrosion microcells in RC structures ? According to several authors 3, 5, the polarizationresistance method provides an effective means forestimating the corrosion rate of steel in P,C ； the method isquite rapid, convenient, non-destructive, quantitative andrea
12、sonably precise. However, it is uncertain whether itmay give rise to serious errors with highly-polarizedelectrodes by the effect of passive/active area galvanic macrocouples in the reinforcements 6. Based on the authors own experience with thebehaviour of galvanic macrocouples in PC, the contributi
13、onof these macrocouples to overall corrosion is verymodest rehtive to that of the corrosion microcells formedin the active areas of reinforcements in the presence ofsufficient oxygen and moisture 2, 7, 8. Thus, it has beenexperimentally checked that: (a) Galvanic macrocouples have a slight polarizin
14、g effect onanodic areas in wet concrete, whose potential is therebyinfluenced in only a few millivolts. (b) On the other hand, macrocouples have a strongpolarizing effect on passive areas despite the low galvaniccurrents involved relative to the overall corrosion current. (c) As a result, galvanic c
15、urrents can result in grosslyunderestimated icorr values for the active areas since theyare often smaller than 10% of the ico= values estimatedfrom polarization resistance measurements. (d) The corrosive effect ofcoplanar macrocouples on RCstructures only proves dangerous within a small distancefrom
16、 the boundary of active and passive areas. Fig. 3 compares the estimated icorr and ig values, inmortar containing 3 o A CaC12, per anode surface unit fora number of anode/cathode surface ratios for AISI 304stainless steel/carbon steel macrocouples in support ofthe above conclusions 9. 3.2 By what mechanism do reinforcementscorrode in an active state ? When the passive state is lost, the rate of reinforcementcorrosion in inversely proportional to the resistivity ofconcrete over a wide resistivity range 10. Because
