Corrosion behaviour of non-ferrous metals embedded in mortar砂浆中有色金属的腐蚀行为外文文献英文

Corrosion behaviour of non-ferrous metals embedded in mortar G.S. Duff a,b,c, S.B. Farinaa,b,c, F.M. Schulz Rodrguezb,c aGerencia de Materiales Comisin Nacional de Energa Atmica, Av. Gral. Paz 1499, (1650), Bs. As., Argentina bUniversidad Nacional de San Martn, Av. Gral. Paz 1499, (1650), Bs. As., Argentina c Consejo Nacional de Investigaciones Cientfi cas y Tecnolgicas CONICET, Av. Gral. Paz 1499, (1650), Bs. As., Argentina h i g h l i g h t s ?Corrosion rates of non-ferrous metals embedded in mortar were measured. ?Copper and lead showed low to moderate corrosion level. ?Aluminium showed a high corrosion level. ?First attempt to use classical electrochemical techniques to non-ferrous metals. a r t i c l ei n f o Article history: Received 24 May 2018 Received in revised form 9 November 2018 Accepted 16 March 2019 Available online 21 March 2019 Keywords: Aluminium Copper Lead Concrete Corrosion Polarization Weight loss a b s t r a c t In structural engineering, aluminium, copper and lead usually get into contact with cementitious mate- rials and may be prone to corrode. From weight loss tests a ranking of these materials in terms of their corrosion resistance in mortar was established. To evaluate whether the use of conventional electro- chemical techniques is appropriate to assess the corrosion rate of these metals when embedded in mor- tar, weight loss determinations were followed by measurements of the polarization resistance to compare the electrochemical results with the actual corrosion rate of the metals. Consistent results were obtained only on copper and lead. ? 2019 Elsevier Ltd. All rights reserved. 1. Introduction While ferrous alloys such as carbon steels and stainless steels are the most frequently used materials in the construction indus- try, non-ferrous metals and alloys are also available as building components, being their main advantage over ferrous alloys their malleability. Aluminium, copper and lead are among the most commonly used non-ferrous metals in the building industry and, as basic components of a building, they usually get in contact with concrete. Due to its strength, fl exibility and light weight, alu- minium is ideal to construct window frames, door rails, roofs and building structures in general. It develops an almost invisible coat- ing of oxide when exposed to the atmospheric air. This fi lm, if not damaged, prevents further corrosion attack. When aluminium is exposed to neutral, weakly acid or weakly alkaline aqueous media (pH range 49) the protection of this oxide layer is still guaranteed and that is the reason for being chosen as material in the building industry. But, when aluminium is exposed to highly alkaline envi- ronments, the protection is lost due to the dissolution of the oxide layer. Owing to its high resistance to corrosion and attack by many chemicals, copper is used for the production of pipes and tubes in the construction industry. Copper is a noble metal with a superior ability to develop protective layers in several media such as acid and weakly alkaline solutions. However, its corrosion resistance is lost in high alkaline media. The use of lead has diminished con- siderably as a consequence of its toxic effect to the human body, but it is still frequently used for roofs, cornices, tank linings and electrical conduits in the construction industry, and many old buildings still have water pipelines made of lead. As regards its cor- rosion resistance, lead has the ability to develop protective layers of lead compounds in the pH range between 5 and 9. However, it is weakly protected in acid and alkaline media. As stated above, the components of a building are usually in contact with concrete. It is well known that the aqueous solution /10.1016/j.conbuildmat.2019.03.208 0950-0618/? 2019 Elsevier Ltd. All rights reserved. Corresponding author at: Gerencia de Materiales Comisin Nacional de Energa Atmica, Av. Gral. Paz 1499, (1650), Bs. As., Argentina. E-mail address: .ar (S.B. Farina). Construction and Building Materials 210 (2019) 548554 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: retained in the pores of mortar and concrete is highly alkaline, with pH values ranging between 12.5 and 13.5. Corrosion of metals may occur in these strong alkaline solutions and, due to the fact that the products of corrosion normally occupy a greater volume than the metal that has corroded, cracking and spalling of the concrete may occur. This phenomenon has extensively been studied for reinforced concrete 14, where steel bars are used to obtain a material with good tensile and compressive resistance. However, the degradation of non-ferrous metals embedded in concrete has received little attention 57. Aluminium reacts in fresh concrete principally with alkali hydroxide present in the cement 57. As a result of this reaction hydrogen gas and aluminium oxides are pro- duced. The addition of chlorides (present in the cement paste or in the admixed sand or water) may greatly increase corrosion. In gen- eral, copper will not corrode in fresh concrete, unless soluble chlo- rides are present but, the combined effect of chloride and moisture, may lead to a very poor corrosion resistance 58. Lead in contact with damp concrete is attacked by the calcium hydroxide present in the cement and mixture of lead oxides are produced 57,9. This corrosion process tends to stop as concrete dries, but in the presence of moisture the reaction will continue. The few studies on the corrosion susceptibility of non-ferrous metals like aluminium, copper and lead embedded in concrete have been undertaken by means of gravimetrical tests. No attempt to use the classical electrochemical techniques, such as linear polarization resistance measurements, electrochemical impedance spectroscopy tests, galvanostatic pulse measurements, etc. to determine corrosion rates have been reported. These electrochem- ical techniques are successfully used to assess the corrosion rate of steel reinforcements in concrete. They have the advantage of being non-destructive methods that yield results in short periods with proven accuracy. The objective of the present work is to evaluate the corrosion susceptibility of aluminium, copper and lead when they are embedded in mortar with and without the presence of chloride ions. To this purpose the same electrochemical techniques used to assess the corrosion rate of steel in reinforced concrete were applied. The corrosion potential and the polarization resistance of the above mentioned non-ferrous metals, as well as the electrical resistivity of mortar, were periodically measured. In a previous publication 10 results corresponding to 700 days of exposure were shown. In the present work electrochemical results up to 1200 days of exposure are analysed together with results from gravimetrical tests. The ultimate goal of the present work is to compare the corrosion rates obtained using the Stern-Geary equa- tion 11 with those obtained directly through gravimetrical tests in order to validate, for non-ferrous metals, the electrochemical technique frequently used to this purpose for carbon and stainless steels embedded in cementitiuos materials. 2. Experimental techniques Bars (5 mm in diameter and 60 mm in length) of aluminium, copper and lead were embedded in prismatic sample mortars (7x7x6 cm3) after being degreased and weighed. Stainless steel AISI 304 bars were also included as counter electrodes, and a tita- nium rod coated with titanium, iridium and tantalum oxides previ- ously characterized 12 was used as reference electrode. All these bars (electrodes) were arranged in a confi guration shown in Fig. 1: two bars of stainless steel AISI 304 (counter electrodes) and two bars of the metal -aluminium, copper or lead- under study (work- ing electrodes) were located in opposite vertices of the mortar samples; a titanium reference electrode was placed in the centre of the specimen. Results are expressed in the standard hydrogen electrode (SHE) scale. The potential of the titanium reference elec- trode with respect to the SHE was obtained by placing a copper/ saturated copper sulphate electrode (CSE) on top of the prismatic specimens and measuring the potential of the titanium electrode against it. Then, the conversion to the hydrogen scale was straight- forward (VSHE= VCSE+ 0.318 V). To prevent crevice corrosion an insulation tape was used to isolate the metal bars from the mortar-air interface, being the exposed area to mortar equal to 5.5 cm2for each bar. The bars were placed in the mould so as to leave a cover thickness of 1 cm. A mix of Portland Composite Cement CPC 40 (slag + pozzolans + ash 35%), sand and water was used to prepare the mortar, according to ASTM C-305 Standard 13. A sand-cement ratio and a water-cement ratio of 3 and 0.6, respectively, were used. Metallic moulds were used to cast the mortar specimens. Some specimens were contaminated with chloride ions added to the mix as sodium chloride (NaCl) in concentrations of 0.3 wt% and 1 wt% (weight of chloride ion by weight of cement). After 24 h the spec- imens were demoulded and cured for 28 days in a controlled atmo- sphere with 98% relative humidity (RH), reaching a compressive strength of 22.6 2.3 MPa. Then, they were exposed to different exposure conditions, shown in Table 1. Two mortar specimens were exposed to each of the six different conditions (Series 1 to 6). As each specimen contained two bars of the metal under study, tests were performed by quadruplicate for each metal type in each of the six conditions. The following electrochemical variables usually determined to characterise the corrosion behaviour of steel embedded in mortar were periodically measured: the electrical resistivity of the matrix, and the corrosion potential and polarization resistance (Rp) of the metal bars. To assess the matrix electrical resistivity a sinusoidal signal (DV = 10 mV,m= 10 kHz) was applied between the counter electrodes and the internal reference electrode. The ratio between the peak voltage and the peak current was computed to obtain the resistance. Resistivity values were then obtained through a calibra- tion with KCl solutions of known resistivity. The corrosion poten- tialwasmeasuredagainsttheinternaltitaniumreference electrode. The galvanostatic pulse technique was applied to deter- mine Rp14. The galvanostatic pulse duration was 60 s and the current density 1 mA.cm?2. All electrochemical measurements wereperformedusingaGamryInstrumentPotentiostat- Galvanostat. Repetitive results were obtained in all cases. Fig. 1. Distribution of electrodes in the mortar specimens. Table 1 Exposure conditions of the specimens tested. Chloride Content (wt.% Cl?) Exposure conditions Series 10Laboratory environment (approx. 60% RH) Series 21Laboratory environment (approx. 60% RH) Series 30.398% RH Series 40Immersed in aqueous NaCl solution (3.5 wt%) Series 5098% RH Series 6198% RH G.S. Duff et al./Construction and Building Materials 210 (2019) 548554549 In Series 14 the exposure time was more than 3 years; in Ser- ies 5 and 6 the tests started one year later, so that the exposure time was of the order of two years. After the exposure to the differ- ent conditions, the metal bars were removed from the mortar spec- imens and cleaned of rust using cleaning chemical procedures according to ASTM G1-03 standard 15 in order to obtain the weight loss produced by the corrosion test. The weight loss of metal per unit surface area and time (weight loss rate: WLR) is determined with the following equation: WLR Wi? Wf p:D:L:t 1 where Wiis the initial weight of the bar before the test; Wfis the weight of the bar after the time test t; D is diameter of the rebar; and L is the length of the rebar sample. The corrosion rate (CR) is calculated as: CR WLR q 2 whereqis the density of the metal. Taking into account the weight loss rate, the corrosion current density (icorr) can be determined with the following equation: icorr WLR:n:F A 3 where A is the atomic weight the metal involved, n is the valence of the metal and F is the Faradays constant (96487 C.mol?1). Finally, icorrcalculated with the equation (3) and the integrated value of Rpalong the whole length of the tests, are related via the Stern-Geary equation 11: icorr B Rp 4 where B is a metal-medium dependent constant 11. One of the objectives of the present work is to determine the B value for each system, and compare it with the theoretical B values in order to establish if the values of Rpare representative of the real corrosion rate. It is well known that carbonation of concrete is one of the main causes of corrosion of steel reinforcing bars. The question arises whether it also plays a role on the corrosion processes of other metals. Then, it is important to determine the transport parameters of CO2. To this purpose mortar cylindrical specimens (without metal bars) were prepared. These specimens were exposed to: i) the laboratory environment with an approximately RH of 60%, which favours the carbonation process (corresponding to the con- dition of Series 1 and 2) and; ii) an environment with RH of 98% (corresponding to Series 3, 5 and 6). After a period of time, samples were broken apart and sprayed with phenolphthalein. From the carbonated thickness of the mortar (XCO2) and the exposure time (t), the carbonation rate (kCO2) was calculated by the following equation 13: XCO2 kCO2:t1=25 On the other hand, the effective chloride diffusion coeffi cient was determined using the Nordtest Method Build 443 16. Cylin- drical specimens were coated on all sides but one and submerged in a 16.5 g.L?1NaCl aqueous solution. After 3 months, samples from incremental depths were taken by sawing slices and analysed for chloride. The obtained chloride profi le is analysed by optimis- ing the fi t of equation (6) to the experimental data, with Def(effec- tive diffusion coeffi cient) and Cs(chloride concentration at the surface of the specimen) as parameters: Cs ? Cx Cs ? Co erf x 2 ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffiffi Def ? t p6 In Eq. (6), Cxis the chloride concentration at the depth x at time t, and Cois the initial chloride concentration of the matrix. 3. Results and discussion Fig. 2 shows the evolution of the electrical resistivity of mortar (q) as a function of time. The electrical resistivity was measured along time in all specimens (two counter electrodes in each spec- imen and 6 different specimens) yielding a total of 12 measure- ments at a given time and for a given Series. The reported values are the average of these 12 results (standard deviations are not shown for the sake of simplicity). The resistivity values increase with time to reach an almost stable value due to the process of dry- ing of mortar. In Series 1 and 2 (with approx. 60% RH)qvalues are nearly two orders of magnitude higher than in the other Series, indicating that resistivity is strongly dependent on the humidity. Resistivity values increase from values close to 103X.cm to values between 105and 106X.cm. Specimens with 1 wt% Cl?and RH approx. 60% (Series 2), showqvalues slightly lower than those in Series 1 (no chloride addition). The effect is ascribed to the hygroscopic nature of NaCl that retains more moisture in the mor- tar pores, yielding lower electrical resistance values. On the other hand, when the relative humidity is high (98%), or when the spec- imens are immersed in a NaCl solution (Series 3 to 6), the electrical resistivity is almost the same, and the concentration of chlorides has no infl uence onqin these cases. The initialqvalues are of the order of 103X.cm and they increase less than one order of magnitude by the end of the tests. Figs. 3, 5 and 7 show the evolution of the corrosion potential (Ecorr) as a function of time for aluminium, copper and lead, respec- tively. Figs. 4, 6 and 8 show the evolution of the polarization resis- tance (Rp) as a function of time for aluminium, copper and lead, respectively. In all these fi gures the data reported are average values over a total number of n = 4 repetitions, at a given time and for a given Series (two metal bars in each specimen and two specimens per Series). Error bars are not shown in these fi gures for the sake of clarity, but dispersion was calculated and in all cases is in the order of 35%. In the case of aluminium (Fig. 3), the initial Ecorrvalues are approximately ?1.0 VSHE for all Series, but in Series 26 large fl uc- tuations in the Ecorrare observed with time. In these cases the Ecorr tends to increase slightly with time, and by the end of the tests, values between ?0.4 and 0.0 VSHEare obtained. In the case of Series 1, the Ecorris almost constant along the whole test-time and Fig. 2. Electrical resistivity of mortar as a function of time. 550G.S. Duff et al./Construction and Building Materials 210 (2019) 548554 remains lower than in the other Series. The laboratory environ- ment (no chloride and low RH) yields the less noble Ecorr(Series 1). When chloride is added (Series 2) the Ecorrincreases. On the other hand, when the media has high humidity (Series 36), the Ecorris independent of the chloride content. As regards the polar- ization resistance of aluminium (Fig. 4), some fl uctuations are observed in all Series, particularly at the beginning of the tests. There is a slight tendency in Rpto increase with time in all Series with the exception of Series 4. In fact, the lowest values in Rp(in the order of 105X.cm2) are observed in NaCl solution (Series 4), and the rest of the Series show similar Rpvalues (in the order of 106X.cm2). In the case of copper (Fig. 5) the initial Ecorrvalues are approx- imately 0.0 VSHEin all Series. As time goes by the Ecorrvalues in Ser- ies 1 and 2 (dry conditions) tend to increase slightly a