corrosion control

A Corrosion Control Plan for Saint Lawrence Seaway Navigation Locks

The Saint Lawrence Seaway is a series of locks, canals and navigable channels that allow oceangoing vessels to travel from the Great Lakes to the Atlantic Ocean. To facilitate this transport, construction on the St. Lawrence Seaway along the St. Lawrence River began in 1954 and was completed in 1959. The existing gates were installed during that construction.

As seen in the November 2019 Edition of the Journal of Protective Coatings & Linings (JPCL) and on Paintsquare.com

In Canada, the locks are managed by the St. Lawrence Seaway Management Corporation (SLSMC), while in the United States the locks are managed by the St. Lawrence Seaway Development Corporation (SLSDC).

The SLSMC manages five locks in the Montreal region—Locks 1, 2, 3, 4 and 7. Locks 5 and 6 are managed by the SLSDC in the United States. Additionally, SLSMC manages all of the locks on the Welland Canal in the Niagara region.

THE WELLAND CANAL LOCKS AND GATES

Construction of the current Welland Canal began in 1913 and was completed in 1932, and the Welland Canal gates in existence today are the original gates from the construction completed in 1932.

Fifty of the gates at the locks at the Welland Canal are miter gates consisting of two leaves. All of the miter gates on the Welland Canal are the double-skinned type, meaning that they have solid steel plate on both sides. Several sections on the bottom of each gate are designed to be airtight. These air chambers provide buoyancy to the gate, taking pressure off of the gate hinges and making them easier to operate. The top chambers of the gate are filled with water and are referred to as water chambers.

The Welland Canal is in continuous operation throughout the year except for an approximately 10-week period that typically begins in January and continues through March. During that time, the canal is dewatered to facilitate maintenance. The Montreal section of the St. Lawrence Seaway is on a different dewatering schedule that occurs once every three years for approximately 10 weeks during the winter. As a result, all gate painting must be performed during those periods in the dead of winter.

Similar to any steel structure exposed to exterior and/or immersion environments, the gates managed by SLSMC require periodic maintenance including coating all exterior surfaces and the interior surfaces of the double-skinned miter gates. According to SLSMC documents, the exterior surfaces of the gates in the Montreal section of the St. Lawrence Seaway had not been coated since their original installation in 1959. The interior surfaces of the double-skinned miter gates on the Welland Canal have been coated at various times with most of the interiors coated within the last 15–25 years. The exterior surfaces of the individual gates were coated between 1961 and 1989 with the majority of the gates coated in the 1970s (Fig. 1).

Fig. 1: Miter gates from the Welland Canal with a vinyl coating system. Photos courtesy of
KTA-Tator, Inc. unless otherwise noted.

THE STUDY

To facilitate efficient and effective corrosion protection on the existing gates as well as secure the functionality of the Seaway for oceangoing vessels, SLSMC commissioned a study, the goal of which was to develop a program to extend the life of the existing gates on the St. Lawrence Seaway at least until 2059.

Analysis Of Existing Coatings

The first step involved reviewing the current condition of the existing coatings. Fortunately, SLSMC had an extensive library of photographs of the interior of the double-skinned gates taken between 2010 and 2014 (Fig. 2). In addition to reviewing the photographs, the interior and exterior surfaces of various gates in the Niagara and Montreal regions were examined firsthand. The interior surfaces of the double-skinned miter gates in the Niagara region were accessed through manholes on the top of the gates. Exterior surfaces were generally visually examined from the top of the lock except for a few where the exteriors were closely examined from a crane basket.

Fig. 2: The interior of a double-skinned miter gate with a coal-tar-epoxy coating system. 

The condition of the coatings in the water chambers and the air chambers varied considerably. The coating on all gate interiors was black and had an appearance similar to that of coal-tar epoxy, although, on a few of the gates, the coating had a checking pattern that is more commonly found with coal-tar enamel than with coal-tar epoxy. The degree of corrosion found in the water chambers was somewhat greater than in the air chambers, although the amount of corrosion on the surfaces was not generally found to be excessive considering the length of time that the coatings had been in constant or intermittent immersion service. Although the air chambers are obviously not in immersion service, it was clear that there was some water intrusion into the chambers and frequent condensation that created a moderately corrosive environment.

The thickness of the coatings within the gate chambers varied considerably, from as low as 160 µm up to 700 µm. The thinnest coatings were found in areas where the topcoat had delaminated, leaving only the primer. There was no noticeable correlation between the thickness of the coating in an area and the amount of corrosion on the surface. 

The adhesion of the coating was assessed in accordance with ASTM D3359, Method A (X-cut) and was rated 3A or better in most areas, which is considered good for an aged coating.

The condition of the vinyl coatings on the exterior surfaces of the gates in both the Niagara and the Montreal regions were generally in excellent condition, considering the length of time they had been in service (Fig. 3). Close access to the exterior surfaces of the gates was limited, and the condition survey was mostly done from the top of the locks. In the Montreal region, the coating systems on the exterior surfaces were the original systems applied in 1959, indicating that the coatings had been in service for almost 60 years (Fig. 4). In the Niagara region, the coatings were generally applied in the 1970s, indicating a service life of approximately 40 years. Regardless of extended service lives, many of the gates that were examined had less than 5% corrosion.

Fig. 3: Some pitting on the old vinyl system that was mostly intact.

Fig. 4: A single-skinned miter gate with a 60-year-old vinyl-coating system.

Gates in both regions had a black vinyl topcoat. The condition of the coating on the miter gates was often in noticeably better condition on the upstream side than on the downstream side. Unlike the corrosion found on the interior surfaces, the corrosion on the gate surfaces generally appeared in patches found most predominantly on the plate surfaces. Corrosion was less severe on the rivets. On the upstream side of the double-skinned miter gates, the worst corrosion patches were found predominantly in the middle of individual chamber cells between the rivets, although sometimes the corrosion extended to the riveted surfaces (Fig. 5). The coating thickness here was between 200 µm and 500 µm and the adhesion was rated good (3A or better) in limited areas of access.

Fig. 5: The corrosion in the interior spaces was predominantly on rivets and crevices.

The next step was to estimate the remaining service life of the existing coating system. Because the dewatering process is conducted in winter, a heated enclosure around the gates was necessary to apply coatings on both the interiors and exteriors. Because of the cost of such an enclosure, it was deemed most expedient to recoat both the interior and exterior of double-skinned gates at the same time. Surfaces were prioritized based on the degree of corrosion from greatest to least.

The approximate percentage of corrosion was estimated on each of the gates, with the interiors and exteriors of the double-skinned gates estimated separately. The percentage of coating breakdown within the interiors of the double-skinned miter gates was determined mostly from viewing the nearly 14,000 photographs taken during previous examinations of these surfaces.

There was considerable variation in the condition of the interiors of the double-skinned miter gates. The amount of deterioration ranged from as little as 0.5% up to as much as 20%. The amount of deterioration of the exterior coatings in the Niagara region ranged from 1–20%, while the amount of deterioration on the exterior of the gates in the Montreal region ranged from 2–10% (Fig. 6). Based on the percentages of coating deterioration, a table was prepared for SLSMC prioritizing the gates in order of need for coating replacement. Generally, coatings in immersion service are considered to have reached their service life when the percentage of coating breakdown is between 5 and 10%. There were several gates within this category that required recoating within the next 10 years.

Fig. 6: The exterior of a miter gate showing advanced corrosion.

SELECTING CORROSION-CONTROL SYSTEMS

Various corrosion control systems were investigated to determine the most efficient and cost-effective method for the lock gates. Coating systems including liquid-applied organic systems and thermal-spray metals were considered separately for the gate exteriors and the interiors of the water and air chambers on the double-skinned miter gates.

Moisture Cured Urethanes

Moisture-cured urethanes (MCUs) have been used for many years on dam and lock gates by the U.S. Army Corps of Engineers and other agencies. The MCU coating system is reported to perform very well in freshwater environments similar to those found on the St. Lawrence Seaway. One of the biggest advantages to moisture-cured urethane systems is that they can be applied in cold, damp conditions—temperatures as low as -7 C with humidity of 99%. The primary restriction is that the surfaces to be coated must be dry. The basic system consists of an MCU zinc-rich primer and two barrier coats formulated with micaceous iron oxide and coal-tar resin. This coating system has a relatively thin film applied at 330–480 µm dry-film thickness. At this thickness range, a service life of approximately 20 years can be expected before maintenance painting is required.

Because this system is composed of three coats, it takes somewhat longer to apply than do one- and two-coat systems such as elastomeric urethanes and 100%-solids epoxies. The recoat window between coats is six hours at 21 C and 12 hours at 10 C. The cure time before immersion service is fairly long—seven days with the use of an accelerator.

100%-Solids Epoxy Coatings

The 100% solids epoxy coatings have been known to provide excellent corrosion protection for well over 30 years, generally with only minor maintenance painting required a year or two after installation to touch up pinholes and discontinuities on edges and welds. These coatings were developed in the 1970s in Germany and some of the applications installed at that time are still in service today. The coatings were introduced to the United States in the early 1980s. Many of these installations are still in service after over 30 years.

There are two major advantages of low-VOC or VOC-free formulations. From a performance and application perspective, the lack of solvent prevents the coating from shrinking after application, which in many cases reduces internal stresses and provides greater edge-retentive properties. Additionally, because no solvent is present, the return-to-service time is significantly reduced, although there still is a cure-to-service time period to ensure that the coating has properly cross-linked.

Elastomeric Urethanes

Elastomeric urethanes are 100%-solids products; however, they have a slightly shorter history of successful use compared to their 100%-solids-epoxy counterparts. Elastomeric urethanes have been used in immersion service since the early 1990s and like the epoxy coatings, these applications are still in service. A service life of 25–30 years has been achieved in immersion service and could reasonably be anticipated on hydraulic structures.

The advantages of elastomeric urethanes are that they can be applied at temperatures as low as -32 C and they cure rapidly. The coating is typically dry to the touch in 20 minutes at 21 C (although curing times vary by product) and can be put into service in less than 24 hours. However, because elastomeric urethanes cure so quickly, plural-component spray application is required, which can create a challenge in limited access areas like those found on the interiors of double-skinned miter gates.

Vinyl Systems

In chemistry, vinyl is a term that refers to chemical linkage (-CH=CH2). Many industrial protective coatings contain that linkage, but generally, industrial vinyl coatings are considered to be combinations of vinyl chloride and vinyl acetate. The most significant advantage of a vinyl coating system is its excellent resistance to water permeation. In fact, prior to the 1980s, vinyl coatings were used extensively in potable water tanks, on bridges and on various other steel structures that were exposed to water, including hydraulic structures.

A large amount of solvent is required to formulate a vinyl coating with a low enough viscosity for application. In most cases, these coatings contain solids of less than 30% but because of the high percentage of solvent, they did not conform to VOC-content limits in the U.S. and Canada, and as a result cannot be used in most applications. However, vinyl coatings can still be used on hydraulic structures.

Thermal-Spray Coatings (Metallizing)

Metallizing is a process where a metal wire or powder is heated to the melting point and the melted particles of metal are then transferred to a surface using compressed air. In the protective coatings industry, thermal spray coatings typically consist of 100% zinc, 100% aluminum, or an alloy of approximately 85% zinc and 15% aluminum. There are advantages and limitations to thermal spray coatings. The biggest advantage is that they provide excellent galvanic protection to steel. The applied coating is hard and has good impact resistance. However, thermal- spray coatings are not surface tolerant and therefore, SSPC-SP 5/NACE No. 1, “White Metal Blast Cleaning” (for immersion service) and an angular 75–100 µm surface profile are required.

PROBABLE COST OF SURFACE PREPARATION AND COATING OPTIONS

A cost analysis was prepared for the various surface preparation and coating material options that involved making several assumptions based upon information provided by SLSMC, consultant expertise, and industry experience. The following assumptions were used in generating these cost opinions.

  1. The maximum available working time is 10 weeks during the winter months.
  2. The contractor is able to work seven days per week.
  3. Because the work will be performed during cold ambient conditions, heating units will be required to control temperature and relative humidity for surface preparation and coating application.
  4. Toxic metals such as lead and chromium may be present in existing coatings.
  5. For gates with interior wet and dry surfaces, the contractor will be permitted to cut additional temporary hatches for access and ventilation, the cost of which was included in the estimates.
  6. For gates with timber fenders (lumber used to dampen ship impact), labor costs to remove and reinstall fenders were included in cost estimates. Cost of new fenders and associated new hardware was not included.

Life-cycle costs of various interior and exterior lock gate candidate coating materials were generated. The purpose of the life-cycle analysis was to determine a candidate’s value based not only on initial cost but on future cost as well. For example, an installed coating system may have a very high initial cost, but if this coating system has a life expectancy superior to other candidate systems, an overall cost savings may be realized.

Life-cycle cost was not the only consideration for choosing a corrosion-control system. Environmental impact was also considered, but high VOC content was offset by a long service life and relatively low required thickness. Ease of application in the difficult conditions was also of prime importance. In the end, the two systems with the lowest calculated life-cycle costs were recommended.

It was determined that for the exterior surfaces, a four-coat vinyl system provided the lowest life-cycle cost partially due to its extended service life. For interior surfaces, the lowest life-cycle cost was a moisture-cured urethane system, although the life-cycle cost of the 100%-solids epoxy system was a close second. Moving forward, each year a few gates will undergo complete removal and replacement using the selected coatings.

CONCLUSION

The value of the St. Lawrence Seaway to transportation of cargo cannot be understated. Over 80% of seaway traffic is made up of bulk cargoes such as grain, iron ore, coal, chemicals and oil. Manufactured goods of all kinds, including finished and semi-finished steel products, make up the remainder of seaway cargoes. Ships from more than 50 nations call at Seaway ports in Canada and the U.S.

Since its opening, the St. Lawrence Seaway has moved more than 2.5 billion metric tons of cargo with an estimated value of more than $375 billion. Almost 25% of this cargo travels to and from overseas ports. The SLSMC in Canada and the SLSDC in the U.S. work together to ensure the gates on the locks do not become victim to corrosion and remain operable for decades. Coating-system selection is particularly critical given the time of year and short length of outage when coating work must be executed.

ABOUT THE AUTHORS

Rick Huntley is the Technical Manager of Coatings Consulting and a Senior Coatings Consultant for KTA-Tator, Inc., where he has been employed for over 20 years. He is a NACE-certified Coating Inspector (Level III) and an SSPC-certified Protective Coatings Specialist. Huntley has consulted on major marine and bridge coating projects, condominium and housing-project rehabilitations and parking-garage renovations. He has also conducted coating specification review for highway departments, water districts, A&E firms and various manufacturers. Huntley is a primary instructor for various KTA training courses and holds a B.S. in chemical engineering from Washington State University.

Dan Boich is currently the Continuous Improvement Manager at the St. Lawrence Seaway Management Corporation (SLSMC). He is a Professional Engineer who is proficient at executing and managing Lean Six Sigma Black Belt projects for the Corporation. Boich has spent the last 26 years working in the fields of new product development, mechanical maintenance engineering, asset management and continuous improvement. He previously chaired the Movable Bridge Technical Committee, the Hands-Free Mooring Technical Committee and led large asset-management-related studies for the SLSMC.

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