Feasibility of Replacing Structural Steel with Aluminum Alloys in the Shipbuilding Industry
Scott Brown
University of Wisconsin at Madison
May 4, 1999
Abstract - Structural steel has a long history of providing superior mechanical properties to the ship building industry, but with one major disadvantage: weight. Increasing demands for size have forced ship designers to search for alternative materials which will reduce the weight of the ship without compromising strength. When properly designed, aluminum typically reduces the weight of small structures made of low-carbon steel by over 50%. Dramatic technological advances have allowed aluminum to meet or exceed the minimum strength requirements for normal strength steels currently used in the shipbuilding industry. Another advantage of aluminum is that it corrodes over 100 times slower than conventional structural carbon steel used to build ships. This report investigates weight reduction, strength, corrosion resistance, and cost to determine the feasibility of replacing conventional structural steel with lighter-weight aluminum alloys in the shipbuilding industry.
April 29, 1999
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Subject: Executive Summary for investigating the feasibility of replacing steel with aluminum alloys in the ship building industry.
For over 150 years steel has been the most widely used material in the shipbuilding industry because of superior mechanical properties and low cost. Although steel has many advantages, it has one major drawback: weight. Weight issues have become increasingly important as advanced technology allows us to build larger and larger ships. Since 1910 the maximum weight of ships has more than doubled, increasing from 46,000 tons to 109,000 tons. Increasing demands for size have forced ship designers to search for alternative materials to reduce the weight of the ship without compromising strength. When properly designed, aluminum typically reduces the weight of small structures made of low-carbon steel by over 50%. Dramatic technological advances have allowed aluminum to meet or exceed the minimum strength requirements for normal strength steels currently used in the shipbuilding industry. Another advantage of aluminum is that its resistance to corrosion is superior to steel. Aluminum corrodes over 100 times slower than conventional structural carbon steel used to build ships. During the first year and a half of an eight and a half year study steel corroded at a rate of 120 mm per year, whereas in a similar study aluminum corroded at a rate of only 1 mm per year.
There are two major disadvantages to replacing steel with aluminum in the shipbuilding industry. The first disadvantage is that high-strength, low-alloy steels used in certain applications have yield strengths much higher than any aluminum alloy and are not replaceable by existing materials. The second disadvantage is that aluminum costs roughly five times more than steel. Aluminum costs approximately $1.15/lb whereas steel costs $0.25/lb. Based on weight reduction, strength, corrosion resistance, and cost it was concluded that replacing conventional structural steel with aluminum alloys is feasible in most shipbuilding applications, but not always economical. The attached report investigates weight reduction, strength, corrosion resistance, and cost to determine the feasibility of replacing conventional structural steel with lighter-weight aluminum alloys in the shipbuilding industry.
Introduction
Oceangoing ships are designed, built, and operated to fulfill the mission requirements and limitations specified by the operator. These mission requirements, such as minimum deadweight carrying capacity, measurement tonnage limit, or selected speed at sea, represent the essential considerations which are to form the basis for the design. This report will examine these requirements in terms of the feasibility of replacing conventional steel with lighter-weight aluminum alloys. The incentive for replacing steel with aluminum is to reduce the weight of the ship and increase the corrosion resistance without compromising strength. This feasibility study will compare the yield strengths and corrosion resistance of steels currently used in the shipbuilding industry with possible aluminum alloy replacements.
It should be noted that the main interest in this study is not in the strength and corrosion behavior of commercially pure or refined aluminum but in the performance of specific aluminum alloys that have been designed to possess specific properties. Because the addition of alloying elements to aluminum affects the strength and corrosion resistance separately, it is difficult to make general statements concerning the overall behavior of aluminum. However, this study will take this issue into account when suggesting suitable aluminum alloys for the replacement of steel.
Technical Background
For over 150 years, steel has combined strength, toughness, and cost to become the most widely used material in the shipbuilding industry. Steel may be defined as an alloy of iron and carbon containing less than 2.0 percent of carbon. Steels may be classified into two types, carbon and alloy. Carbon steels, frequently referred to as straight or plain carbon steels, owe their properties primarily to carbon. Alloy steels are those to which one or more alloying elements are added in sufficient amounts to modify certain properties. A composition of five weight percent total non-carbon additions serve as an arbitrary boundary between low-alloy and high-alloy steels [Shackelford, 1992]. These alloy additions improve properties such as strength, toughness, or corrosion resistance. Structural steels, like those used in the shipbuilding industry, are alloy steels designed for toughness and fatigue strength. High-strength, low-alloy (HSLA) steels are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels. For a better explanation of the notation used to index steel alloys in this report, please refer to Appendix A.
Dramatic technological advances in strength have allowed aluminum to emerge as a possible replacement for oceangoing ships. Aluminum is a relatively strong, light weight metal. With a density of 2.70 g/cm3, aluminum is roughly one-third the weight of steel (r = 7.83 g/cm3). Aluminum can be formed through either casting or wrought processes. The designation “wrought” indicates that the alloys are available primarily in the form of worked products, such as sheet, foil, plate, extrusions, tube, forgings, rod, bar, and wire. The working operations and thermal treatments transform the cast ingot structure into a wrought structure. The structure influences the strength, corrosion resistance, and other properties of an aluminum alloy. This study deals only with wrought aluminum alloys because they possess superior strength and corrosion resistance properties to cast aluminum alloys.
Weight Reduction
Since the mid-19th century, steel has effectively satisfied the previously mentioned mission requirements by combining strength, toughness, and cost to become the most widely used material in the shipbuilding industry. Although steel remains the most popular shipbuilding material, it has one major drawback: weight. Generally, the structural design of a ship should seek to minimize weight. This will reduce cost and minimize the loss of cargo deadweight due to structure. In addition, weight is important to consider when designing and building ships because as ships get larger and heavier it becomes increasingly difficult to meet the mission requirements.
Another concern of increasing weight deals with fuel efficiency. For instance, a bulky, heavy ship will not be as fuel efficient as a lighter, more aerodynamic ship. As a ship gets larger it becomes increasingly difficult to design for fuel efficiency without sacrificing other aspects of the mission requirements. In addition, larger ships require larger power plants, which require more fuel. The larger engines and massive quantities of fuel add weight to the already bulky ships. Storage of the fuel also becomes a question. Weight issues have become increasingly important as advanced technology allows us to build larger and larger ships.
Figure 1. Island Princess of Princess Cruises in Glacier Bay, AK. The Island Princess weighs 20,000 tons. [Lycos Image Gallery, 1999].
Since 1900, the size and weight of ships have grown at a dramatic pace. Built in 1910 and weighing 46,328 tons, the White Star line’s RMS Titanic was the largest movable object of its time. The Titanic, perhaps the most famous ship to ever set sail, was impressive because of its sheer size. A more modern example of the enormity of ships is the Queen Elizabeth 2, built in 1969. The Queen Elizabeth 2 weighs 70,327 tons. Built in 1997 by Princess Cruises, The Grand Princess is one of the largest non-military ocean-going vessels in the world. With 14 decks and a capacity of 2600 passengers, The Grand Princess weighs 109,000 tons, more than twice the weight of the Titanic [Ocean Odyssea Cruises, 1999]. As you have read, since 1910 the maximum weight of ships has more than doubled. Increasing demands for size have forced ship designers to search for alternative materials to reduce the weight of the ship without compromising strength.
When properly designed, aluminum typically saves over 50% of the weight required by low-carbon steel in small structures [ASM: Corrosion, 1994]. Aluminum has a density of only 2.7 g/cm3, approximately one-third as much as steel (7.83 g/cm3). This means that one cubic foot of steel weighs about 490 lbs where as one cubic foot of aluminum weighs only 170 lbs. Because aluminum’s low density corresponds to light weight, it has been used for marine structures such as navigation buoys, life boats, motor launches, cabin cruisers, patrol boats, barges, and larger vessels since 1930. In 1960, Rogers summarized the experience of the Canadian Navy in part as follows: [Hatch, 1984]
'It cannot be emphasized too strongly that aluminum as a new shipbuilding material needs treating as such. It has its own design problems, its own maintenance problems, and its own repair problems. It cannot be used everywhere as a substitute for steel or any other alloy, but if the contractors, naval architects, shipwrights, and shipbuilders, and of course suppliers will treat it as something that requires a new approach they will find they have a very fine metal for use in seawater and marine atmospheres.'
Aluminum is commonly used in other marine applications as well. These structures include main strength members such as hulls, deckhouses, and other applications such as stack enclosures, hatch covers, windows, air ports, accommodation ladders, gangways, bulkheads, deck plates, ventilation equipment, lifesaving equipment, furniture, hardware, fuel tanks, and bright trim [ASM: Aluminum, 1993]. Aluminum-manganese (5xxx), and aluminum manganese-silicon (6xxx) alloys have been widely used for ship superstructures [Hatch, 1984]. High strength aluminum-copper (2xxx) and aluminum-zinc-manganese (7xxx) alloys can also be used in marine atmospheres, but they must be protected by cladding or painting. Properly designed aluminum structures can reduce the weight of the superstructure and hull of a ship by 67%. In other words, one kilogram of weight saved by the use of the lighter aluminum structures often leads to an overall decrease in displaced weight of three kilograms [ASM: Aluminum, 1994].
Strength
The structural steel specifications for commercial ships are developed and put into operation by a number of ship classification societies, including the American Bureau of Shipping (ABS) and the American Society for Testing and Materials (ASTM). These organizations have unified their requirements for structural steels into two classes: normal strength (34 ksi yield strength) and higher strength (46 and 51 ksi yield strength) [ASM: Carbon & Alloy Steels, 1996]. The normal strength class consists of four grades of carbon-manganese steel, with the grading based on toughness. The higher strength class is also based on toughness, but they belong to a separate family of microalloyed high-strength low-alloy (HSLA) steels. Precipitation hardening mechanisms and grain refinement through the presence of small amounts of vanadium, niobium, and/or copper elevate their yield strength. For this investigation, the structural steel specifications will be applied to aluminum alloys in order to determine which alloys have strength levels that meet or exceed the required standards.
Figure 2. RMS Titanic. Built in 1910, the White Star Line’s RMS Titanic was the largest movable object of its time, weighing 46,328 tons. The Titanic was remarkable because of its sheer size and reputation as “unsinkable” [Adapted from Government of Nova Scotia, Halifax, Nova Scotia, Canada, 1998].
In 1912 the “unsinkable” RMS Titanic, pictured in Figure 2, set sail as the largest moving man-made object in history. The hull of the Titanic was constructed of low carbon steel, almost identical to today’s AISI 1018 steel. Today the most common material used to build ships is structural steel, which has better mechanical properties than the steel used in 1910 to construct the Titanic. The most popular structural steel is ASTM A36, which is usually referred to as A36 steel. A36 steel is normal strength carbon steel with a yield strength of 36 ksi. Through minimal additions of alloying elements such as manganese, vanadium, and chromium, the properties of A36 steel can be altered to improve certain properties such as strength, toughness, and corrosion resistance. A36 steel is widely used in the shipbuilding industry in both modified and unmodified compositions. Other normal strength structural steel alloys include A131 (yield strength of 34 ksi), A242 (yield strength of 42 ksi), A441 (40 ksi), and A573 (35 ksi) steels.
High-strength low-alloy steels such as ASTM A710 (yield strength of 50 ksi), HY-80 (80 ksi), HY-100 (100 ksi), and HY-130 (130 ksi) are designed to provide specific desirable combinations of properties such as strength, toughness, formability, weldability, and corrosion resistance. They are used primarily in shipbuilding applications where high strength is critical, such as the hull and superstructure of battleships, aircraft carriers, submarines, coast guard patrol boats like the one pictured in Figure 3, or other heavy military ships. These steels have yield strengths much higher than any aluminum alloys and are not replaceable by existing materials.
Figure 3. US Coast Guard patrol boat with steel hull and superstructure [Naval Vehicle Register, 1999].
Table 1: Comparative strength-to-weight ratios for various materials [adapted from ASM: Aluminum, 1993].
|
Material |
Typical Ultimate Tensile Strength, ksi |
Density, g/cm3 |
Strength-to-Weight Ratio |
|
7075-T6 |
83 |
2.80 |
822 |
|
2024-T361 |
72 |
2.80 |
713 |
|
5056-H18 |
63 |
2.66 |
656 |
|
6061-T6 |
45 |
2.71 |
459 |
|
3004-H38 |
41 |
2.71 |
418 |
|
Fiberglass |
19 |
1.43 |
367 |
|
6063-T5 |
27 |
2.74 |
273 |
|
1020 Carbon Steel |
60 |
7.86 |
211 |
|
PVC Plastic |
7.5 |
1.40 |
149 |
As Table 1 illustrates, there are many aluminum alloys that reach the minimum yield strength level of normal steels, 34 ksi. For over forty years aluminum has been used in the superstructure of military ships such as guided missile cruisers, destroyers, frigates, or any other military vessel where speed at sea is important. For example, the guided missile destroyer, Scott, has a steel hull and an aluminum superstructure while the patrol combatant missile hydrofoil, Gemini, has an aluminum hull and superstructure [Naval Vehicle Register, 1999]. Series 5xxx, average yield strength of 40 ksi, were developed as marine alloys, highly resistant to corrosion even in saltwater environments. These alloys are the most popular for seawater environments because of their excellent combination of strength and corrosion resistance. Series 2xxx and 7xxx alloys are the strongest aluminum alloys, with yield strengths of 65-70 ksi. They are strengthened by solution heat treatment and age hardened to obtain yield strength levels comparable to mild steel. The yield strength values of series 1xxx and 3xxx alloys lie below the minimum for steel required by ASTM and ABS, and will; therefore, not be included in this investigation. Table 1 illustrates the general range of strengths available within each of the major wrought aluminum alloy families and compares the strength-to-weight ratios of various aluminum alloys with other materials.
The next section, Corrosion Resistance, will examine the corrosion resistance of the aluminum alloys with yield strengths that meet or exceed those required for structural steel by ABS and ASTM to determine the optimum alloys to replace steel in the structure and hulls of ships.
Corrosion in Marine Environments
The general marine environment includes a great diversity of sub-environments, such as full-strength open-ocean water, coastal seawater, brackish and estuarine waters, bottom sediments, and marine atmospheres. Because of the high degree of variability of seawater, it is difficult to simulate this environment in laboratories. Often, stored seawater is used rather than synthetic replacements in order to try and create a laboratory atmosphere as close to a natural environment as possible. However, stored seawater has been known to exhibit corrosive behavior that is different than that of the open ocean from which it was taken [Craig, 1989]This difference is due to the fact that the minor constituents, including the living organisms and their dissolved organic nutrients, are in a delicate balance in the natural environment. The laboratory cannot simulate this balance and the equilibrium is upset as soon as the sample is isolated from the parent water mass. It is important to recognize the difference in corrosion behavior between laboratory created tests and open ocean field tests as the corrosion behavior of the materials introduced in this report are based on either lab tests or field tests. Whenever possible, changes in the corrosion behavior of a material is monitored and accounted for as corrosion properties are being determined. With that in mind, this section will characterize the corrosion behavior of steel and aluminum alloys in terms of overall corrosion resistance.
As stated previously, the selection of alloy steels for ship hulls and structure is based on strength, toughness, and weldability, with corrosion performance being of secondary concern. [Craig, 1989]. Without some sort of surface protection, carbon and low-alloy steels are poor material selections for resisting attack by very aggressive environments such as seawater. Types of surface protection consist of a range of options from, oiling and paint for low-cost, temporary protection to vapor deposition coating for long-term corrosion, heat, and wear resistance. The optimum coating is dependent on the location and application of the part. As a rule, only low-carbon, normal strength steels, 0.08-0.28 wt.% C, are in any way considered for corrosion resistance. In aqueous solutions, low-alloy steels behave essentially the same as carbon steels. However, because low-alloy steels are frequently used at much higher strength levels than carbon steels there is an increase in the tendency for environmentally assisted cracking such as hydrogen embrittlement and stress corrosion cracking.
Aluminum, as indicated by its position in the electromotive force series, is a thermodynamically reactive metal. Among structural metals, only beryllium and magnesium are more reactive. However, aluminum has excellent corrosion resistance due to an extremely adherent oxide film that forms on the surface whenever it is exposed to air or water. This oxide film is highly protective and because it is more thermodynamically inactive, prevents aluminum from corroding further. When exposed to extremely corrosive materials, such as salt water, the oxide film may break down and further aluminum corrosion or pitting may occur. In contrast, steel’s oxide layer, rust, does not provide a highly protective layer, and as a result, steel continues to corrode. A comparison of the corrosion behavior of aluminum and steel is exemplified by comparing home screen door frames. After a few years of use, painted steel doors often rust. This causes the paint to chip or flake off. However, aluminum door frames, even in seacoast environments, do not corrode and the paint remains smooth and flat.
The chemical composition and fabrication practice of both aluminum alloys and steel play a role in determining the corrosion properties of the metal. The composition of an alloy determines the type of microstructure the material will have. The type of microstructure will affect both the amount of localized corrosion, and the method of the corrosive attack. For example, the most important mechanism for corrosion of aluminum deals with the electrochemical cell created by the potential differences of different alloy-constituents of a microstructure. If an alloy- constituent in the microstructure has greater electrochemical potential than aluminum, corrosion of the alloy will occur. Heat treating and hardening processes are fabrication techniques that play a role in determining the corrosion properties of aluminum alloys and steel because they directly determine the microstructure of the metal and affect intergranular corrosion. Design characteristics can also have an important influence on a metal’s corrosion behavior. The design of bolts and joints and the presence of other metals are important factors. For instance, aluminum-to-steel coupling creates an ideal electrochemical cell and increases the galvanic corrosion of aluminum
The most popular aluminum alloys for use in corrosive environments such as seawater are the 5xxx and 6xxx series alloys because of their adequate strength and excellent corrosion resistance. Series 5xxx alloys are the most resistant and most widely used because of their favorable strength and good weldability. Series 6xxx alloys are generally stronger, but less corrosion resistant than 5xxx series. Although no general thinning occurs, weight loss may be two to three times that for 5xxx alloys. The more severe corrosion is reflected in larger and more numerous pits. Alloys of the 2xxx and 7xxx series are considerably less resistant to corrosion in seawater and are generally not used unprotected. Protective measures, such as the use of Alclad products, coating the less resistant aluminum with a more resistant aluminum alloy, and coating by metal spraying or by painting, provide satisfactory service in certain situations.
Specific Types of Corrosion
There are five types of corrosion important to this feasibility study. They are uniform, galvanic, intergranular, stress, and fouling corrosion. Exfoliation corrosion is also an important type of corrosion, but only for certain alloys. Low carbon structural steels used in the shipbuilding industry are generally immune to exfoliation corrosion, but certain aluminum alloys are vulnerable. The susceptibility of aluminum to exfoliation corrosion is discussed in appendix C. The following section of the report will describe each of the five significant types of corrosion and then explain how they affect the steel currently used to build ships and aluminum alloys that are feasible replacements.
Uniform Corrosion: “Uniform corrosion is an attack of the metal when an electrochemical reaction proceeds uniformly over the entire surface of a metal.” Unprotected steel forms iron oxide (rust) when exposed to an oxygen containing environment. The oxygen in the air or dissolved oxygen in the seawater reacts with the neutral iron on the surface to form iron oxide. Although the formation of this oxide film doesn’t prevent further corrosion, it can slow the rate of further corrosion. This was proven in an 8.5 year study at Wrightsville Beach in North Carolina. In the first 1.5 years the corrosion rate was 120 mm/yr. After 2.5 years the corrosion rate slowed to 105 mm/yr and after 4.5 the corrosion rate was 85 mm/yr. At the conclusion of the study the corrosion rate was 70 mm/yr [Craig, 1989]. Notice the decrease in the corrosion rate of steel as the study progressed. This is due to the layer of corrosion product that formed on the surface as the steel oxidized.
In a ten-year study with various aluminum alloys the corrosion rate was found to be much less than that of steel. The results of the studies on the corrosion of carbon steel and various aluminum alloys can be found in Table 3.
Table 3: Corrosion behavior of various aluminum and steel alloys in seawater [Adapted from Craig, 1989].
|
Aluminum Alloy |
Corrosion Rate, mm/yr |
% Change in Tensile Strength |
Steel Alloy |
Corrosion Rate, mm/yr |
|
5083-O |
0.9 |
0.0% |
Structural Carbon Steel |
120 |
|
5086-O |
0.9 |
-2.7% |
105 |
|
|
5454-H34 |
1.0 |
-0.7% |
85 |
|
|
5456-H321 |
1.6 |
-1.1 |
70 |
|
|
5456-O |
0.4 |
-0.4% |
|
For the first 1.5 years of the study aluminum was 133 times more corrosion resistant that structural carbon steel. In general, the rate of corrosion based on weight loss does not exceed about 5mm/yr, which is generally less than 5% of the rate for unprotected low carbon steel in seawater [ASM: Aluminum, 1993]. Figure 5 is an example of typical uniform pitting corrosion of aluminum. The round, white indentations are pits in the surface of the metal.
Figure 5. Uniform pitting corrosion of aluminum. Photograph of typical localized pitting corrosion of aluminum [Adapted from InterCorr International, 1996-1999].
Galvanic Corrosion: Galvanic corrosion occurs when two different metals or alloys are in electrical contact in the presence of a corrosive environment. The coupling of the materials causes one of the metals to corrode more rapidly than it would under normal conditions. The rate of galvanic attack depends on (1) the difference in corrosion potentials between the two metals, (2) the electrical resistance between the two metals, (3) the conductivity of the electrolyte, (4) the cathode-anode area ratio, and (5) the polarization characteristics of the two metals [Hatch, 1984]. The differences in potential between dissimilar metals or alloys cause electron flow between them when they are electrically coupled in a conductive solution. The direction of flow, and therefore galvanic behavior depends on which metal is more active. The more active metal becomes anodic and loses material, while the more noble metal becomes cathodic, and is unaffected by the couple.
Figure 4. Aluminum-to-steel galvanic coupling. Electron flow from aluminum anode to steel cathode ionizes the aluminum. The aluminum ions dissolve in the seawater electrolyte and leave deep pits on the surface of the aluminum.
Galvanic corrosion is especially important for this study because an aluminum-to-steel coupling creates an ideal galvanic cell. In the aluminum-to-steel galvanic coupling, the aluminum is the anode and the steel is the cathode. This causes current flow from the steel to the aluminum and through the seawater electrolyte in a manner pictured in Figure 4. In such an environment, aluminum in contact with the more cathodic steel greatly increases the corrosion current and, therefore, causes the loss of aluminum into the electrolyte. The aluminum dissolves in the seawater electrolyte and leaves deep pits on the surface of the aluminum. The steel cathode remains unaffected during the corrosion of the aluminum. Painting the cathode and leaving the anode un-coated can easily prevent galvanic corrosion. It is important not to paint the aluminum because any scratches in the coating will dramatically increase the cathode-anode area ratio and create an extremely high, localized current density through the exposed aluminum. These high current densities causes severe localized pitting of the surface and, depending on the thickness of the part, complete perforation.
Intergranular Corrosion: Intergranular corrosion occurs when grain boundaries are attacked and corroded more easily than the grains themselves. The mechanism of intergranular corrosion is similar to that of galvanic corrosion. This mechanism is driven by an electrochemical potential difference between the constituents on the grain boundaries and adjacent grain bodies. That potential difference is caused by impurities collected at grain boundaries or by enrichment or depletion of elements in and near grain boundaries. Cells are formed between second-phase constituents on the grain boundaries and the depleted grains bodies. In some alloys the precipitates are more anodic than the adjacent grain bodies and in other alloys the precipitates are more cathodic. In either case, selective attack of the grain boundary region occurs.
Intergranular corrosion is not a significant form of corrosion in steel. However, intergranular stress corrosion cracking (SCC) along prior austenite grain boundaries in quenched and tempered steels is commonly observed in acidic solutions, such as aqueous chloride solutions and seawater.
Stress Corrosion Cracking: Stress corrosion cracking (SCC) is a slow, environmentally induced, crack propagation that results from the interaction of continuous tensile stress and a corrosive environment. SCC is caused by intergranular corrosion, often occurring at relatively low stress levels as compared with the stress needed for mechanical failure. In addition, SCC usually occurs at relatively low concentrations of chemicals. Environments that cause SCC are usually aqueous and are made up of layers of moisture or bulk solutions. Typically, SCC of an alloy is the result of the presence of a specific chemical specie in the environment.
Stress corrosion cracking in low to medium strength steels is restricted to those alloys with ferritic-pearlitic microstructures. SCC is also restricted for steels with yield strengths greater than 90 ksi, and those with tempered martensite microstructures. Steels with yield strengths less than 90 ksi, like those normal strength steels used in the shipbuiliding industry, are generally considered resistant to SCC. For carbon and low-alloy steels, two mechanisms are thought to be operative: anodic (active path) SCC and cathodic (hydrogen embrittlement) SCC. Their electrochemical potential, pH, and temperature effects differentiate these mechanisms. When negative potentials, lower pH values, and lower temperatures promote SCC, a cathodic mechanism is active, and the reverse trends promote anodic behavior. Steels with yield strengths greater than 180 ksi, like some high-strength low-alloy steels used to build ships, fall into a grouping in which hydrogen embrittlement is the predominant mechanism for SCC. At this strength level, susceptibility to SCC is acute. Environments that cause minimal general corrosion promote the incidence of SCC.
The probability that aluminum alloys will develop stress corrosion cracking (SCC) is dependent on the magnesium (Mg) content of the alloy, grain structure, amount of strain hardening, and subsequent time/temperature history. The 2xxx and 7xxx series alloys are susceptible to SCC for the same reason they are susceptible to intergranular corrosion: because of the presence of second phase constituents on the grain boundaries. Both alloy series can be strengthened by solution heat treatment and age hardening techniques. The processing sequence used to increase the strength may cause increased vulnerability to cracking. During heat treatment precipitates form at the grain boundaries, creating a copper-depleted zone within the grain bodies. The susceptibility of the alloy increases as the magnitude of the corrosion potential difference between
Figure 6. Cracking of low carbon steel plate caused by intergranular stress corrosion [During, 1997].
the depleted zone and the grain bodies increases. For the alloys of the 5xxx series, those with magnesium contents greater than 3.0 wt%, such as 5083, 5086, and 5254, contain a continuous film of anodic Al2Mg3 along the grain boundaries rather than in a solid solution within the grains [Hatch, 1984]. This decreases the toughness, the ability of a material to withstand an impact, by making the alloy brittle. The most detrimental precipitates form at room temperature in heavily cold worked material. Those precipitates form over a number of years, or after prolonged exposure to slightly elevated temperatures of 150°F to 350°F like those found near the engine and exhaust system of a ship. Alloys in the 6xxx series are generally resistant to SCC; however, temperatures approaching 350°F coupled with the presence of chloride ions, found in seawater, may create conditions favorable for SCC. The effects of SCC in all aluminum alloys can be minimized with proper heat treatments, appropriate alloy selection, and reduction of non-design stresses like residual and assembly stresses.
Fouling Corrosion: Fouling corrosion occurs when marine organisms, such as barnacles and mussels, attach themselves to surfaces, grow, and then effectively seal off a small part of the surface from its environment. Concentration cells form underneath the barnacles and cause pitting of the underlying metal. Fouling on ship hulls occurs most severely in relatively warm, shallow water. The warm water temperature favors long breeding seasons and rapid multiplication of the macro-organisms that cause fouling. If a film provides either a complete or spotty coverage of the metal surface, this coverage influences the extent of fouling corrosion. A spotty film, or one that forms in discrete colonies of organisms with bare metal in between, will be more likely to induce structurally significant corrosion than a film that produces a continuous layer. A continuous layer acts as a barrier film
Figure 7. Biologically induced fouling corrosion of low carbon steel plate located on a ship at the bottom of a bilge compartment.
limiting the amount of dissolved oxygen that can reach the metal surface. A spotty film is more likely to cause the initiation of localized corrosion by creating oxygen concentration cells. Figure 7 is a photograph of fouling corrosion on the bottom plate of a bilge compartment found on a ship. Extensive pitting and wall thinning, caused by oil and wastewater from the engine room, that collected in the bilge compartment and remained there for extended periods characterize the corrosion. Fouling corrosion occurs by the same mechanism for both steel and aluminum.
Cost Analysis
This feasibility study investigates weight, strength, and corrosion resistant properties of steel and aluminum alloys. Many other variables must be considered before aluminum can be used as a replacement for steel. Toughness, stiffness, and fatigue strength must also be compared before selecting feasible aluminum alloy replacements. Because of the lower density and elastic modulus of aluminum alloys, in order to achieve minimal stiffness requirements, a section of aluminum must be three times thicker than the equivalent steel part. Based on the fact that aluminum costs approximately $1.15/lb, whereas steel costs $0.25/lb, replacing steel with aluminum in higher strength applications dramatically increases the material cost at a minimal reduction in weight. However, in certain applications, the corrosion resistant aluminum alloys now used, permit designs that save about 50% of the weight as compared to similar designs in steel [ASM: Aluminum, 1993]. The relatively low modulus of elasticity for aluminum alloys offers advantages in structures built on a steel hull. Flexure of the steel hull results in low stresses in an aluminum superstructure as compared to those stresses induced in a similar steel superstructure. Consequently, continuous aluminum structures, such as deckhouses, can be built without special design features. In addition, cumulative savings in weight improve the stability and allow the width of the ship to be decreased.
Conclusions and Recommendations
Demands for greater ship size have forced designers to search for alternative materials to reduce ship weight while maintaining strength. When properly designed, aluminum typically reduces the weight of small structures made of low-carbon steel by over 50 percent. Dramatic technological advances have allowed aluminum to meet or exceed the minimum strength requirements for normal strength steels currently used in the shipbuilding industry. Aluminum has the additional advantage of superior resistance to corrosion, since it corrodes over 100 times more slowly than conventional structural carbon steel used to build ships. During the first year and a half of an eight-and-a-half-year study, steel corroded at a rate of 120 mm per year, while in a similar study, aluminum corroded at a rate of only 1 mm per year.
Using aluminum to replace steel has two major disadvantages. The first disadvantage is that aluminum alloys cannot meet the maximum yield strengths required in certain shipbuilding applications—only high-strength, low-alloy steels meet these strength requirements. The second disadvantage is that aluminum, at about $1.15 per pound, costs roughly five times more than steel, at about $0.25 per pound.
The feasibility of replacing steel with aluminum in the shipbuilding industry depends primarily on the application and cost constraints. Because aluminum alloys meet or exceed the minimum yield strength requirements for normal strength steels and have superior corrosion resistance properties, using aluminum to replace steel is feasible in most structural applications. However, because of higher costs, aluminum is not always economical. For high-strength applications, ship builders should sacrifice corrosion resistance and weight reduction in favor of the greater strength provided by HSLA steels. When normal strength materials are adequate—and costs not prohibitive—ship builders should consider using aluminum alloys to reduce ship weight and improve corrosion resistance.
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Glossary
bow: the front section of a ship. (Back)
bulkheads: the upright partitions dividing a ship into compartments. (Back)
Charpy test: a common test of brittleness in structural materials. A Charpy test is run by placing a specimen against a steel backing and striking it with a large pendulum. (Back)
coupon: a cigarette-sized sample of material. Coupons are the test specimens used with the Charpy test. (Back)
davits: the small cranes that project over the side of a ship and are used to raise and lower lifeboats. (Back)
grain structure: the arrangement or pattern of the particles composing a substance. (Back)
ice field: a large, level expanse of floating ice that is more than 5 miles in its greatest dimension. (Back)
lumen: the unit of luminous flux equal to the light given off by one candle. (Back)
stern: the rear section of a ship. (Back)
wireless: a radio telegraph or radiotelephone system. (Back)
References
Division of the History of Technology, Transportation Collections, National Museum of American History, in cooperation with the Public Inquiry Mail Service, Smithsonian Institution, "The Titanic," http://www.si.edu/resource/faq/nmah/titanic.htm (Washington, DC: Smithsonian Institution, May 1997).
Gannon, Robert, "What Really Sank the Titanic," Popular Science, vol. 246, no. 2 (February 1995), pp. 49-55.
Garzke, William H., David K. Brown, and Arthur Saniford, "The Structural Failure of the Titanic," Oceans Conference Record (IEEE), vol. 3 (1994), pp. 138-148.
Hill, Steve, "The Mystery of the Titanic: A Case of Brittle Fracture?" Materials World, vol. 4, no. 6 (June 1996), pp. 334-335.
Manning, George, The Theory and Technique of Ship Design (New York: John Wiley and Sons, Inc., 1956), pp. 25-53.
Muckle, William, Modern Naval Architecture (London: W.P. Griffith & Sons, 1951), pp. 121-125.
Refrigerator, Mister, "R.M.S. Titanic," http://www.scv.net/~fridge/index.htm (May 1998).
Rogers, Patrick, Anne-Marie O'Neill, and Sophfronia S. Gregory, "Sunken Dreams," People, vol. 49, no. 10 (March 1998), pp. 44-51.
Society of Naval Architects and Marine Engineers, Principles of Naval Architecture, 4th ed. (New York: The Society of Naval Architects and Marine Engineers, 1977), pp. 121-133.
Author's Note: Vicki Bassett is a senior in Mechanical Engineering at the University of Wisconsin. She studied technical communication under Professor Michael Alley.
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