Effect of Post-Rolling Surface Condition on the Corrosion Performance of UNS 32205 Stainless Steel Rebar
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Date
2022-04-29
Authors
Yang, Fook Yee
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
During the winter season, ice accumulation on roads and sidewalks is a major problem and
poses a hazard to road users. The most common method of addressing this issue is by applying
de-icing salts or spraying a brine solution on road surfaces. Unfortunately, the use of de-icing
salts and anti-icing brines has a detrimental effect on concrete infrastructure by causing
corrosion of steel reinforcement and deterioration of the concrete matrix.
The issue of corrosion can be addressed by using more corrosion resistant material as
reinforcement (rebar), such as stainless steel. Currently, the manufacturing process of stainless
steel rebar involves a hot rolling process followed by shotblasting to remove mill scale formed
during the high temperature rolling process and then acid pickling to remove the chromium
depleted layer below the mill scale. Since this process is generally required by many authorities,
there is little literature on the effect of mill scale on the corrosion resistance of stainless steel.
The main goal for this project is to determine the effect of surface condition due to post-rolling
processes on the corrosion performance of UNS 32205 duplex stainless steel rebar.
Furthermore, this project examines the condition of the mill scale after exposure to the high pH
environment in concrete for an extended time. Four different surface conditions of rebar were
tested as part of this project: as-rolled (no treatment), shotblasted-only, pickled-only, and both
shotblasted-and-pickled.
This project is divided into three parts: rebar surface characterization, a rapid corrosion
screening test, and a longer-term corrosion exposure test. Firstly, surface characterization was
carried out by optical microscopy, scanning electron microscope (SEM) paired with energy-dispersive
X-ray spectroscopy (EDX), and secondary ion mass spectrometry (SIMS).
The rapid screening test provides only a qualitative comparison of the corrosion resistance. For
this test, six replicate lengths of rebar of each type of surface condition were cast into concrete
cylinders. Sodium chloride was dissolved in the mixing water to induce active corrosion. The
concrete cylinders were cured for 24 hours before being demoulded and immersed in saturated
calcium hydroxide solution. The open circuit potential of the bars was monitored for 24 hours; after which, they were anodically polarized for 96 hours. The current response in the rebar was
recorded during this time. Only two of the 30 specimens exhibited active corrosion during the
96 hours of polarization. These were two of the six shotblasted-only bars. There are several
possible reasons for the corrosion found in only these bars. Firstly, the shotblasting process was
done using carbon steel chips as a medium. Any embedded carbon steel in the mill scale would
be more susceptible to corrosion. Secondly, microscopy showed incomplete removal of mill
scale from these bars. This would lead to a galvanic effect between areas with mill scale
remaining and areas where mill scale had been removed, further aggravating any corrosion.
The as-rolled and pickled-only bars exhibited similar corrosion rates to those which were fully
treated, i.e., shotblasted-and-pickled bars. This was unexpected because the mill scale was
cracked in many places and still had a chromium depleted layer in the underlying steel. These
results show that, at least in this test, as-rolled UNS 32205 rebar performs as well as those with
conventional post-rolling treatments.
For more quantitative data, a modified version of the ASTM A955 macrocell corrosion test was
used. In this test, rebar specimens were cast in concrete mortar rectangular blocks with a
ponding well. Four replicates of each surface condition were prepared. Saturated calcium
hydroxide was added to the ponding well to prevent calcium hydroxide from leaching out and
maintaining a high pH environment. After 200 days, 15% sodium chloride solution was added to
the ponding well of three replicates in an attempt to initiate corrosion. One replicate of each
surface condition was kept without exposure to chlorides to allow observation of the effect of
the high pH on the mill scale. Throughout this time, the macrocell potential of the bar was
measured. Electrochemical tests were performed periodically to monitor for any corrosion
activity. After 440 days, two of the three replicates exposed to chlorides were autopsied to
examine the condition of the rebar. None of the bars exhibited active corrosion rates and there
were no visible signs of corrosion on the autopsied bars. It was concluded from this test that a
15% sodium chloride solution is insufficient to initiate corrosion in UNS 32205 rebar in any of
the surface conditions.
Description
Keywords
corrosion, stainless steel, mill scale, concrete, reinforcing bars, oxide layer