Reinforcing Wire in Solutions with Chloride and Cement

Transcription

1 Reinforcing Wire in Solutions with Chloride and Cement Protection of reinforcing steel from corrosion is usually attributed, in the first instance, to a protective layer which forms at the high ph provided by hydrated cement, but which may be destroyed by the presence of chloride. For example (from a widely-read textbook) 1 : Steel embedded in hydrating cement paste rapidly forms a thin passivity layer of oxide which strongly adheres to the underlying steel and gives it complete protection from reaction with oxygen and water, that is formation of rust or corrosion -. However, chloride ions destroy the film and, in the presence of water and oxygen, corrosion occurs. The series of tests described here was initiated to illustrate the process as described, but using water as the medium instead of concrete, with cement powder initially dispersed in the solution to provide the high ph. It was expected that at chloride concentrations high enough to destroy the protective film, corrosion would take place in much the same way as would be observed in a neutral-ph solution containing chloride. In above-ground structures depassivation is commonly observed in concrete containing chloride at average concentrations which, predicated on a moisture content typical of partially saturated concrete, would correspond to a solution strength similar to that in seawater. It was anticipated, therefore, that if reinforcing wire was immersed in air-saturated water mixed with cement and containing dissolved salt at a concentration above that in seawater (about 3%), the steel would corrode, exhibiting a similar pattern to that resulting from immersion in salt solution without the cement. Reference to photographs on the following pages will show what actually occurs. Without cement the wire corrodes freely, producing rust, at a salt concentration of only 0.5%. For solutions with added cement and salt concentrations of 10% and 20% (about three and six times the concentration in sea water, respectively) there is nothing to suggest a general breakdown of a passivating layer. The sample (Figure 5) in the 20% solution exhibits a nodule suggestive of a corrosion product, but the situation has no resemblance to the general corrosion occurring in solutions without the cement. There s a further point of interest from immersion 0.5% chloride alone. Figure 6 shows rustlike discoloration of the solution after just one day. Referring the bottles without cement shown in Figures 1, 2 and 3, rust has settled at the bottom but some is also held in place surrounding the wire. The pattern is in all respects consistent with rust forming not at the steel surface but in the solution, from metal ions released and able to diffuse some distance before being oxidised. Some of the rust so formed is drawn to the wire by an electrostatic field gradient originating from ion interchange at the metal-solution interface. One can take this a step further and suggest that the passivity observed in the solutions containing cement is created not by a passive film but by alteration of the potential of the steel relative to the electrolyte, by a surface reaction with the hydroxyl ions, imposing a protective negative charge on the steel. Norwood Harrison August

2 All tests Materials: Wire as used for the manufacture of spun concrete pipes. Mild steel, profiled, 5 mm nominal diameter. Cement Type GP from Cement Australia. Kitchen salt. Tap water (Melbourne). Arrangement: In each sample the wire is cut to 90 mm length, and has one end encapsulated in a cylinder of epoxy of outer diameter 9.5 mm, leaving 50 mm of wire exposed (see example in Figure 4). The test container is a small plastic bottle, capacity 220 ml. If the exposure medium is to contain cement this is put into the bottle first, followed by the salt solution, leaving a space in the bottle above the surface as shown in the photos. The bottle is shaken or the contents stirred to disperse the cement. The encapsulated end of the sample is assembled into a fixture which fits into the neck of the bottle, allowing free passage of air past it to the liquid surface, and which holds the sample central and vertical in the bottle. All of the metal surface not encapsulated by the epoxy is below the surface level of the liquid. The lid is then screwed on to the bottle but left loose, initially, to allow access of air while limiting the rate of evaporation. The lid remained loose up to the time of the first photo shown here, for each series. At this time the assembly was set aside and for at least part of this later period lids were tightened to prevent evaporation. With a pair of samples, the same treatment was applied to each. As the samples with cement never exhibited any significant corrosion it can be assumed that with these, closing the lid did not result in any depletion of oxygen dissolved in the water. The bottles were stored indoors at ambient temperature, about 20 C. 0.5% salt, 10% salt with cement, started 13/7/07 See Figure 1. Samples of the two solutions taken in December 2007 had composition shown in the table: Chloride mg/l Equivalent NaCl % Salt only 3, Salt+cement 67, Analysis by concrete laboratory CRL The salt+cement exposure has been continued and at 6/8/18 (11 years) shows no change to the exposed wire condition from that in Figure 1. ph 2

3 Figure 1. Condition at 3 month 6 days 0.5% salt, 10% salt with cement, started 9/3/13 See Figures 2 and 3. The wire samples were cleaned using a plastic abrasive brush set in a power drill. Cement batch was 20 g. Figure 2. Condition at 1 month 20 days 3

4 Figure 3. Condition at 3 years 11 months At the conclusion of the test (January 2016), a sample of liquid from the salt+cement exposure had composition shown in the table: Chloride mg/l Equivalent NaCl % ph 66, Analysis by Envirolab 20% salt with cement, started 8/3/16 The wire sample was cleaned using a plastic abrasive brush set in a power drill. The photo of the sample prior to exposure, Figure 4, illustrates how samples were constructed, and their size, for all the tests of the series. Cement batch was 20 g. The condition after immersion for 2 months and 7 days is shown in Figure 5. The test has continued and at 6/8/18 (2y 5m) the there has been no change from that shown in Figure 5. 4

5 Figure 4. Test sample Figure 5. 2 months 7 days 0.5% salt, 4/8/15 The solution was clear and colourless at the start of the test. Figure 6 shows appearance after 1 day. Reference Figure 6. One day 1. A M Neville, Properties of Concrete, 5 th edition, Chapters 10 and 11 under sub- headings, respectively, Carbonation and Chloride attack. 5