Satish Lele

Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxides or salts of the original metal. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.

Galvanic corrosion: Galvanic corrosion occurs when two different metals have physical or electrical contact with each other and are immersed in a common electrolyte, or when the same metal is exposed to electrolyte with different concentrations. In a galvanic couple, the more active metal (the anode) corrodes at an accelerated rate and the more noble metal (the cathode) corrodes at a retarded rate. When immersed separately, each metal corrodes at its own rate. What type of metal to use is readily determined by following the galvanic series. Zinc is often used as a sacrificial anode for steel structures. Galvanic corrosion is of major interest to the marine industry and also anywhere water containing salts, contacts pipes or metal structures.
Factors such as relative size of anode, types of metal, and operating conditions such as temperature, humidity, salinity, etc., affect galvanic corrosion. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials. Galvanic corrosion is often utilized in sacrificial anodes.
Galvanic series: In a given environment one metal will be either more noble or more active than others, based on how strongly its ions are bound to the surface. Two metals in electrical contact share the same electrons, so that the "tug-of-war" at each surface is analogous to competition for free electrons between the two materials. Using the electrolyte as a host for the flow of ions in the same direction, the active metal will take electrons from the noble one. The resulting mass flow or electrical current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called a galvanic series and is useful in predicting and understanding corrosion.
Corrosion removal: Often it is possible to chemically remove the products of corrosion. For example phosphoric acid in the form of jelly is often applied to ferrous tools or surfaces to remove rust. Corrosion removal should not be confused with electro polishing, which removes some layers of the underlying metal to make a smooth surface. For example, phosphoric acid may also be used to electro polish copper but it does this by removing copper, not the products of copper corrosion. Some metals are more intrinsically resistant to corrosion than others. There are various ways of protecting metals from corrosion including painting, hot dip galvanizing, and combinations of these.
Passivation: It refers to the spontaneous formation of an ultra thin film of corrosion products known as passive film, on the metal's surface that act as a barrier to further oxidation. The chemical composition and microstructure of a passive film are different from the underlying metal. Typical passive film thickness on aluminum, stainless steels and alloys is within 10 nanometers. The passive film is different from oxide layers that are formed upon heating and are in the micrometer thickness range. The passive film recovers if removed or damaged whereas the oxide layer does not. Passivation in natural environments such as air, water and soil at moderate pH is seen in such materials as aluminum, stainless steel, titanium, and silicon. Passivation is primarily determined by metallurgical and environmental factors. The effect of pH is summarized using Pourbaix diagrams, but many other factors are influential. Some conditions that inhibit passivation include high pH for aluminum and zinc, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal, rather than the electrolyte) and fluoride ions for silicon. On the other hand, unusual conditions may result in passivation of materials that are normally unprotected, as the alkaline environment of concrete does for steel rebar. Exposure to a liquid metal such as mercury or hot solder can often circumvent passivation mechanisms. Passivation is extremely useful in mitigating corrosion damage, however even a high-quality alloy will corrode if its ability to form a passivating film is hindered. Proper selection of the right grade of material for the specific environment is important for the long-lasting performance of this group of materials. If breakdown occurs in the passive film due to chemical or mechanical factors, the resulting major modes of corrosion may include pitting corrosion, crevice corrosion and stress corrosion cracking.
Pitting corrosion: Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which compete as anions, can interfere with a given alloy's ability to re-form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an autocatalytic process. In extreme cases, the sharp tips of extremely long and narrow corrosion pits can cause stress concentration to the point that otherwise tough alloys can shatter. A thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in passivated alloys, but it can be prevented by control of the alloy's environment.
SS Corrosion: Stainless steel can pose special corrosion challenges, since its passivating behavior relies on the presence of a major alloying component of at least 11.5% chromium. Because of the elevated temperatures of welding and heat treatment, chromium carbides can form in the grain boundaries of stainless alloys. This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries in the heat affected zones) in highly corrosive environments.
A stainless steel is said to be sensitized if chromium carbides are formed in the microstructure. A typical microstructure of a normalized type-304 stainless steel shows no signs of sensitization while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries.
Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knife line attack. As its name implies, corrosion is limited to a very narrow zone adjacent to the weld, often only a few micrometers across, making it even less noticeable.
Crevice corrosion: Crevice corrosion is a localized form of corrosion occurring in confined spaces (crevices), to which the access of the working fluid from the environment is limited. Formation of a differential aeration cell leads to corrosion inside the crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles.
Crevice corrosion is influenced by the crevice type (metal-metal, metal-nonmetal), crevice geometry (size, surface finish), and metallurgical and environmental factors. The susceptibility to crevice corrosion can be evaluated with ASTM standard procedures. A critical crevice corrosion temperature is commonly used to rank a material's resistance to crevice corrosion.
Microbial corrosion: Microbial corrosion, or commonly known as microbiologically influenced corrosion (MIC), is a corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metallic and non-metallic materials, in the presence or absence of oxygen. Sulfate-reducing bacteria are active in the absence of oxygen (anaerobic). They produce hydrogen sulfide, causing sulfide stress cracking. In the presence of oxygen (aerobic), some bacteria may directly oxidize iron to iron oxides and hydroxides, other bacteria oxidize sulfur and produce sulfuric acid causing biogenic sulfide corrosion. Concentration cells can form in the deposits of corrosion products, leading to localized corrosion.
Accelerated low-water corrosion (ALWC) is a particularly aggressive form of MIC that affects steel piles in seawater near the low water tide mark. It is characterized by an orange sludge, which smells of hydrogen sulfide when treated with acid. Corrosion rates can be very high and design corrosion allowances can soon be exceeded leading to premature failure of the steel pile. Piles that have been coating and have cathodic protection installed at the time of construction are not susceptible to ALWC. For unprotected piles, sacrificial anodes can be installed local to the affected areas to inhibit the corrosion or a complete retrofitted sacrificial anode system can be installed. Affected areas can also be treated electrochemically by using an electrode to first produce chlorine to kill the bacteria, and then to produced a calcareous deposit, which will help shield the metal from further attack.
High-temperature Corrosion: High-temperature corrosion is chemical deterioration of a material (typically a metal) as a result of heating. This non-galvanic form of corrosion can occur when a metal is subjected to a hot atmosphere containing oxygen, sulfur or other compounds capable of oxidizing (or assisting the oxidation of) the material concerned. For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion.
The products of high temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperatures in hostile conditions. Such high temperature corrosion products, in the form of compacted oxide layer glazes, prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.
Metal Dusting: Metal dusting is a catastrophic form of corrosion that occurs when susceptible materials are exposed to environments with high carbon activities, such as synthesis gas and other high carbon monoxide (CO) environments. The corrosion manifests itself as a break-up of bulk metal to metal powder. The suspected mechanism is firstly the deposition of a graphite layer on the surface of the metal, usually from carbon monoxide (CO) in the vapor phase. This graphite layer is then thought to form metastable M3C species (where M is the metal), which migrate away from the metal surface. However, in some regimes no M3C species is observed indicating a direct transfer of metal atoms into the graphite layer.
Protection from corrosion, Surface treatments
Applied coatings: Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the structural material. Aside from cosmetic and manufacturing issues, there are trade offs in mechanical flexibility versus resistance to abrasion and high temperature. Platings usually fail only in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. For this reason, it is often wise to plate with active metal such as zinc or cadmium. Painting either by roller or brush is more desirable for tight spaces, spray would be better for larger coating areas such as steel decks and water front applications. Flexible polyurethane coatings, like Durabak-M26 for example, can provide an anti-corrosive seal with a highly durable slip resistant membrane. Painted coatings are relatively easy to apply and have fast drying times although temperature and humidity may cause dry times to vary.
Reactive coatings: If the environment is controlled (especially in recirculating systems), corrosion inhibitors can often be added to it. These form an electrically insulating or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in the coating, since extra inhibitors can be made available wherever metal becomes exposed. Chemicals that inhibit corrosion include some of the salts in hard water (Roman water systems are famous for their mineral deposits), chromates, phosphates, polyaniline, other conducting polymers and a wide range of specially designed chemicals that resemble surfactants (i.e. long chain organic molecules with ionic end groups).
Anodization: Aluminium alloys often undergo a surface treatment. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal's oxide film. These pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.
Anodizing is very resilient to weathering and corrosion, so it is commonly used for building facades and other areas that the surface will come into regular contact with the elements. Whilst being resilient, it must be cleaned frequently. If left without cleaning, panel edge staining will naturally occur.
Biofilm coatings: A new form of protection has been developed by applying certain species of bacterial films to the surface of metals in highly corrosive environments. This process increases the corrosion resistance substantially. Alternatively, antimicrobial producing biofilms can be used to inhibit mild steel corrosion from sulfate-reducing bacteria.
Cathodic protection: Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with a piece of another more easily corroded "sacrificial metal" to act as the anode of the electrochemical cell. The sacrificial metal then corrodes instead of the protected metal. For structures where passive galvanic CP is not adequate, for example in long pipelines, an external DC electrical power source is sometimes used to provide current. Cathodic protection systems are used to protect a wide range of metallic structures in various environments. Common applications are: steel, water or fuel pipelines and storage tanks such as home water heaters, steel pier piles, ship and boat hulls, offshore oil platforms and onshore oil well casings and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust. Cathodic protection can, in some cases, prevent stress corrosion cracking.
Sacrificial anode protection: For effective Cathodic protection, the potential of the steel surface is polarized (pushed) more negative until the metal surface has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For galvanic Cathodic protection systems, the anode material corrodes under the influence of the steel, and eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference in electrochemical potential between the anode and the cathode.
Anodic protection: Anodic protection impresses anodic current on the structure to be protected (opposite to the cathodic protection). It is appropriate for metals that exhibit passivity (e.g., stainless steel) and suitably small passive current over a wide range of potentials. It is used in aggressive environments, e.g., solutions of sulfuric acid.
Pipelines are routinely protected by a coating supplemented with cathodic protection. System for a pipeline would consist of a DC power source, which is often an AC powered rectifier and an anode, or array of anodes buried in the ground (the anode ground bed). The DC power source would typically have a DC output of between 10 and 50 amperes and 50 volts, but this depends on several factors, such as the size of the pipeline. The positive DC output terminal would be connected via cables to the anode array, while another cable would connect the negative terminal of the rectifier to the pipeline, preferably through junction boxes to allow measurements to be taken. Anodes can be installed in a vertical hole and backfilled with conductive coke (a material that improves the performance and life of the anodes) or laid in a prepared trench, surrounded by conductive coke and backfilled. The choice of grounded type and size depends on the application, location and soil resistivity. The output of the DC source would then be adjusted to the optimum level after conducting various tests including measurements of electrochemical potential. It is sometimes more economically viable to protect a pipeline using galvanic anodes. This is often the case on smaller diameter pipelines of limited length.
Galvanizing generally refers to hot-dip galvanizing which is a way of coating steel with a layer of metallic zinc. Galvanized coatings are quite durable in most environments because they combine the barrier properties of a coating with some of the benefits of cathodic protection. If the zinc coating is scratched or otherwise locally damaged and steel is exposed, the surrounding areas of zinc coating form a galvanic cell with the exposed steel and protect it from corrosion. This is a form of localized cathodic acts as a sacrificial anode.
Galvanizing, while using the electrochemical principle of cathodic protection, is not actually cathodic protection. Cathodic protection requires the anode to be separate from the metal surface to be protected, with an ionic connection through the electrolyte and an electron connection through a connecting cable. This means that any area of the protected structure within the electrolyte can be protected, whereas in the case of galvanizing, only areas very close to the zinc are protected. Hence, a larger area of bare steel would only be protected around the edges.
Rate of corrosion: A simple test for measuring corrosion is the weight loss method. The method involves exposing a clean weighed piece of the metal or alloy to the corrosive environment for a specified time followed by cleaning to remove corrosion products and weighing the piece to determine the loss of weight. The rate of corrosion (R) is calculated as
R = KW/(den A t)
where K is a constant, W is the weight loss of the metal in time t, A is the surface area of the metal exposed, and den is the density of the metal (in g/cm3).
Corrosion in nonmetals: Most ceramic materials are almost entirely immune to corrosion. The strong chemical bonds that hold them together leave very little free chemical energy in the structure. They can be thought of as already corroded. When corrosion does occur, it is almost always a simple dissolution of the material or chemical reaction, rather than an electrochemical process. A common example of corrosion protection in ceramics is the lime added to soda-lime glass to reduce its solubility in water. Though it is not nearly as soluble as pure sodium silicate, normal glass does form sub-microscopic flaws when exposed to moisture. Due to its brittleness, such flaws cause a dramatic reduction in the strength of a glass object during its first few hours at room temperature.
Corrosion of glasses: Glass disease is the corrosion of silicate glasses in aqueous solutions. It is governed by two mechanisms: diffusion controlled leaching (ion exchange) and hydrolytic dissolution of the glass network.
Corrosion of polymers: Polymer degradation involves several complex and often poorly understood physiochemical processes. These are strikingly different from the other processes discussed, So the term "corrosion" is only applied to them in a loose sense of the word. Because of their large molecular weight, very little entropy can be gained by mixing a given mass of polymer with another substance, making them generally quite difficult to dissolve. While dissolution is a problem in some polymer applications, it is relatively simple to design against. A more common and related problem is swelling, where small molecules infiltrate the structure, reducing strength and stiffness and causing a volume change. Conversely, many polymers (notably flexible vinyl) are intentionally swelled with plasticizers, which can be leached out of the structure, causing brittleness or other undesirable changes. The most common form of degradation, however, is a decrease in polymer chain length. Mechanisms which break polymer chains are familiar to biologists because of their effect on DNA: ionizing radiation (most commonly ultraviolet light), free radicals, and oxidizers such as oxygen, ozone, and chlorine. Ozone cracking is a well-known problem affecting natural rubber for example. Additives can slow these processes very effectively, and can be as simple as a UV-absorbing pigment (i.e., titanium dioxide or carbon black). Plastic shopping bags often do not include these additives so that they break down more easily as litter.
Steel Abrasive Cleaning: Abrasive materials are steel, glass or sand particles that are used as abrasive media. They are usually available in two different shapes (shot and grit) that address different industrial applications. Steel shot refers to spherical grains made of molten steel through an atomization or granulation process, available in different sizes and hardness. Steel grit characterizes grains with a predominantly angular shape. These grains are obtained by crushing steel shot, therefore they exhibit sharp edges and broken sections. Harder than steel shot, it is also available in different sizes and hardness. Surface preparation is a series of operations including cleaning and physical modification of a surface. Steel shot and grit are used in surface preparation process for cleaning metal surfaces which are covered with mill scale, dirt, rust, or paint coatings and for physically modifying the metal surface such as creating roughness for better application of paint and coating. The steel shots are generally employed in shot blasting machines.
Shot Blasting: While many people regard any blast cleaning process as shot blasting, strictly speaking shot blasting refers to blasting with small steel balls or pellets, as distinct from other types of blast cleaning materials.
Abrasive Blast Cleaning: Abrasive blast cleaning is the general term given to a range of blast cleaning processes designed to remove old paint and coatings, debris, oil, grime, rust and other contaminants from a variety of metal surfaces and components. Blast cleaning is conducted by propelling abrasive materials at high speed through a jet nozzle using pressurized air onto the surface of the component, leaving it clean, dry, profiled and ready for applying new coatings. Blast cleaning can be carried out manually, or by using automated blast cabinets and the pressure can be varied to suit the type of metal or treatment required. Specialized materials or blast media in a variety of sizes or grades are used for treating different types of metals, surfaces and components, depending on the finish required. Following materials are used for Abrasive Blast Cleaning.
  • Chilled Iron: Chilled iron grit is an angular abrasive material suitable for general blast cleaning applications such as the removal of paint and oxides from carbon steels and ferrous castings. Blasting with chilled iron is a fairly aggressive cleaning process which makes it unsuitable for use with softer metals such as aluminum. It should not be used on machinery that is exposed to moisture, or items which are prone to corrosion such as engine parts or stainless steel. Chilled iron grit is ideal for the removal of paint and contaminants from carbon steels and ferrous castings. It is commonly used for de-rusting, deburring, roughening and surface preparation prior to coating or painting.
  • Steel Balls: Steel ball shot blasting is one of the most widely used methods for cleaning and stripping metal surfaces and components. The process involves firing small steel balls (1-6 mm in diameter) at high speed against the surface of the metal or component. The finish is determined by the size of the steel shot. Larger shot has a more aggressive cleaning action and produces a rougher finish. Smaller steel shot creates a smoother, more polished surface. Larger steel shot is ideal for removing rust, scale and other contaminants from heavy steel, malleable iron and gray iron castings. Smaller steel shot is suitable for cleaning small to medium sized ferrous and non-ferrous castings and machined parts.
  • Steel Grit: Steel grit blasting is used for aggressive cleaning projects such as stripping contaminants from steel and other industrial metals. The cleaning action of steel grit produces an etched surface providing excellent adhesive properties for a variety of paints and coatings. Steel grit blasting is suitable for steel and foundry metals and is also used for aircraft and aero-space components.
  • Aluminum Oxide: Aluminum oxide is a sharp, abrasive blasting material used in sand blast finishing. It is harder than most common dry abrasive blast media and will cut even the hardest metals and surfaces. Aluminum oxide is ideal for removing rust from cast iron parts like manifolds and water pumps.
  • Garnet: Garnet is a hard, heavy abrasive used to prepare surfaces on non-ferrous metals. It is an inert natural mineral which cuts faster than many other blast abrasives while producing lower dust emissions. The deep cleaning action of garnet ensures the removal of rust, debris and other contaminants, producing a clean substrate and a uniform surface profile. Garnet abrasives are suitable for the treatment of stainless steel, aluminium and other non-ferrous metals.
  • Glass Beads: Glass bead or dry bead blasting uses spherical beads for cleaning metal parts without damaging the surface. This media offers a gentle cleaning process creating a softer, more cosmetic finish than angular abrasives. Glass bead abrasives provide a silica-free option for blast cleaning, peening, honing, descaling and light deburring. Glass bead cleaning is suitable for soft metals such as aluminum and brass. Ideal for pistons, engine blocks and for light rust removal. A good choice for the restoration of car parts, motorcycles and other components where a gentle cleaning action is required.
  • Plastic Abrasives: Plastic Abrasives such as Urea, Acrylic, Polyester and Melamine deliver a highly effective stripping rate, removing coatings and contaminants without damaging the base metal. They are ideal for paint stripping, cleaning, deflashing and deburring operations on aluminum and other soft metals. Plastic abrasives are widely used for restoring components in the aerospace and automotive industries.
Painting of Steel Pipe: Steel pipes can be painted by any color needed for its intended purpose. The pipe must be clean and dry before painting to improve how well the primer and paint adhere to the surface. Choose a primer and paint that are specifically designed for metal, with a rust retardant formulated into the product to prevent rust from developing under the paint. With proper preparation, paint will stick to metal. Compatible primers and paints also play a key role. Surface contamination is the main culprit that prevents the paint from sticking. Even rusted metal pipes can be painted with proper preparation. Taking just a few minutes to read paint and primer instructions is also important so that you know exactly when you need to apply the top coat and second top coat for best performance.
Metal Primers: A primer or undercoat is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted.
When primers are used: Primer is a paint product that allows finishing paint to adhere much better than if it were used alone. For this purpose, primer is designed to adhere to surfaces and to form a binding layer that is better prepared to receive the paint. Because primers do not need to be engineered to have durable, finished surfaces, they can instead be engineered to have improved filling and binding properties with the material underneath. Sometimes, this is achieved with specific chemistry, as in the case of aluminum primer, but more often, this is achieved through controlling the primer's physical properties such as porosity, tackiness, and hygroscopy.
Priming is mandatory if the material is not water resistant and will be exposed to the elements. Primers can usually be tinted to a close match with the color of the finishing paint. If the finishing paint is a deep color, tinting the primer can reduce the number of layers of finishing paint that are necessary for good uniformity across the painted surface. Some metals, such as untreated aluminum, require a primer. Others may not. A primer designed for metal is still highly recommended, if a part is to be exposed to moisture. Once water seeps through to the bare metal, oxidation will begin (plain steel will simply rust). Metal primers might contain additional materials to protect against corrosion, such as sacrificial zinc.
Metal hydroxides/oxides do not provide a solid surface for the paint to adhere to, and paint will come off in large flakes. Using a primer will provide extra insurance against such a scenario. An additional reason for using a primer on metal could be the poor condition of the surface. A steel part can be rusty, for example. Of course, the best solution is to thoroughly clean the metal, but when this is not a viable option, special kinds of primers can be used that chemically convert rust to the solid metal salts. And even though such surface is still lacking in comparison to the shiny clean metal, it is yet much better than weak, porous rust. Painting and gluing aluminum is especially important in the aircraft industry, which uses toxic zinc chromate primers and chromating to add the necessary adhesion properties.
Metal Pickling: Pickling is the chemical removal of rust and scale from a wide range of products. It ensures 100% cleaning and rust prevention. All parts are separated to ensure 100% cleaning and coating. Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale from ferrous metals, copper, and aluminum alloys. A solution called pickle liquor, which contains strong acids, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steel making processes.
  • Process: Many hot working processes and other processes that occur at high temperatures, leave a discoloring oxide layer or scale on the surface. In order to remove the scale, the work piece is dipped into a vat of pickle liquor. The primary acid used is hydrochloric acid, although sulfuric acid was previously more common. Hydrochloric acid is more expensive than sulfuric acid, but it pickles much faster while minimizing base metal loss. The speed is a requirement for integration in automatic steel mills that run production at high speed. Speeds as high as 800 ft/min (~240 meters / min) have been reported. Carbon steels, with an alloy content less than or equal to 6%, are often pickled in hydrochloric or sulfuric acid. Steels with an alloy content greater than 6% must be pickled in two steps and other acids are used, such as phosphoric, nitric and hydrofluoric acid. Rust resistant and acid resistant chromium-nickel steels are pickled in a bath of hydrochloric and nitric acid. Most copper alloys are pickled in dilute sulfuric acid, but brass is pickled in concentrated sulfuric and nitric acid mixed with sodium chloride and soot. Sheet steel that undergoes acid pickling will oxidize (rust) when exposed to atmospheric conditions of moderately high humidity. For this reason, a thin film of oil or similar waterproof coating is applied to create a barrier to moisture in the air. This oil film must later be removed for many fabrication, plating or painting processes.
  • Disadvantages: Acid cleaning has limitations in that, it is difficult to handle because of its corrosiveness, and it is not applicable to all steels. Hydrogen embrittlement becomes a problem for some alloys and high-carbon steels. The hydrogen from the acid reacts with the surface and makes it brittle and causes cracks. Because of its high reactance to treatable steels, acid concentrations and solution temperatures must be kept under control to assure desired pickling rates.
  • Waste products: Pickling sludge is the waste product from pickling, and includes acidic rinse waters, metallic salts and waste acid. Spent pickle liquor is considered a hazardous waste. Pickle sludge from steel processes is usually neutralized with lime and disposed of in a landfill. After neutralization the Environmental Protection Agency no longer deems it a hazardous waste. The lime neutralization process raises the pH of the spent acid and makes heavy metals in the sludge less likely to leach into the environment. Since the 1960s, hydrochloric pickling sludge is often treated in a hydrochloric acid regeneration system, which recovers some of the hydrochloric acid and ferric oxide. The rest must still be neutralized and disposed of in land fills. The by-products of nitric acid pickling are marketable to other industries, such as fertilizer processors.
  • Alternatives: Smooth clean surface (SCS) and eco pickled surface (EPS) are more recent alternatives. In the Smooth clean surface process, surface oxidation is removed using an engineered abrasive and the process leaves the surface resistant to subsequent oxidation without the need for oil film or other protective coating. Eco pickled surface is a more direct replacement for acid pickling. Acid pickling relies on chemical reactions while eco pickled surface uses mechanical means. The eco pickled surface process is considered "environmentally friendly" compared with acid pickling. It imparts to carbon steel a high degree of rust resistance, eliminating the need to apply the oil coating that serves as a barrier to oxidation for acid-pickled carbon steel.

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