For more than 100 years steel bridges have been erected
throughout the world--fascinating structures with wonderful histories like the
Brooklyn Bridge in New York and the Golden Gate Bridge in San Francisco.
Structures that families put their blood, sweat and tears to erect, and states
and cites have used to develop reputations.
However, these incredible structures are deteriorating in
front of our eyes due to the devastating effects of--not just corrosion, but of
more importance--crevice corrosion and pack rust.
Case in point, the Quebec Bridge, Quebec, Canada, considered
the "Eighth Wonder of the World" when construction was completed in
1919, is the longest cantilever steel railway bridge in the world. A riveted
steel structure with an overall length of 3,239 ft, width of 94 ft and height
of 340 ft above the water, the bridge includes one rail line, two pedestrian
walkways and three vehicle lanes.
In 1987, both the Canadian and American Society of Civil
Engineers declared it a Historic Monument and on Jan. 24, 1996, the Department
of Canadian Heritage declared the Quebec Bridge a National Historic Site.
Unfortunately, the Quebec Bridge is now suffering from the
ravages of weather, time and heavy-vehicle loading. In April 1999 the Quebec
Bridge Reclamation Project was initiated. To date the work has consisted of
cleaning and coating half of the south and north arms and the portals.
The initial work plan was to use sand blasting to remove all
of the existing lead-based paint. Due to the complexity of the structure and
the wind load limits, a negative air containment and abrasive blasting would
have cost hundreds of millions of dollars. By using high-pressure water
cleaning and selective UHP water jetting followed by an overcoat system
supplied by Termarust Technologies, the owner was able to substantially reduce
the cost of cleaning and coating the historic Quebec Bridge.
Breaking through paint
Crevice corrosion and pack rust create serious structural
problems and can seriously affect the load capacity and structural stability of
bridges. The rate of corrosion within crevices can be 400 times greater than
corrosion on flat surfaces that are open to the ambient atmospheric conditions.
Research on reinforcing bars in concrete bridge decks has found that for every
mil loss of steel cross section there is 10 mil (i.e., 10x) increase in
corrosion product. This is why pack rust can rapidly develop and cause
considerable forces to be exerted against adjacent steel members and their
rivet or bolt fasteners.
Metallic corrosion can produce very corrosive environments
through the chemical change of water into acid, called hydrolysis. This phenomenon
is particularly noticeable when the environment is confined, such as in most
forms of localized corrosion (pitting, crevice, environmental cracking).
As in a pitting corrosion attack, coating systems which rely
on passive surface films for corrosion resistance can be particularly
vulnerable to this mode of corrosion. The highly corrosive micro environment of
crevices tends to be similar to the micro environment which exists at the base
of corrosion pits. Crevice corrosion is usually a result of a differential
oxygen concentration cell in which the mouth of the crevice is richer in oxygen
than the metal interface within the crevice. It then becomes anodic and
dissolves. Subsequent pH shifts within the crevice may lead to an even more
intensified attack associated with the induction (initiation) and propagation
phases of the corrosion cycle.
The chemical change in question is true of most metals since
the metallic ions produced by the corrosion proc-ess are not soluble in their
ionic forms. The ions will then react and form more stable species such as
oxides and hydroxides. In aerated environments iron oxidizes to ferric ions
that subsequently react with water.
Many of our older (especially truss) bridges have not been painted
for many years, specifically because painting does not stop the development of
crevice corrosion and pack rust in inaccessible places, such as between steel
members that are connected with bolts or rivets.
Could pack rust contribute to the collapse of a bridge? The
answer is yes. From the analysis of structural failures it has been found that
in almost all cases failures are the result of a combination of effects. There
is no question that pack rust can severely distort steel members and overstress
fasteners--to the point that a combination of load effects can be critical
(e.g., overweight vehicles, wind loads, cold temperatures and earthquakes).
The old National Bridge Inventory System (NBIS) bridge
inspection criteria did not include an assessment of the level and severity of
crevice corrosion and pack rust, and thus its ramification as a structural
problem was often not recognized as being important.
The fact that pack rust can be a structural problem is
highlighted by the new PONTIS bridge evaluation criteria, which included a
'Smart Flag' for pack rust. Now, when severe pack rust is found there is a
requirement that the bride be analyzed to determine the effect of pack rust on
the load capacity and structural stability.
Lots of connections, and rust
It is important to recognize why crevice corrosion and pack
rust cannot be stopped by sand blasting to remove existing paint and cleaning
the steel members before using inactive film forming coatings or sealants on
First, sand blasting will not remove the corrosion products
in inaccessible areas. Second, the active corrosion cells (within crevices)
include water, oxygen and hydrochloric acid (and sometimes also nitric and
sulfuric acids). The corrosion
process will not be stopped by caulking or sealing up of the exterior edges of
the connections. In fact, within the connections there are oxygen concentration
cells, which are created when sealing up connections. If connections are sealed
up without treating them with an active chemistry to neutralize the acids,
displace the moisture and scavenge the oxygen it will actually accelerate the
development of protons which in turn creates more acid, thereby driving the
corrosion process within the space between the steel members. Sealing up the
joint without doing something to neutralize the active corrosion product in the
joint actually accelerates the corrosion.
This is why meticulous cleaning of the outer steel surfaces
of bridges then painting them with a three-coat zinc-based coating system does
not work for maintenance painting of crevice corroded bridges. The three-coat
zinc-based type of coating system does not chemically deal with the corrosion
process that exists within connections, and this type of coating system
frequently fails at the joints and connections within six months to a year. The
result is rust leaking from within the connections onto the exterior surfaces
of the bridge. It is important to recognize that there is a solution to this
Corrosion in inaccessible places that can reduce the load
capacity and structural stability of structures that contain built-up members,
splices and connections can be stopped.
The widespread existence and the potential severity of
crevice corrosion and pack rust on steel bridges and structures can be
chemically stopped with a Reacted Alkaline Viscolastic Calcium Sulfonate
(RAVCS) coating system.
This system has been used on major highway and railroad
bridges throughout the U.S. and Canada--on structures that had major pack rust
problems. Where the coatings are between five and 11 years old there is no
corrosion, including the crevice corroded joints and connections. Development
of pack rust has been stopped.
The Termarust Technologies' Reacted Alkaline Viscolastic
Calcium Sulfonate=based coating system (which has a pH of 10.5) includes:
1. A RAVCS penetrant/sealer which pulls itself into the
connections (by polar attraction) and then neutralizes the acid in the crevice
corroded joint; and
2. A RAVCS topcoat material that stops corrosion on the
surface of the steel. In addition this is an excellent one-coat (three-step)
"overcoat" coating system that is applied wet-on-wet and requires
only minimal surface preparation to remove only loose paint and debris.
structural stability of bridges is important to the safety of all and it is
important to recognize that there is an affordable solution to the problem of
crevice corrosion and pack rust.