Preservation of Iron Ships in the Marine EnvironmentDon Birkholz
What Hath Man Wrought?
Long-term preservation is a difficult objective for anything that resides in an exposed environment, and this is particularly true for ships in the marine environment. Wooden ships are often considered the most problematic, being of materials that are intended by Mother Nature to rapidly degrade, and to which task she dispatches legions of insects, funguses, bacteria, and marine animals. Yet iron ships, while not subject to such organic onslaught, are hardly less ephemeral. To understand why, we need to look at the material, iron, in both a general and specific sense, and to consider the realities inherent in any preservation effort.
When man produces iron to build a ship he is taking a relatively stable substance - iron ore - and purifying it until it becomes an unstable substance, so unstable in fact that it wants to decompose in the presence of oxygen. A nearly omnipresent element, oxygen is hard to get away from on this planet (some ships have done so, but only by great misfortune) and, as if that is not bad enough, these creations are then tossed into one of the most corrosive environments there is - sea water.
Of course the men who built these ships were not overly concerned with this -- they were intent on transporting cargoes or fighting battles on the high seas. They valued the superior strength of iron and probably expected no more than a 20 to 30-year life cycle for their creations. But this is the reality we face as preservationists today, given a mandate to preserve "in perpetuity" objects that were never expected to significantly outlive their creators. Despite a generally high quality of initial construction, most historic ships built of iron or steel have already suffered appreciable deterioration, and many of these have yet to reach the 100th anniversary of their launching -- relative youngsters.
The Nature of the Problem
The historic ships under discussion here range from iron sailing ships of the last century to the welded steel ships of the W.W.II era. The vessels that are the most troublesome and idiosyncratic are the riveted ships of the 19th and early 20th centuries. For one thing, these ships often come into the hands of their preservation-minded caretakers only after suffering years of abuse or neglect (the Balclutha, Eppleton Hall, Wavertree, and Great Britain are among the many). Their partially deteriorated state, in itself, makes them more difficult to maintain. This is in contrast to the more recent military ships that were often (though not always) "mothballed" and carefully preserved for future use. The earlier ships are also more subject to loss of historic integrity, as their forged and riveted structures are harder to repair in kind. The result is that, over time, repeated repairs using alternative methods will alter the original character of the vessel. The following discussion primarily focuses on these ships, although most of the preservation problems, and possible cures, will apply to all iron and steel ships, whether riveted or welded.
The Enemy: Corrosion
The old saying "corrosion never sleeps" is definitely true, but may be a little understated; corrosion actually appears to go on binges. Coatings are the primary line of defense against corrosion; they work to keep oxygen and moisture away from the metal. Fortunately we are living in the golden age of protective coatings; zinc-based primers and high-build epoxies have vastly improved the effectiveness, and extended the life span, of anti-corrosive coatings. Perhaps more importantly, many of the new coatings are "surface tolerant", meaning they can be applied over a less-than-well prepared surface. In some cases, this can eliminate the headache of having to sandblast. A detailed discussion of coatings is beyond the scope of this paper. Suffice to say that one should stay abreast of the advances being made in the field, either by contacting the technical staff of the major paint companies specializing in marine coatings, or by contacting a professional trade organization, such as the Steel Structures Painting Council.
The Underwater Hull
The portion of an iron ship most vulnerable to corrosion is the underwater hull, and this is where corrosion can have the most catastrophic effect. Along with coatings, an effective weapon for preserving the underwater hull is cathodic protection. This is another area where major advances have been made -- the traditional system of using sacrificial anodes has been largely replaced with the "impressed current" system, which pumps electrical current into the hull, turning it into a giant cathode and thereby preventing corrosion. Although impressed current systems have been around for many years, recent advances have been made in the automatic control systems that regulate them. The systems are now fairly reliable and easy to maintain. They will reduce corrosion to a minimum and significantly extend the period between maintenance dry dockings. Any ships afloat in sea water should be under the protection of an impressed current system.
The impressed current system will not, however, protect a ship's topsides, which must continue to rely on anti-corrosive coatings. The area of greatest concern is the wind-and-water line, where the combination of wind and splashing sea water invariably causes severe corrosion and wastage (this is often the thinnest area of a hull and where leaks usually begin). The best and thickest epoxy coating is called for here, and will effectively protect the wind-and-water line unless it is mechanically damaged by ice, flotsam, or contact with a pier or another vessel. Measures should be taken to prevent such damage to the waterline area.
Bilges are a common corrosion problem because they remain damp. If sloshing bilge water is present, the lower interior of the hull can suffer corrosion rates as severe as those of the wind-and-water line. Bilge water should be kept to an absolute minimum, and bilge areas ventilated to remove moisture. Even with these measures, getting paint to stick to bilge surfaces can be difficult. Some areas may call for a soft-film coating. Once applied, these coatings will make all surfaces extremely slippery. This type of coating is best used in areas that people are not expect to access frequently, such as engine beds or ballast tanks.
Many builders of iron vessels in the late 19th or early 20th century poured cement in the bilges as a protective coating. Compared to the best paint available at the time, cement served far better in protecting hulls from the corrosive effects of sloshing bilge water. Over time though, the system can fail as water gets in through fractures in the cement or along the thin edges where the cement terminates, usually at the turn-of-the-bilge. Moisture starts the corrosion process, and rust expansion lifts the cement in a vicious cycle that eventually spreads throughout the cemented area. This can lead to severe wastage of shell plating that, coupled with exterior wastage, can hole the hull. It may be difficult to tell whether this process is occurring - sounding the cement with a hammer will, in some cases, produce a hollow sound, indicating the cement has lifted; in other cases, particularly where the cement is thick, the cement may sound solid while active corrosion is taking place beneath. Once water has gotten beneath the cement, it never dries out, ensuring that the corrosion will continue. About the only sure way of determining the condition of shell plating beneath cement is to remove the cement.
The term "galvanic action" is often used to describe a particular form of corrosion that takes place when dissimilar metals are placed in contact with each other, particularly in the presence of sea water. The result is wastage of the less noble metal. Due to minor dissimilarities between the composition of plates and rivets, galvanic action is often seen in the shell plating of riveted hulls that are afloat in sea water. This generally results in wasting of rivets, which tend to be less noble than the surrounding plate. This form of galvanic action is slow acting, but can result in severe wastage over time. One solution is to encapsulate the rivets and surrounding plate with a thick coating, such as a high-build epoxy, that prevents sea water contact, another is to employ the aforementioned cathodic protection system. Electrolysis
Electrolysis occurs due to ground faults in a ship's electrical system. It can also occur due to stray currents from electrical sources near the ship. In either case, as current passes through a ship's hull, it takes some metal with it, resulting in pitting of the hull exterior similar to that caused by galvanic corrosion. With most permanently moored historic ships, electrolysis results from ground faults in the shore power system. Finding all the ground faults in a ship's electrical system can be daunting. An alternate solution is to install an "isolation transformer" in the electrical shore power line. This effectively keeps the faulty current from passing through the hull as it tries to get back to ground ashore, and does not require that all ground faults are corrected. Stray current from outside sources are more difficult to detect and correct, although the effects can be largely mitigated by the use of an impressed current cathodic protection system.
One of the weaknesses of riveted construction is the vulnerability of plate seams to corrosion. The joints between plates and other structural members cannot be completely sealed from moisture, even in shell plates that have been caulked. The result is that, over time, corrosion can lead to expanding rust scale that draws joints apart with tremendous force, shearing rivets or pulling the heads though the plate. This can present a problem for the surveyor, as it is often difficult to tell whether rivets still have attachment. For the preservationist, removing rust scale from joints can be difficult, and sealing off the joints nearly impossible. About the only lasting solution, although a costly one, is to remove the rivets, clean out and coat the seam, and re-rivet the joint. A hermetic seal, such as the thick epoxy coatings mentioned earlier, can then be used to seal off the seam
Wood Decks on Iron Ships
Planked weather decks are a feature of most early iron ships. An ongoing conflict often exists between the maintenance needs of wooden decks and those of the surrounding ship -- wood decks are traditionally washed down with sea water to take advantage of the fungicidal properties of salt. This is robbing Peter to pay Paul in a big way because these same chlorides invariably result in corrosion of adjacent iron structures (hatch coamings, bulwarks, fittings). Wherever possible, an alternative to sea water should be used for deck maintenance. Unfortunately, some of the most effective wood preservatives are also toxic to humans. One safe alternative is sodium borate (sold as TIMBOR (R), a water-soluble preservative that is not corrosive to ferrous metals. Like any water-soluble product, sodium borate will tend to leach out of the wood over time. It therefore must be applied in an ongoing maintenance program in order to be effective.
As every wood deck that was ever laid upon a ship eventually leaks, corrosion is often found on the underlying plate or deck beams. There are few good solutions to this problem and once the corrosion has pitted the tops of beams or plates, the steel-to-wood faying surfaces become irregular and hard to seal. One solution that appears to work is the use of a soft film coating to bed the planking -- Eureka Fluid, a brand of lanolin-based anti-corrosive coating, was used in this manner during deck repairs on the ship Balclutha in the 1960s, and again in the 1980s, and appears to have been successful.
Monitoring the Rate of Corrosion
To determine the degree of success of our preservation efforts, the rate of corrosion and attendant wastage of a vessel should be monitored closely. Visual inspections should be carried out on a routine basis, with particular attention to some of the problem areas noted above. Measuring shell plate thickness is probably the single most important method for establishing the overall condition and rate of deterioration of an iron hull. The tool of choice for this is the ultrasonic caliper, which measures the thickness of metal by sending an ultrasonic signal though the material. These tools have become cheaper and more sophisticated in recent years and would be a useful addition to the maintenance tool kit of any large vessel. The process of ultrasonic testing (UT) is straightforward, but difficulties are sometimes encountered when attempting to measure some types of malleable iron plate due to imbedded layers of silicone slag. These layers tend to bounce the signal back, thereby giving only a partial reading. An experienced operator can learn to accurately decipher these signals and some newer UT gauges can be calibrated to overcome this problem entirely.
Enough is Enough
If we are to be successful in preserving iron ships, we must accept that even a minor degree of corrosion is too much. A one-percent annual corrosion rate would perhaps have been acceptable to most ship owners a century ago, but will not achieve the goal of long-term preservation if we are looking beyond the next one-hundred years. Of course, the idea of preserving something "in perpetuity" is patently absurd unless we acknowledge the necessity of constant renewal. Yet, one of the principle values we place in these ships is the craftsmanship they exhibit, something that is not easily reproducible. Who can say what future technologies will bring to the field of maritime preservation -- perhaps a solution will be found for this dilemma, but until then, we should do all that we can to not loose our grip on what we have.
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