پالایشگاه نفت یک واحد صنعتی است که در آن نفت خام به مواد مفیدتری مانند گاز مایع، نفت سفید، بنزین، گازوئیل، نفت کوره، آسفالت، قیر و دیگر فراوردههای نفتی تبدیل میگردد. پالایشگاههای نفت بهطور معمول واحدهای صنعتی بزرگ و پیچیدهای میباشند که در آنها واحدهای مختلف توسط مسیرهای لولهکشی متعددی به هم پیوند داده شدهاند.
نفت بهصورت خام یا فراوری نشده خیلی مفید نیست و بهصورتی که از دل زمین بیرون آمده کاربرد چندانی ندارد. با اینکه نفت شیرین (با لزجت کم و نیز با گوگرد کم) به صورت تصفیه نشده در وسایل محرکه با قوه بخار به کار برده میشد، گازها و سایر محلولهای سبکتر آن معمولاً داخل مخزن سوخت جمع شده و باعث بروز انفجار میگردید. غیر از مورد گفته شده برای استفاده از نفت، برای تولید محصولات دیگر مانند پلاستیک، فومها و ... نفت خام بهطور حتم باید پالایش گردد. فراوردههای سوختی نفتی در گستره وسیعی از کاربردها، سوخت کشتی، سوخت جت، بنزین و بسیاری دیگر موارد استفاده میشود. هر کدام از مواد یادشده دارای نقطه جوشی متفاوت میباشند از این رو میتوان آنها را توسط فرایند تقطیر از همدیگر جدا نمود. از آنجائیکه تقاضای زیادی برای اجزای مایع سبکتر وجود دارد از این رو در یک پالایشگاه مدرن نفتی هیدروکربنهای سنگین و اجزای گازی سبک در طی فرایندهای پیچیده و انرژی بری به مواد با ارزش تری تبدیل میشوند.
نفت به خاطر دارا بودن هیدروکربنهایی با وزن و طولهای مختلف مانند پارافین، آروماتیکها، نفتا، آلکنها، دینها و آلکالینها میتواند در موارد متعددی مفید واقع گردد. هیدروکربنها مولکولهایی با طولهای متفاوت هستند که تنها از هیدروژن و کربن تشکیل شدهاند، ساختارهای مختلف به آنها خواص متفاوتی میدهد. فن پالایش نفت در واقع عبارت است از جداکردن و بالابردن درجه خلوص اجزا تشکیل دهنده نفت از هم.
همینکه اجزا از هم جدا گردیده و خالص شدند میتوان ماده روغنکاری یا سوخت را بهطور مستقیم روانه بازار مصرف کرد. میتوان با ترکیب مولکولهای کوچکتر مانند ایزوبوتان و پروپیلن یا بوتیلن طی پروسههایی همانند آلکالنین کردن یا دیمرازسیون میتوان سوختی با اکتان موردنظر تهیه نمود. همچنین درجه اکتان بنزین را میتوان طی فرایند بهسازی توسط کاتالیزور بهبود بخشید که طی آن هیدروژن از هیدروکربن جداشده و هیدروکربن آروماتیکی تشکیل میگردد که درجه اکتان بسیار بیشتری دارد. تولیدات میانی برج جداکننده را میتوان طی پروسههای کراکینگ گرمایی، هیدروکراکینگ یا کراکینگ کاتالیزوری سیالی به محصولات سبک تری تبدیل نمود. مرحله نهایی در تولید بنزین ترکیب مواد هیدروکربن مختلف با درجههای اکتان متفاوت با همدیگر است تا به مشخصات محصول موردنظر دست یابیم.
معمولاً پالایشگاههای بزرگ توانایی پالایش از صدهزار تا چندین صدهزار بشکه نفت در روز را دارا میباشند. به دلیل ظرفیت بالای مورد نیاز، بسیاری از پالایشگاهها به صورت دائم برای مدت طولانی از چندین ماه تا چندین سال بطور مداوم کار میکنند.
پالایشگاههای نفت بسته به نوع خوراکی که بر مبنای آن طراحی شده اند دارای پیچیدگیهای متفاوتی هستند که بر اساس آن میزان و نوع محصولات متفاوتی تولید میکنند. میزان سرمایه گذاری برای احداث یک پالایشگاه با پیچیدگی متوسط بین ۱۸ تا ۱۹ هزار دلار به ازای هر بشکه ظرفیت پالایش است و با افزایش درجه پیچیدگی گاه تا ۲۴ تا ۲۵ هزار دلار در هر بشکه سرمایه گذاری نیاز دارد.
تولیدات پالایشگاه به ترتیب سبک به سنگین عبارتند از:
در شکل پروسسهای معمول در یک پالایشگاه نفت نشان داده شدهاست. پالایشگاههای نفت شامل واحدهای پروسس مختلفی است که در ذیل به توضیح مختصر هر یک از موارد میپردازیم:
اولین پالایشگاه جهان در پولشیتی، رومانی در سال ۱۸۵۶ ساخته شده، تا قبل از اشغال رومانی توسط نازیها چندین پالایشگاه دیگر نیز با سرمایه گذاری شرکتهای آمریکایی در همان محل تأسیس گردید. اکثر این پالایشگاهها در طی بمباران نیروی هوایی آمریکا در عملیات تیدال ویو در ۱۹۴۳ از کار افتادند. پس از این حادثه این پالایشگاهها دوباره سازی گردیدهاند. پالایشگاه قدیمی دیگر پالایشگاه Oljeon میباشد که هم اکنون به عنوان موزه در میراث جهانی یونسکو به عنوان سایت انگلسبرگ (Engelsberg) ثبت شدهاست. این پالایشگاه در ۱۸۷۵ افتتاح گردیدهاست. پالایشگاههای اولیه درایالات متحده تنها نفت سفید را پالایش میکردند و مابقی محصولات پالایش نشده به نزدیکترین رودخانه تخلیه میگردید. با اختراع اتومبیل پالایشگاهها به سمت تولید بنزین و سوخت دیزل هدایت شدند که تا به امروز به عنوان اصلیترین فراورده پالایشگاه شناخته میشوند. پس از نیمه دوم قرن بیستم در ایالات متحده سازمانهای محیط زیست قوانین بسیار محدودکنندهای برای احداث پالایشگاههای جدید (از نظر آلودگیهای آب و هوا) وضع نمودهاند. این شرایط احداث پالایشگاههای جدید در آمریکا را بسیار محدود و پرهزینه کردهاست بطوریکه آخرین پالایشگاه نفت احداث شده در ایالات متحده مربوط به سال ۱۹۷۶ میباشد. تشخیص اینکه کدام پالایشگاه هم اکنون بزرگترین پالایشگاه جهان است کار مشکلی است. زمانی ادعا میشد پالایشگاه راس التانورا در عربستان سعودی متعلق به آرامکو صاحب این عنوان است. با این حال میتوان ادعا نمود که در قرن بیستم پالایشگاه آبادان واقع در ایران بزرگترین پالایشگاه جهان بودهاست. این پالایشگاه در طی جنگ ایران و عراق متحمل خسارات فراوانی شد. هم اکنون کتاب ثبت رکوردهای گنیس (اکتبر ۲۰۰۶) پالایشگاه BP Ameco واقع در تگزاس ایالات متحده را به عنوان پالایشگاه دارای بیشترین ظرفیت پالایش (با ظرفیت ۴۳۳،۰۰۰ بشکه در روز) ثبت کردهاست.
پالایشگاههای نفت ایران[ویرایش]
Oil refinery or petroleum refinery is an industrial process plant where crude oil is transformed and refined into more useful products such as petroleum naphtha, gasoline, diesel fuel, asphalt base, heating oil, kerosene, liquefied petroleum gas, jet fuel and fuel oils.
Oil refineries are typically large, sprawling industrial complexes with extensive piping running throughout, carrying streams of fluids between large chemical processing units, such as distillation columns. In many ways, oil refineries use much of the technology of, and can be thought of, as types of chemical plants.
The crude oil feedstock has typically been processed by an oil production plant. There is usually an oil depot at or near an oil refinery for the storage of incoming crude oil feedstock as well as bulk liquid products.
Petroleum refineries are very large industrial complexes that involve many different processing units and auxiliary facilities such as utility units and storage tanks. Each refinery has its own unique arrangement and combination of refining processes largely determined by the refinery location, desired products and economic considerations.
Some modern petroleum refineries process as much as 800,000 to 900,000 barrels (127,000 to 143,000 cubic meters) per day of crude oil.
According to the Oil and Gas Journal in the world a total of 636 refineries were operated on the 31 December 2014 for a total capacity of 87.75 million barrels (13,951,000 m3).
The Chinese were among the first civilizations to refine oil. During 512 A.D. and 518 A.D., in the late Northern Wei Dynasty, the Chinese geographer, writer, and politician Li Daoyuan introduced the process of refining oil into various lubricants in his famous work Commentary on the Water Classic. During the first century AD, the Chinese were among the first peoples to refine oil for use as an energy source. During the Northern Song Dynasty, a workshop called the "Fierce Oil Workshop", was established in the city of Kaifeng to produce refined oil for the Song military as a weapon. The troops would then fill the iron cans with refined oil and threw them toward the enemy troops, causing a fire - effectively the world's first "fire bomb" The workshop was one of the world's earliest oil refining factories where thousands of people worked to produce Chinese oil powered weaponry.
Prior to the nineteenth century, petroleum was known and utilized in various fashions in Babylon, Egypt, China, Philippines, Rome and Azerbaijan. However, the modern history of the petroleum industry is said to have begun in 1846 when Abraham Gessner of Nova Scotia, Canada devised a process to produce kerosene from coal. Shortly thereafter, in 1854, Ignacy Lukasiewicz began producing kerosene from hand-dug oil wells near the town of Krosno, Poland. The first large petroleum refinery was built in Ploesti, Romania in 1856 using the abundant oil available in Romania.
In North America, the first oil well was drilled in 1858 by James Miller Williams in Ontario, Canada. In the United States, the petroleum industry began in 1859 when Edwin Drake found oil near Titusville, Pennsylvania. The industry grew slowly in the 1800s, primarily producing kerosene for oil lamps. In the early twentieth century, the introduction of the internal combustion engine and its use in automobiles created a market for gasoline that was the impetus for fairly rapid growth of the petroleum industry. The early finds of petroleum like those in Ontario and Pennsylvania were soon outstripped by large oil "booms" in Oklahoma, Texas and California.
Samuel Kier established America's first oil refinery in Pittsburgh on Seventh avenue near Grant Street, in 1853. Polish pharmacist and inventor Ignacy Łukasiewicz established oil refinery in Jasło, then part of the Austro-Hungarian Empire (now in Poland) in 1854. The first large refinery opened at Ploiești, Romania, in 1856-1857. After being taken over by Nazi Germany, the Ploiești refineries were bombed in Operation Tidal Wave by the Allies during the Oil Campaign of World War II. Another close contender for the title of hosting the world's oldest oil refinery is Salzbergen in Lower Saxony, Germany. Salzbergen's refinery was opened in 1860.
At one point, the refinery in Ras Tanura, Saudi Arabia owned by Saudi Aramco was claimed to be the largest oil refinery in the world. For most of the 20th century, the largest refinery was the Abadan Refinery in Iran. This refinery suffered extensive damage during the Iran–Iraq War. On the 31 December 2014, the world's largest refinery complex is the Jamnagar Refinery Complex, consisting of two refineries side by side operated by Reliance Industries Limited in Jamnagar, India with a combined production capacity of 1,240,000 barrels per day (197,000 m3/d). PDVSA's Paraguaná Refinery Complex in Paraguaná Peninsula, Venezuela with a capacity of 940,000 bbl/d (149,000 m3/d) and SK Energy's Ulsan in South Korea with 840,000 bbl/d (134,000 m3/d) are the second and third largest, respectively.
Prior to World War II in the early 1940s, most petroleum refineries in the United States consisted simply of crude oil distillation units (often referred to as atmospheric crude oil distillation units). Some refineries also had vacuum distillation units as well as thermal cracking units such as visbreakers (viscosity breakers, units to lower the viscosity of the oil). All of the many other refining processes discussed below were developed during the war or within a few years after the war. They became commercially available within 5 to 10 years after the war ended and the worldwide petroleum industry experienced very rapid growth. The driving force for that growth in technology and in the number and size of refineries worldwide was the growing demand for automotive gasoline and aircraft fuel.
In the United States, for various complex economic and political reasons, the construction of new refineries came to a virtual stop in about the 1980s. However, many of the existing refineries in the United States have revamped many of their units and/or constructed add-on units in order to: increase their crude oil processing capacity, increase the octane rating of their product gasoline, lower the sulfur content of their diesel fuel and home heating fuels to comply with environmental regulations and comply with environmental air pollution and water pollution requirements.
Oil refining in the United States
In the 19th century, refineries in the U.S. processed crude oil primarily to recover the kerosene. There was no market for the more volatile fraction, including gasoline, which was considered waste and was often dumped directly into the nearest river. The invention of the automobile shifted the demand to gasoline and diesel, which remain the primary refined products today.
Today, national and state legislation requires refineries to meet stringent air and water cleanliness standards. In fact, oil companies in the U.S. perceive obtaining a permit to build a modern refinery to be so difficult and costly that no new refineries were built (though many have been expanded) in the U.S. from 1976 until 2014, when the small Dakota Prairie Refinery in North Dakota is set to begin operation. More than half the refineries that existed in 1981 are now closed due to low utilization rates and accelerating mergers. As a result of these closures total US refinery capacity fell between 1981 and 1995, though the operating capacity stayed fairly constant in that time period at around 15,000,000 barrels per day (2,400,000 m3/d). Increases in facility size and improvements in efficiencies have offset much of the lost physical capacity of the industry. In 1982 (the earliest data provided), the United States operated 301 refineries with a combined capacity of 17.9 million barrels (2,850,000 m3) of crude oil each calendar day. In 2010, there were 149 operable U.S. refineries with a combined capacity of 17.6 million barrels (2,800,000 m3) per calendar day. By 2014 the number of refinery had reduced to 140 but the total capacity increased to 18.02 million barrels (2,865,000 m3) per calendar day. Indeed, in order to reduce operating costs and depreciation, refining is operated in less sites but of bigger capacity.
In 2009 through 2010, as revenue streams in the oil business dried up and profitability of oil refineries fell due to lower demand for product and high reserves of supply preceding the economic recession, oil companies began to close or sell the less profitable refineries.
Raw or unprocessed crude oil is not generally useful in industrial applications, although "light, sweet" (low viscosity, low sulfur) crude oil has been used directly as a burner fuel to produce steam for the propulsion of seagoing vessels. The lighter elements, however, form explosive vapors in the fuel tanks and are therefore hazardous, especially in warships. Instead, the hundreds of different hydrocarbon molecules in crude oil are separated in a refinery into components which can be used as fuels, lubricants, and as feedstocks in petrochemical processes that manufacture such products as plastics, detergents, solvents, elastomers and fibers such as nylon and polyesters.
Petroleum fossil fuels are burned in internal combustion engines to provide power for ships, automobiles, aircraft engines, lawn mowers, dirt bikes, and other machines. Different boiling points allow the hydrocarbons to be separated by distillation. Since the lighter liquid products are in great demand for use in internal combustion engines, a modern refinery will convert heavy hydrocarbons and lighter gaseous elements into these higher value products.
Oil can be used in a variety of ways because it contains hydrocarbons of varying molecular masses, forms and lengths such as paraffins, aromatics, naphthenes (or cycloalkanes), alkenes, dienes, and alkynes. While the molecules in crude oil include different atoms such as sulfur and nitrogen, the hydrocarbons are the most common form of molecules, which are molecules of varying lengths and complexity made of hydrogen and carbon atoms, and a small number of oxygen atoms. The differences in the structure of these molecules account for their varying physical and chemical properties, and it is this variety that makes crude oil useful in a broad range of several applications.
Once separated and purified of any contaminants and impurities, the fuel or lubricant can be sold without further processing. Smaller molecules such as isobutane and propylene or butylenes can be recombined to meet specific octane requirements by processes such as alkylation, or more commonly, dimerization. The octane grade of gasoline can also be improved by catalytic reforming, which involves removing hydrogen from hydrocarbons producing compounds with higher octane ratings such as aromatics. Intermediate products such as gasoils can even be reprocessed to break a heavy, long-chained oil into a lighter short-chained one, by various forms of cracking such as fluid catalytic cracking, thermal cracking, and hydrocracking. The final step in gasoline production is the blending of fuels with different octane ratings, vapor pressures, and other properties to meet product specifications. Another method for reprocessing and upgrading these intermediate products (residual oils) uses a devolatilization[permanent dead link] process to separate usable oil from the waste asphaltene material.
Oil refineries are large scale plants, processing about a hundred thousand to several hundred thousand barrels of crude oil a day. Because of the high capacity, many of the units operate continuously, as opposed to processing in batches, at steady state or nearly steady state for months to years. The high capacity also makes process optimization and advanced process control very desirable.
Petroleum products are usually grouped into four categories: light distillates (LPG, gasoline, naphtha), middle distillates (kerosene, jet fuel, diesel), heavy distillates and residuum (heavy fuel oil, lubricating oils, wax, asphalt).
These require blending various feedstocks, mixing appropriate additives, providing short term storage, and preparation for bulk loading to trucks, barges, product ships, and railcars. The primary end-products produced in petroleum refining may be grouped into four categories: light distillates, middle distillates, heavy distillates and others. This classification is based on the way crude oil is distilled and separated into fractions.
Further products include:
Oil refineries also produce various intermediate products such as hydrogen, light hydrocarbons, reformate and pyrolysis gasoline. These are not usually transported but instead are blended or processed further on-site. Chemical plants are thus often adjacent to oil refineries or a number of further chemical processes are integrated into it. For example, light hydrocarbons are steam-cracked in an ethylene plant, and the produced ethylene is polymerized to produce polyethene.
Because technical reasons and environment protection demand a very low sulfur content in all but the heaviest products, it is transformed to hydrogen sulfide via catalytic hydrodesulfurization and removed from the product stream via amine gas treating. Using the Claus process, hydrogen sulfide is afterwards transformed to elementary sulfur to be sold to the chemical industry. The rather large heat energy freed by this process is directly used in the other parts of the refinery. Often an electrical power plant is combined into the whole refinery process to take up the excess heat.
Chemical processes found in a refinery
Flow diagram of typical refinery
The image below is a schematic flow diagram of a typical oil refinery that depicts the various unit processes and the flow of intermediate product streams that occurs between the inlet crude oil feedstock and the final end products. The diagram depicts only one of the literally hundreds of different oil refinery configurations. The diagram also does not include any of the usual refinery facilities providing utilities such as steam, cooling water, and electric power as well as storage tanks for crude oil feedstock and for intermediate products and end products.
There are many process configurations other than that depicted above. For example, the vacuum distillation unit may also produce fractions that can be refined into end products such as: spindle oil used in the textile industry, light machinery oil, motor oil, and various waxes.
The crude oil distillation unit
The crude oil distillation unit (CDU) is the first processing unit in virtually all petroleum refineries. The CDU distills the incoming crude oil into various fractions of different boiling ranges, each of which are then processed further in the other refinery processing units. The CDU is often referred to as the atmospheric distillation unit because it operates at slightly above atmospheric pressure.
Below is a schematic flow diagram of a typical crude oil distillation unit. The incoming crude oil is preheated by exchanging heat with some of the hot, distilled fractions and other streams. It is then desalted to remove inorganic salts (primarily sodium chloride).
Following the desalter, the crude oil is further heated by exchanging heat with some of the hot, distilled fractions and other streams. It is then heated in a fuel-fired furnace (fired heater) to a temperature of about 398 °C and routed into the bottom of the distillation unit.
The cooling and condensing of the distillation tower overhead is provided partially by exchanging heat with the incoming crude oil and partially by either an air-cooled or water-cooled condenser. Additional heat is removed from the distillation column by a pumparound system as shown in the diagram below.
As shown in the flow diagram, the overhead distillate fraction from the distillation column is naphtha. The fractions removed from the side of the distillation column at various points between the column top and bottom are called sidecuts. Each of the sidecuts (i.e., the kerosene, light gas oil and heavy gas oil) is cooled by exchanging heat with the incoming crude oil. All of the fractions (i.e., the overhead naphtha, the sidecuts and the bottom residue) are sent to intermediate storage tanks before being processed further.
Location of petroleum refineries
A party searching for a site to construct a refinery or a chemical plant needs to consider the following issues:
Refineries which use a large amount of steam and cooling water need to have an abundant source of water. Oil refineries therefore are often located nearby navigable rivers or on a sea shore, nearby a port. Such location also gives access to transportation by river or by sea. The advantages of transporting crude oil by pipeline are evident, and oil companies often transport a large volume of fuel to distribution terminals by pipeline. Pipeline may not be practical for products with small output, and rail cars, road tankers, and barges are used.
Petrochemical plants and solvent manufacturing (fine fractionating) plants need spaces for further processing of a large volume of refinery products for further processing, or to mix chemical additives with a product at source rather than at blending terminals.
Safety and environment
The refining process releases a number of different chemicals into the atmosphere (see AP 42 Compilation of Air Pollutant Emission Factors) and a notable odor normally accompanies the presence of a refinery. Aside from air pollution impacts there are also wastewater concerns, risks of industrial accidents such as fire and explosion, and noise health effects due to industrial noise.
Many governments worldwide have mandated restrictions on contaminants that refineries release, and most refineries have installed the equipment needed to comply with the requirements of the pertinent environmental protection regulatory agencies. In the United States, there is strong pressure to prevent the development of new refineries, and no major refinery has been built in the country since Marathon's Garyville, Louisiana facility in 1976. However, many existing refineries have been expanded during that time. Environmental restrictions and pressure to prevent construction of new refineries may have also contributed to rising fuel prices in the United States. Additionally, many refineries (more than 100 since the 1980s) have closed due to obsolescence and/or merger activity within the industry itself.
Environmental and safety concerns mean that oil refineries are sometimes located some distance away from major urban areas. Nevertheless, there are many instances where refinery operations are close to populated areas and pose health risks. In California's Contra Costa County and Solano County, a shoreline necklace of refineries, built in the early 20th century before this area was populated, and associated chemical plants are adjacent to urban areas in Richmond, Martinez, Pacheco, Concord, Pittsburg, Vallejo and Benicia, with occasional accidental events that require "shelter in place" orders to the adjacent populations. A number of refineries are located in Sherwood Park, Alberta, directly adjacent to the City of Edmonton. The Edmonton metro area has a population of over 1,000,000 residents.
Petroleum refineries run as efficiently as possible to reduce costs. One major factor that decreases efficiency is corrosion of the metallic components found throughout refining process. Corrosion causes the failure of equipment items as well as dictating the maintenance schedule of the refinery, during which part or all of the refinery must be shut down. The corrosion-related direct costs in the U.S. petroleum industry as of 1996 was estimated as US$3.7 billion per year.
Corrosion occurs in various forms in the refining process, such as pitting corrosion from water droplets, embrittlement from hydrogen, and stress corrosion cracking from sulfide attack. From a materials standpoint, carbon steel is used for upwards of 80 per cent of refinery components, which is beneficial due to its low cost. Carbon steel is resistant to the most common forms of corrosion, particularly from hydrocarbon impurities at temperatures below 205 °C, but other corrosive chemicals and environments prevent its use everywhere. Common replacement materials are low alloy steels containing chromium and molybdenum, with stainless steels containing more chromium dealing with more corrosive environments. More expensive materials commonly used are nickel, titanium, and copper alloys. These are primarily saved for the most problematic areas where extremely high temperatures and/or very corrosive chemicals are present.
Corrosion is fought by a complex system of monitoring, preventative repairs and careful use of materials. Monitoring methods include both off-line checks taken during maintenance and on-line monitoring. Off-line checks measure corrosion after it has occurred, telling the engineer when equipment must be replaced based on the historical information they have collected. This is referred to as preventative management.
On-line systems are a more modern development, and are revolutionizing the way corrosion is approached. There are several types of on-line corrosion monitoring technologies such as linear polarization resistance, electrochemical noise and electrical resistance. On-Line monitoring has generally had slow reporting rates in the past (minutes or hours) and been limited by process conditions and sources of error but newer technologies can report rates up to twice per minute with much higher accuracy (referred to as real-time monitoring). This allows process engineers to treat corrosion as another process variable that can be optimized in the system. Immediate responses to process changes allow the control of corrosion mechanisms, so they can be minimized while also maximizing production output. In an ideal situation having on-line corrosion information that is accurate and real-time will allow conditions that cause high corrosion rates to be identified and reduced. This is known as predictive management.
Materials methods include selecting the proper material for the application. In areas of minimal corrosion, cheap materials are preferable, but when bad corrosion can occur, more expensive but longer lasting materials should be used. Other materials methods come in the form of protective barriers between corrosive substances and the equipment metals. These can be either a lining of refractory material such as standard Portland cement or other special acid-resistant cements that are shot onto the inner surface of the vessel. Also available are thin overlays of more expensive metals that protect cheaper metal against corrosion without requiring lots of material.