Thermal Spraying for Maintenance
of Naval Shipyard Facilities

Stephen W. Vittori, James D. Herbstritt
Puget Sound Naval Shipyard
Bremerton, Washington USA

Abstract
Puget Sound Naval Shipyard (PSNS) has built an experienced thermal spray production, engineering and quality assurance workforce. The most publicized efforts to date have been on spraying for shipboard equipment during vessel overhauls. PSNS has also used thermal spraying to maintain, repair and upgrade equipment necessary to keep shipyard plant facilities and operations running properly. Corrosion control and machinery repair applications have been common for power plant, dry dock and production support equipment. An extensive work package is now in planning for aluminum corrosion control spraying of all dry dock and pier components that have previously been painted. Examples of past machinery repairs include ceramic coating of shaft sealing surfaces and relining of babbitt bearings.

Thermal spraying of plant facilities saves money and assures maximum up time of equipment. This paper will report on cost analysis and coating performance to explain why PSNS is one of its own best customers.

THERMAL SPRAYED COATINGS have been used in industry for decades for corrosion control and machinery repair applications. Thermal spraying has earned its place because of good service performance and substantial cost savings. The U.S. Navy is benefiting from this experience by using coatings on highly-visible shipboard applications. This paper attempts to publicize less well- known applications on plant equipment to illustrate thermal spray possibilities for public and private industrial facilities.

Applications for Corrosion Control Coatings
The thermal spray facility, which is part of the Welding Shop at PSNS, first provided corrosion control services to the Public Works Department in 1978. Corrosion is prevalent due to the shipyard environment, which is one of salt air and high humidity much of the year. Some components are also submerged in salt water, either intermittently or continuously. In addition, parts may be mechanically abused during installation or service.

The earliest work was the thermal spraying of aluminum coatings on 250 dry dock drainage trough covers. These had originally been fabricated from carbon steel, painted for corrosion prevention, and placed in service until the paint coatings failed. Failures were due to the salty, humid atmosphere, occasional submersion during dry dock flooding, and scuffing and abrasion from foot and vehicle traffic. Before instituting thermal spraying at PSNS, covers either were blast cleaned to remove corrosion, and repainted, or were replaced. A 1985 inspection of the thermal sprayed trough covers showed all were intact. Covers that were protected by painting were so severely rusted that they had to be replaced. The thermal spray shop is now spraying 1,800 trough covers for the other dry docks. Figures 1 - 3 show rusted trough covers, cleaned covers set up for thermal spraying, and finished aluminum-sprayed covers. Typical coating thickness is 0.25 mm (0.010 inch), and the aluminum is sealed after spraying.

The Public Works Department has a goal to eventually thermal spray with aluminum all corrosion- prone carbon steel equipment. Some components currently made out of stainless steel will be replaced with carbon steel components coated with thermal sprayed aluminum. This goal will take some time to achieve, as maintenance of pier and dry dock equipment must be scheduled around ship overhauls.

Figures 4-9 show other components that are now thermal sprayed with aluminum. Deck grates, pumpwell covers and sludge boxes are shown in rusted conditions and during or after spraying.

Waterborne marker buoys provided data on thermal sprayed aluminum coating performance in submerged applications. The buoys are painted bright yellow with large black numbers for good visibility and easy identification in the water. Much of the surface is under water, with the portion above the water line exposed to salt air and salt water splash. The buoys are commonly gouged by forklift arms and scraped along rough beaches and barge hulls before placement in the water.

Paint coatings on buoys often failed within six months. Severe rust bled out through porosity in the paint, and thick rust formed rapidly on steel areas that were exposed to the environment by mechanical damage to the paint. Failed coatings resulted in the loss of easy visibility and identification.

A program to apply arc sprayed aluminum as an undercoat beneath the yellow paint was begun to give the dual benefit of corrosion protection of the substrate and a good bonding surface for the paint. Better-adhered paint proved less likely to come off under abuse, and where it did wear away, the aluminum provided excellent barrier and galvanic protection.

Figure 10 shows typical buoy surface condition after removal from salt water. Once cleaned and abrasive blasted, the sprayer applied a 0.25 mm to 0.388 mm (0.010 inch to 0.015 inch) thick aluminum coating (Figure 11). This provided a surface well suited for the sealer and the finish paint coating (Figure 12).

The PSNS Welding Engineering Division did an on-site inspection of the buoys when they were removed from the water to verify the improved performance. Buoys with and without thermal sprayed aluminum undercoats were available for comparison. The inspected buoys had been in the water up to two years. They were visually examined to estimate the area with paint intact, with paint lost, and with significant rust. Aluminum coating thickness was measured with a magnetic coating thickness gauge. The inspection concentrated on the surface below the water line.

The inspection showed that the aluminum undercoat prevented rust bleed-through from the steel substrate to the paint surface. Rust bleed-through was common for buoys with no aluminum undercoat. Thermal sprayed aluminum prevented rusting on many spots where its thickness had been scraped or gouged to less than 0.13 mm (0.005 inch). No or minimal rust was present even where the steel substrate was exposed by complete local thermal spray coating removal. Paint by itself did not provide the galvanic protection offered by the aluminum undercoat. Thick rust layers on these buoys spread under adjacent paint to dislodge more pain from the substrate, resulting in flaking, lifting and blistering.

As-sprayed aluminum coating thickness was 0.25 mm to 0.38 mm (0.010 inch to 0.015 inch) by procedure. All in-service areas where the aluminum thickness was measured at less than 0.25 (0.010 inch) had signs of gouging or scraping. Thus, during the service of the buoys, there was no measurable loss of aluminum due to corrosion.

Cost Estimates for Corrosion Control Coatings
Accurate estimates are based on historical data when available. Lacking this, estimators must make assumptions for preparing initial estimates.

PSNS made use of engineering performance standards for painting to estimate application costs for thermal spraying with aluminum. Painting and thermal spraying compare closely because they both entail surface preparation and coating as the primary production steps. Figures have been published that report costs for painting, arc spraying and flame spraying as all within 15% of each other [1].

In some instances, thermal spray application costs at PSNS may be 10% to 30% higher per application cycle than the engineering performance standard estimates if additional steps are carried out. For example, more expensive aluminum oxide grit is sometimes used in an extra step for surface roughening. Other components, such as the buoys, require a complete paint system over the thermal sprayed coatings. Also, thermal sprayed coatings are sealed. The size and complexity of the components influence the process selection. PSNS commonly uses arc spraying for large volume work and flame spraying for smaller, more complex shaped parts.

Table 1 - Eight-Year Cost Estimate Comparison Between Painting and Thermal Spraying with Aluminum for Dry Dock Components (1)(2)

(1) Components included in these estimates are trough covers, air drops, grating, drain tunnel access doors, hangers/supports, and handrails.  The SAVINGS figures in the bottom row represent the 8-year cost of maintenance by thermal spraying subtracted from the 8-year cost of maintenance by painting.  This is based on two years of service life for painting and eight years of service life for thermal spraying.  The per-cycle application cost for painting and thermal spraying are the same, based on engineering performance standards.
(2)  Cost figures are in thousands of dollars.

Referring to Table 1, offsetting the increased application cost is the probability that eight years of service is a conservative estimate. This time span is used because this was the longest period thermal sprayed aluminum coatings had been in service in PSNS facilities when the estimates were made. However, nineteen years of corrosion protection of test specimens have been reported [2]. This length of service life would greatly increase the savings figures reported in Table 1. Paint estimates are based on use of polyurethane enamel or epoxy paints, which historically have provided no more than two years of service. PSNS is tracking production costs and service performance, so in coming years they will be able to report figures based on historical data instead of estimates.

Repair of Power Plant Equipment
Thermal spraying has provided restoration of steel shafting and babbitt bearings. Steam turbines in the power plant help convert steam to electricity, and the turbine shafts rotate at several thousand revolutions per minute. Under proper continuous operation, shaft journals run on well-lubricated babbitt bearings. In such conditions, wear on shafts and bearings is minimal.

Startups and shutdowns set up conditions of maximum wear on components. At rest, there is metal to metal contact between shaft and bearing, which must be overcome as rotation starts. In the brief interval before the lubricant film is established, steel shaft and babbitt bearing rub. This results in wear of the softer babbitt. There may also be scoring of the shaft journals from foreign material embedded in the babbitt.

Many of the shipyard's turbine bearings have been repaired by arc spraying with babbitt. (See Figures 13 and 14.) Other effective, reliable processes are wire flame spraying and powder flame spraying. The condition of the bearings when received for repair measured typically 0.38 mm to 0.50 mm (0.015 inch to 0.020 inch) oversized on the diameter. The alternative to spraying was to melt out all the existing babbitt, re-tin the shell surface, and recast the babbitt. Recasting often involved several pours before a satisfactory bond was achieved.

By using soundly-bonded existing babbitt as a base for a sprayed babbitt layer, adequate bonding to the shell was assured. Metallographic examination of test samples has shown that sprayed babbitt can fuse into existing babbitt. While most machinery repair work for the power plant has been for repairing bearings, there has also been repair work on shafting. Seal areas have occasionally become worn or eroded. Ceramic coatings, using alumina-titania, have repaired seals.

Quality Benefits
The application of sound thermal sprayed coatings requires the skills of competent, experienced mechanics. Retention of operator skills is best assured by keeping mechanics employed in thermal spray production work. At PSNS, prudent scheduling of plant facility work among ship overhaul jobs maintains a constant workload.

Summary
Plant facility equipment and components have been thermal sprayed at Puget Sound Naval Shipyard since 1978. Thermal spraying was begun because of the belief that quick turnaround repairs were achievable, and that coatings would meet demanding service conditions. Periodic inspections have proven the soundness of the coatings. Estimates have shown that thermal sprayed aluminum application costs are reasonable compared to painting, and that the long service life assures reduced maintenance costs over a period of years. The result is that the shipyard has been a satisfied customer.

Acknowledgments

The authors wish to acknowledge the assistance of the PSNS Public Works Department in preparing the estimates, and the PSNS Photographic Laboratory. The authors also extend appreciation to the Shipyard Commander, Puget Sound Naval Shipyard, for support of the work reported.

References

1. W.C. Cochran, Metal Progress, 122 (1982) 37
2. American Welding Society Committee on Thermal Spraying, AWS C2.14, Corrosion Tests of Flame-Sprayed Coated Steel, 19-Year Report, American Welding Society, Miami, FL, 1974

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