CABRILLO Historic Structures Report

REPAIR PROCEDURES (continued)

Ferrous Metal

Description

Ferrous metals are those containing iron. They are widely used at Cabrillo National Monument, in a variety of applications. These include the following:

• Reinforcing steel. Used as reinforcement for concrete. Examples: all concrete structures.

• Hatch covers. Examples: Calef & Wilkeson.

• Window shutters. Examples: All base-end and battery commander stations, and underground searchlight stations.

• Ladders. Bent reinforcing steel embedded in concrete. Examples: All base-end and battery end stations, and underground searchlight stations.

• Searchlight shelter covers. Steel, hipped roof placed on tracks. Examples: Searchlight Stations No. 15 and No. 19.

• Curved steel roofs covered with concrete and stone with steel anchors for netting. Examples: Battery Commander Station Humphreys, all base-end and battery end stations.

• Metal tracks and rails. Tracks opened and closed the searchlight shelter covers. Metal rails on the Panama Mounts positioned guns. Examples: Searchlight No. 15 and No. 19, Battery Point Loma Gun Mount.

• Metal-covered wood doors. Examples: Searchlight Shelter No. 18.

• Solid metal doors. Examples: Generator Station for Searchlight No. 18 and No. 19.

• Vertical steel bars. Examples: Generator Station for Searchlights No. 18 and No. 19.

• Four-foot steel pipe filled with concrete. Examples: Part of the transit system near Battery Bluff Gun Emplacements.

• Galvanized corrugated iron barrel vault. Examples: Battery Point Loma Bunker Complex.

• Pipe railing. Used for handrails. Examples: Base-End Station Battery Ashburn,

• Piping and conduit. Examples: Most structures.

• Counter-weighted elevator mechanism with chain sprocket drive. Used to raise the searchlight platform above ground. Examples: Searchlight Shelters No. 15 and No. 19.

Figure 36. Coating failure and corrosion at Searchlight Shelter No. 15.	Figure 37. Metal corrosion of hatch at the Battery Commander Station, Battery Humphreys.

Conditions

Surveyed metals display the following conditions: failed or missing coatings, corrosion, mechanical breakdown, connection failure, and missing elements.

Failed or Missing Coatings

The two primary protective coating types used at Cabrillo are paint and galvanization. Coating loss or deterioration exposes the metal, which begins to oxidize or corrode. Missing or failing coatings were observed on hatch covers, window shutters, searchlight shelter covers, and pipe railings. Consequently, these features display varying stages of corrosion.

Corrosion

When ferrous metal oxidizes, a chemical reaction between the metal and oxygen occurs. Contributing to the deterioration are many of the natural salts, including those found in mud, marine plants and salt water, common to the coastal environment. These salts include halides such as fluorides, chlorides, bromides, and iodides, and are a major cause of metal corrosion. When corrosion occurs, the metal begins to break down and return to its natural ore state. In addition, corroding metal expands. In the case of corroding reinforcing rod, this expansion leads to the spalling of adjacent concrete.

Corrosion rates depend not only upon the environment, but also upon material thickness. Thick metal plate, which is typical of the shelter covers, hatch covers and window shutters, may take years to wear through. Deferred maintenance might not appear to cause immediate problems, but in the long run, the metal will eventually corrode away, leaving little or nothing to preserve.

In addition, corrosion products are unsightly. Rust mixed with rainwater washes onto concrete and other surfaces, causing material staining.

Mechanical Breakdown

Metal elements can fail from physical abuse such as abrasion, fatigue, and stress corrosion cracking. Metal piping filled with concrete, as is found at Battery Bluff transit pole, results in mechanical breakdown, in addition to corrosion from moisture build-up in the concrete, when the concrete expands and contracts at different rates than the metal. Protective coatings may also delaminate because of mechanical breakdown.

Connection Failure

Connectors used at Cabrillo include bolts, braces, rivets, pins and welds. Connection failure may result from overloading, fatiguing, and corrosion. Corrosion will also make connectors more susceptible to stress failure, as is the case in some hatch covers.

Missing Elements

Missing metal elements include those which have been misplaced, vandalized, or deliberately removed. These elements include metal shutters, hardware, non-functioning plumbing pipes, flue pipes, etc.

Figure 38. Missing roof vent and metal corrosion at Searchlight Shelter No. 15.

Recommendations

General Metal Repair Methodology:

1. Begin with a field survey to identify and locate all problems. This survey can and should be done in conjunction with the survey of other materials. Missing coatings, corrosion, mechanical breakdown, connection failure, missing elements, and other conditions should be mapped on elevation, floor plan and roof plan drawings.

2. Test metal with a magnet to confirm metal type. Ferrous metal will respond to magnetism, other metals will not.

3. Determine the degree of failure for the element and the degree of repair required. For example, if a missing coating has caused limited surface corrosion, then only cleaning and replacement of the coating will be necessary. If severe corrosion has worn away sections of pipe railing, then patching or full replacement may be required.

4. On structures where repair work is not immediately scheduled, mark, cover, or rope off dangerous areas to prevent risk to visitors or park staff.

5. Make sure any new metal is physically and visually compatible with surrounding existing metal. Repair material should match the original metal type, composition, dimension, surface texture and coating appearance.

Ferrous Metal Stabilization

The following recommendations are appropriate for structures with designated ultimate treatments of stabilization, preservation, restoration and rehabilitation. For restoration treatments, additional work may be required to return the structure to its appearance during the period of significance.

Missing Coatings and Corrosion

See Concrete, above, for more about reinforcing rod. See Paint, below, for more about paint analysis and coating selection.

1. Before removing any paint, conduct paint analysis to determine the historic paint sequencing and presence of lead-based paint (see Paint, below).

2. Remove rust and most of the surrounding paint. Determine the extent of failure, corrosion and surface detailing before determining the removal method. Potential methods include wire brushing, grit blasting, flame cleaning, or chemical methods. Grit blasting in inappropriate for in situ cleaning at Cabrillo National Monument, but may be appropriate for elements which may be removed for off-site treatment. The presence of lead-based paint will also affect the choice of removal method (refer to the Paint section for recommendations).

3. Remove all loose, flaking, and deteriorated paint and corrosion to bare metal.

4. Degrease surface and prime immediately.

5. Paint Selection: Option One - Two coats of Alkyd rust-inhibitive primer, followed by alkyd enamel finish coats. Do not use latex or other water-based paints. Option Two - high-performance coatings, such as zinc-rich primers containing zinc dust, and modern epoxy coatings, should be considered to allow for longer-lasting protection. Note: These coatings typically require highly clean surfaces and special application conditions that can be difficult to achieve at some sites. Flashings or new metal may need three primer coats followed by compatible finish coats.

6. Apply paint. Ensure that all surfaces are dry. Do not paint when temperatures are expected to fall below 50 degrees Fahrenheit within 24 hours or when relative humidity is above 80 percent. Brush apply or use an airless sprayer, being careful to protect adjacent surfaces and landscape from overspray. Do not use rollers for primer coat applications.

Connection Failure

1. Repair failed attachments at hatches, doors, or shutters by tightening or replacing connectors. Match connector size and type, using stainless steel connectors if possible to avoid future corrosion. Avoid combining dissimilar metals, such as copper and steel. Repair failed riveted connections by spot-welding.

Severe Deterioration and Mechanical Breakdown

1. Severely deteriorated metal: cut out the failed metal and replace with new castings spliced in place by welding or brazing. Use an epoxy-patching compound designed for ferrous metal repairs to patch small voids.

2. Where concrete infill has resulted in mechanical breakdown or corrosion, take apart if possible and remove concrete. Clean away rust, prime the interior, paint elements, and reassemble. If the element is severely deteriorated, replicate the piece with new material.

Missing Elements

1. Locate any elements that may have been put in storage or placed nearby and reattach to structure.

2. Duplicate and attach elements that are missing, such as window shutters. For small elements, such as hardware, these can be used as casting patterns. For large elements, create patterns made slightly larger in size to compensate for 1/8 inches per foot shrinkage during casting.

3. Coat the replaced metal element following the paint procedure described in Missing Coatings and Corrosion.

4. Substitute materials, including aluminum and zinc castings, should be considered only as a last resort. These materials may have different expansion and contraction coefficients, and may require different future care than the original iron and steel products.

Recommended Tests

1. Paint Analysis. See Paint, below.
2. In situ magnetic analysis.

Ferrous Metal References:

Look, David, Margot Gayle, and John Waite. Metals in America's Historic Buildings. U.S. Department of the Interior, National Park Service Cultural Resources, Preservation Assistance, Washington, D.C., 1992.

Ashurst, John, and Nicola Ashurst with Geoff Wallis and Dennis Toner. Practical Building Conservation: English Heritage Technical Handbook; Volume 4 Metals. Aldershot, Hants: Gower Technical Press, 1988.

Park, Sharon C., A.I.A. Preservation Briefs 16: The Use of Substitute Materials on Historic Building Exteriors. Washington D.C.: Preservation Assistance Division, National park Service, U.S. Department of the Interior, 1988.