The corrosion rate slows and stabilizes as the protective
patina is formed over the life of well-performing weathering steel. After the development of the protective
patina, the corrosion rate of weathering steel was measured to be approximately four times less than that
of plain carbon steel without copper. Extended periods of wetness and/or heavy exposure to deicing salts
will prevent the formation of a dense patina on the weathering steel and the initial corrosion rate will
continue, similar to that of plain carbon steel. Because the corrosion rate, or oxidation, of weathering steel
is greatest at the beginning of its life, the environment and chloride exposure at the time the structure is
put into service has a significant impact on patina performance over the remainder of the service life.
The study of Michigan bridges in the 1980’s found weathering steel bridges were performing well if the
steady-state corrosion rates were between 0.2 and 0.6 mils/year. Ideally, the corrosion rate will remain
below 0.2 mils/year. Research performed in other countries, including the United Kingdom, confirmed
this desired corrosion rate. The currently accepted steady-state corrosion rate was initially presented in
NCHRP Report 314, which recommends a rate of 0.3 mils/year as an acceptable upper limit for patina
development and pit growth. Due to the higher initial corrosion rate, section loss on the order of 10 mils
(0.01 inches) could be expected as the patina develops, but this level of section loss is generally negligible
when considering structural performance.
AISI performed a follow-up study in the early 1990’s revisiting the original 49 bridges surveyed during the initial research. The intent was to evaluate the performance of these bridges after thirteen more years in service. The bridges were inspected from the ground and the slope walls adjacent to the abutments, so visual observations were not performed on the top side of horizontal surfaces over traffic. These inspections revealed:
There was a noticeable difference between the exposed fascia girder and the sheltered interior
elements. Exposed surfaces appeared to develop the patina rapidly and had small rust flakes. The
interior surfaces also appeared to have a dense patina, but the rust flakes were much larger.
When debris was allowed to build up, corrosion could occur if the debris was allowed to remain
A transverse drip groove on the underside of the concrete deck at the joints appeared to help keep
water away from the girder ends.
Overall, the bridges in Iowa were performing in “textbook” fashion; however, some bridges exhibited some micro-environment concerns.
The primary cause of observed corrosion deterioration was inadequate deck drainage through the
joints and scuppers.
A white appearance was noted on the girder bottom flanges of the US30 Bridge in Ames and the
I-35 Bridge in Wright County. It should be noted that these two bridges were visited by WJE
engineers as part of this project.
The AISI report issued in 1993 reiterated and confirmed much of the research performed in the 1980’s
with regard to location selection and design details to achieve satisfactory performance of weathering
steel structures. It was also noted that bridges constructed in accordance with the 1989 FHWA Technical
Advisory were generally performing adequately. Several examples were given where bridges were
constructed with heavy traffic and salting on the bridge deck, but the weathering steel was not exposed to
traffic and salt spray from below. With the exception of leaking joints and drains, these weathering steel
girders were performing in “textbook” fashion.
However, one particular example stood out in the 1993 AISI report. Structures S34-82123 and S35-82123
in Michigan were constructed (in 1972) in an urban area over heavy traffic and are exposed to heavy salt
use from above and below. During the initial AISI inspection in 1981, the lower sheltered surfaces of the
bridge were experiencing flaky/laminar rust, but it had not yet resulted in any measurable section loss.
When the bridge was revisited in 1994 after being in service for 22 years, severe corrosion was observed
over essentially the entire length of the bridge. Painting the corroded weathering steel was recommended.
This example will be discussed again later in this report.
Weathering Steel Corrosion Product Phases
Carbon Steel, Corten Steel, Stainless steel
Rust, Bare, Powder Coated, Polished, Black Heat-resisting Paint, Iron Oxide
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Thickness : 1.2mm--3.0mm
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