Weathering Steel

In June 1980 the Michigan Department of Transportation reported serious problems with the use of unpainted weathering steel, especially when exposed to deicing salt. [Research Report No. R-1142.] Recently there has been a resurgence of its use for roadway bridges. The fact that this steel reacts badly in the presence of chlorides has not changed. It is probable that older, weathered, weathering steel structures will need protective coating especially those exposed to corrosive environments.

Metalized zinc is recommended to protect both salt contaminated and new weathering steel. This special use of metalizing is covered in the following report and guidelines.

As the FHWA report that follows indicates, owners should consider metalizing new weathering steel structures when this steel is used for high strength and will be exposed to salt, humidity, etc.

1. Classification Schemes

The recommendations for coating systems are based on the classification of the steel condition and the exposure conditions, as follows:

• New A 588
• Noncorroded, weathered A 588 (low chloride)
• Corroded, weathered A 588 (high chloride)


2. Bridge Exposure Conditions
• Severe: These are structures or zones, which receive deposits of salt and high humidity or long-term wet conditions.
• Mild/Moderate: These are structures not subject to the conditions for a severe exposure. NOTE: A non-corroded substrate would be present in a severe environment if the structure had only been exposed to the severe conditions (i.e. leaky joints) for a short period or if it had received some other protection (e.g., temporary coating).


3. Coating Systems

The systems recommended are as follows:

Inorganic zinc, organic zinc-rich system (epoxy or urethane zinc-rich primer) and thermal spray zinc system. Depending on whether the steel is to be exposed to severe or mild/moderate conditions, an additional protective topcoat or sealer may be required. Additional details are given in appendix 1.

Following are some factors that favor the various maintenance options.

1. No Maintenance Option (includes deferring maintenance painting)
• Little or no chloride deicing salts used and absence of marine environments.
• Non-leaking joints, the absence of joints, very light traffic.
• Dry climates, rural areas, no prolonged wet conditions.
• Open structure, with minimal angles, joints, and faying surfaces (e.g., box girders at 20-ft [6m] or more above the roadway).
• Inspection indicates very little corrosion (e.g., intact mill scale) or very light, small-grained scale on top flange and other locations.
• Long term maintenance program includes future painting plans.
• Note: Reference FHWA Technical Advisory 5140.22 for guidance.


2. Paint Corroded Areas Only
• Evidence of severe localized corrosion, including heavy salt deposits.
• Little salt spray exposure from below (e.g., no truck traffic or high clearance, light traffic, or non-highway [e.g., river] crossing).

NOTE: In the presence of extensive truck spray, it may be more prudent to paint the entire structure rather than the corroded areas only, because of the difficulty of isolating the high corrosion areas (see next option).

• Loose scale continues to develop in localized areas, even after 5 or more years, evidence of scale loss.
• Configuration presents areas that tend to collect moisture and debris, which are not readily cleaned by rain and are not readily accessible to drying conditions.


3. Paint Entire Structure
• Corrosion and scale evident in many parts of structure (e.g., evidence of salt running along the entire bottom flange or salt spray from traffic on bottom of flange).
• Humid or salt-laden environments (e.g., near salt marshes, bays, or coastal areas).
• Aesthetics important (e.g., desirable to have uniform appearance of the entire bridge).
• Difficult to isolate corrosion-prone areas.
• Rigging and mobility cost very high so that it would not be cost-effective to paint entire bridge.
• Corrosion rate data indicates that eventually entire structure will require painting to perform safely for its design life.

Option 1: Protective Coating on Corroded Areas Only

The most susceptible areas of a structure are the areas beneath open joints where leakage can occur. It should be assumed that eventually all joints will leak, so the treatment should be applied to all areas subjected to joint leaking. In addition, there is an area adjacent to the joints, which should also be protected because of the tendency of running water to carry salt to these areas. This ranges from about 6 to 10 ft (2 to 3 m) on either side of the joints, and generally includes the entire web and flange area. As with any protective coating system, it is necessary to consider the following components of the system: surface preparation, application techniques, coating materials, film thickness and quality control.

Option 2: Full Repaint

In some cases it will be possible to distinguish between the corroded and the non-corroded areas. The former will often consist of the joints and areas, such as those along the top of the bottom flange, where water runoff is present, or the bottom flange, which is subjected to salt spray from trucks.

The recommended metalized coating system consists of blast cleaning to SSPC-SP 10, metalized zinc plus epoxy polyamide sealer.

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