Accepted Manuscript
Effects of surface washing on the mitigation of concrete corrosion under sewer
conditions
Xiaoyan Sun, Guangming Jiang, Tsz Ho Chiu, Mi Zhou, Jurg Keller, Philip L. Bond
PII: S0958-9465(16)30025-7
DOI: 10.1016/j.cemconcomp.2016.02.013
Reference: CECO 2607
To appear in:
Cement and Concrete Composites
Received Date: 31 July 2015
Revised Date: 25 November 2015
Accepted Date: 10 February 2016
Please cite this article as: X. Sun, G. Jiang, T.H. Chiu, M. Zhou, J. Keller, P.L. Bond Effects of surface
washing on the mitigation of concrete corrosion under sewer conditions, Cement and Concrete
Composites (2016), doi: 10.1016/j.cemconcomp.2016.02.013.
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Effects of surface washing on the mitigation of concrete corrosion under
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sewer conditions
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Xiaoyan Sun, Guangming Jiang*, Tsz Ho Chiu, Mi Zhou, Jurg Keller, Philip L. Bond
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Advanced Water Management Centre, Gehrmann Building, Research Road, The University
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of Queensland, St. Lucia, Queensland 4072, Australia
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* Corresponding author. Tel.: +61 (0) 7 334 67205; fax: +61 (0) 7 336 54726.
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Email: x.sun@awmc.uq.edu.au; g.jiang@awmc.uq.edu.au; tsz.chiu@uqconnect.edu.au;
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mi.zhou2@uq.net.au; phil.bond@awmc.uq.edu.aul; j.keller@awmc.uq.edu.au
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Abstract
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This study systematically investigated the potential of mitigating sulfide induced sewer
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concrete corrosion by surface washing. Washing interrupted the corrosion activity of concrete
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coupons by increasing the surface pH and decreasing the H
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S uptake rates (SUR). The SUR
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recovered to the level prior to washing within 60-140 days. The slowest recovery rate was
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from the most severely corroded coupon. However, no significant difference was observed
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for concrete mass loss of the washed and unwashed coupons after 54 months. The results
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suggest that frequent washing at short intervals of a few months might be needed to control
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corrosion over a long term.
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Key words: Sewer; Concrete; Corrosion control; Sulfide; Washing
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Abbreviations
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BSE Backscattered electron
MLA Mineral Liberation Analyzer
SEM Scanning electron microscopy
SOB Sulfide oxidizing bacteria
SUR Sulfide uptake rate
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1. Introduction
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Urban sewer networks collect and transport domestic and industrial wastewater (sewage) to
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centralized facilities for treatment prior to discharge of the treated effluent into receiving
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waters. In industrialised countries, the establishment of sewer networks has been achieved
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through continuous public investment for more than a century. However, the deterioration of
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sewer concrete pipes, caused by sulfide induced concrete corrosion, is a major economic and
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infrastructure burden in many countries [1]. It shortens the service life of sewer pipes and
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results in expensive replacement of prematurely failed structures. For example, the estimated
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corrosion caused cost in wastewater catchment infrastructure per year is around $20 billion in
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the USA [1]. As such, the development of effective technologies to mitigate corrosion is
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imperative for extending the service life of sewer pipes and reducing the huge annual
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maintenance expense.
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Sulfide is produced within the anaerobic regions of the sewer, mainly in the fully filled
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pressure pipes [2, 3]. During the pumping of sewage from a pressure pipe to a gravity pipe,
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the sulfide is partially released into the sewer atmosphere of the gravity pipe [4]. In the
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presence of oxygen, gaseous H
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S is taken up by the moist concrete surface exposed to gas
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phase and there it is oxidized to sulfate and other sulfur species by sulfur oxidizing bacteria
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(SOB) [5, 6]. The acid formed during sulfide oxidation will react with alkaline compounds in
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the concrete and form corrosion products [7]. This process causes decreased concrete surface
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pH (from 12-13 to below 1-2), loss of concrete mass, cracking and weakened sewer structure
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[6, 8]. A corrosion layer develops on the concrete surface that is largely gypsum, this has a
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soft texture and can be several mm in thickness. Severe corrosion will eventually result in the
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structural collapse of the sewer network.
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To achieve effective corrosion control, various technologies have been developed. These
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technologies mitigate the corrosion through either preventing the build up of sulfide in the
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liquid phase or by preventing the concrete surface from H
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S attack [9-14]. In particular, one
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treatment is to directly remove the corrosion layer by washing the concrete surface. This is
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considered a cost efficient approach and hence is of great interest to water utilities [15].
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However, surprisingly there has been no comprehensive evaluation on the effectiveness of
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surface washing for the purpose of mitigating concrete corrosion.
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Currently, there is disagreement on whether the formed concrete corrosion layer accelerates
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or slows the corrosion process [16-18]. It is proposed that the thick corrosion layer acts as a
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barrier that slows the sulfuric acid attack onto the intact concrete surface, and consequently it
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is suggested that the removal of the corrosion layer would accelerate the corrosion activity
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[16, 18]. In contrast it is argued that the flushing of the concrete pipe with sewage removes
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the corrosion layer and this disturbs and removes the low pH environment of the sulfur
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oxidizing bacteria [15, 17, 19]. Therefore, there are contradictory theories of the effect that
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surface washing may have on the corrosion processes and it is not clear whether washing
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would mitigate the corrosion activity.
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It is indicated that gentle flushing is not a successful control measure. Two early studies
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flushed concrete samples with wastewater for a few seconds and repeated it 1-3 times daily or
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weekly [15, 19]. This immediately increased the concrete surface pH, although, this returned
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to its low level found prior to flushing, in a few hours. Likely the gentle flushing only
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removed soluble components (e.g. acid) but not the corrosion layer containing the acid-
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generating microbes. They measured the surface pH and indirectly measured the corrosion
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rates from the polarization resistance. It was postulated that heavy and frequent washing is
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necessary to effectively reduce corrosion rates (in terms of producing high surface pH) [15,
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19].
A more recent study washed concrete pipes using a hose and a brush, but this only
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temporarily reduced the corrosion activity (measured as sulfide oxidation rate). In just 10
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days the sulfide oxidizing rate was seen to increase and by 30-40 days this had reached the
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