Passivation of aluminum with nitric acid

Aluminum is an amphoteric metal.
The electronic configuration of the aluminum atom is 1s22s22p63s23p1. Thus, it has three valence electrons on its outer electron layer: 2 on the 3s and 1 on the 3p sublevel. Due to this structure, it is characterized by reactions as a result of which the aluminum atom loses three electrons from the outer level and acquires an oxidation state of +3.

Aluminum is a highly reactive metal and exhibits very strong reducing properties.

with oxygen

When absolutely pure aluminum comes into contact with air, aluminum atoms located in the surface layer instantly interact with oxygen in the air and form a thin, tens of atomic layers thick, durable oxide film of Al2O3 composition, which protects aluminum from further oxidation. It is also impossible to oxidize large samples of aluminum even at very high temperatures. However, fine aluminum powder burns quite easily in a burner flame:

4Al + 3O2 = 2Al2O3

with halogens

Aluminum reacts very vigorously with all halogens.

Thus, the reaction between mixed aluminum and iodine powders occurs already at room temperature after adding a drop of water as a catalyst. Equation for the interaction of iodine with aluminum:

2Al + 3I2 =2AlI3

Aluminum also reacts with bromine, which is a dark brown liquid, without heating. Simply add a sample of aluminum to liquid bromine: a violent reaction immediately begins, releasing a large amount of heat and light:

2Al + 3Br2 = 2AlBr3

The reaction between aluminum and chlorine occurs when heated aluminum foil or fine aluminum powder is added to a flask filled with chlorine. Aluminum burns effectively in chlorine according to the equation:

2Al + 3Cl2 = 2AlCl3

with sulfur

When heated to 150-200 °C or after igniting a mixture of powdered aluminum and sulfur, an intense exothermic reaction begins between them with the release of light:

- aluminum sulfide

with nitrogen

When aluminum reacts with nitrogen at a temperature of about 800 oC, aluminum nitride is formed:

with carbon

At a temperature of about 2000oC, aluminum reacts with carbon and forms aluminum carbide (methanide), containing carbon in the -4 oxidation state, as in methane.

with water

As mentioned above, a stable and durable oxide film of Al2O3 prevents aluminum from oxidizing in air. The same protective oxide film makes aluminum inert towards water.

When removing the protective oxide film from the surface by methods such as treatment with aqueous solutions of alkali, ammonium chloride or mercury salts (amalgiation), aluminum begins to react vigorously with water to form aluminum hydroxide and hydrogen gas:

2Al + 6H2O = 2Al(OH)3 + 3H2↑

with metal oxides

After igniting a mixture of aluminum with oxides of less active metals (to the right of aluminum in the activity series), an extremely violent, highly exothermic reaction begins.

Thus, in the case of interaction of aluminum with iron (III) oxide, a temperature of 2500-3000°C develops.

As a result of this reaction, high-purity molten iron is formed:

2AI + Fe2O3 = 2Fe + Al2O3

This method of obtaining metals from their oxides by reduction with aluminum is called aluminothermy or aluminothermy.

with non-oxidizing acids

The interaction of aluminum with non-oxidizing acids, i.e. with almost all acids, except concentrated sulfuric and nitric acids, leads to the formation of an aluminum salt of the corresponding acid and hydrogen gas:

a) 2Al + 3H2SO4(dil.) = Al2(SO4)3 + 3H2↑

2Al0 + 6H+ = 2Al3+ + 3H20;

b) 2AI + 6HCl = 2AICl3 + 3H2↑

-concentrated sulfuric acid

The interaction of aluminum with concentrated sulfuric acid under normal conditions and at low temperatures does not occur due to an effect called passivation.

When heated, the reaction is possible and leads to the formation of aluminum sulfate, water and hydrogen sulfide, which is formed as a result of the reduction of sulfur, which is part of sulfuric acid:

Such a deep reduction of sulfur from the oxidation state +6 (in H2SO4) to the oxidation state -2 (in H2S) occurs due to the very high reducing ability of aluminum.

- concentrated nitric acid

Under normal conditions, concentrated nitric acid also passivates aluminum, which makes it possible to store it in aluminum containers.

Just as in the case of concentrated sulfuric acid, the interaction of aluminum with concentrated nitric acid becomes possible with strong heating, and the reaction predominantly occurs:

- dilute nitric acid

The interaction of aluminum with diluted nitric acid compared to concentrated nitric acid leads to products of deeper nitrogen reduction. Instead of NO, depending on the degree of dilution, N2O and NH4NO3 can be formed:

8Al + 30HNO3(dil.) = 8Al(NO3)3 +3N2O↑ + 15H2O

8Al + 30HNO3(ultra dilute) = 8Al(NO3)3 + 3NH4NO3 + 9H2O

with alkalis

Aluminum reacts both with aqueous solutions of alkalis:

2Al + 2NaOH + 6H2O = 2Na[Al(OH)4] + 3H2↑

and with pure alkalis during fusion:

In both cases, the reaction begins with the dissolution of the protective film of aluminum oxide:

Al2O3 + 2NaOH + 3H2O = 2Na[Al(OH)4]

Al2O3 + 2NaOH = 2NaAlO2 + H2O

In the case of an aqueous solution, aluminum, cleared of the protective oxide film, begins to react with water according to the equation:

2Al + 6H2O = 2Al(OH)3 + 3H2↑

The resulting aluminum hydroxide, being amphoteric, reacts with an aqueous solution of sodium hydroxide to form soluble sodium tetrahydroxoaluminate:

Metal passivation: purpose, technology, methods

Although stainless steel is highly resistant to corrosion, the additional protection provided by passivation is desirable. In some cases, when even products made of stainless steel are at high risk of corrosion, the need to perform such a procedure is beyond doubt.

Examples of stainless steel surfaces subjected to corrosion and the results of passivation

What causes the high corrosion resistance of stainless steels?

The essence of such a phenomenon as corrosion is that the surface of the metal begins to deteriorate under the influence of negative external factors and the environment.

Typically, corrosion due to constant oxidation affects the metal layer by layer, gradually destroying the internal structure of the steel.

In many cases, it no longer makes sense to localize the affected areas of the internal structure of the metal, so steel products have to be replaced with new ones.

Passivation (or passivation), as a technology that allows for reliable protection of steel from corrosion, underlies the creation of such a unique metal as stainless steel. The chemical composition of the vast majority of steels belonging to the stainless category may contain various elements:

nickel;
molybdenum;
cobalt;
niobium;
manganese.

However, the main alloying element of such steels, the amount of which in their composition can vary between 12–20%, is chromium.

The addition of various alloying elements to the composition of stainless steels makes it possible to give them the required physical and chemical characteristics, but it is chromium that is responsible for the corrosion resistance of the steel alloy.

The effect of chromium on the properties of stainless steel

Stainless steel alloys, which contain 12% chromium, exhibit high corrosion resistance only when interacting with ambient air.

If the amount of chromium in the chemical composition of stainless steel is increased to 17%, then products made from it can easily interact with nitric acid without losing their performance characteristics.

To make the metal resistant to even more aggressive environments, which include hydrochloric, sulfuric and other acids, the quantitative content of chromium in it is not only increased, but also elements such as copper, molybdenum, nickel, etc. are added to its composition. In other words , they passivate the metal, that is, increase its passivity to corrosion processes.

During the process of passivation of the weld zone, a strong film is formed

Passivation, in which appropriate alloying elements are added to the chemical composition of stainless steel, is not the only condition for high corrosion resistance of the metal.

In order for the protective properties of stainless steel to remain at a high level, the oxide film on its surface, consisting mainly of chromium oxide, must be intact, have a uniform chemical composition and thickness.

Causes of corrosion

Despite the fact that the chemical composition of stainless steel must contain passivators that significantly increase its corrosion resistance, its surface and internal structure can be subject to corrosion.

The main reason why stainless steel begins to deteriorate is insufficient or uneven chromium content in its chemical composition.

Corrosion of aluminum in water

Aluminum corrosion is almost not observed when interacting with clean, fresh, distilled water. Increasing the temperature to 180 °C does not have any special effect. Hot water vapor also has no effect on aluminum corrosion. If you add a little alkali to water, even at room temperature, the corrosion rate of aluminum in such an environment will increase slightly.

The interaction of pure aluminum (not covered with an oxide film) with water can be described using the reaction equation:

2Al + 6H2O = 2Al(OH)3 + 3H2↑.

When interacting with sea water, pure aluminum begins to corrode, because... sensitive to dissolved salts. To use aluminum in seawater, a small amount of magnesium and silicon is added to its composition. The corrosion resistance of aluminum and its alloys when exposed to sea water is significantly reduced if the metal contains copper.

Chemical passivation of aluminum

Aluminum corrosion is the destruction of metal under the influence of the environment.

For the reaction Al3+ +3e → Al, the standard electrode potential of aluminum is -1.66 V.

The melting point of aluminum is 660 °C.

The density of aluminum is 2.6989 g/cm3 (under normal conditions).

Aluminum, although an active metal, has fairly good corrosion properties. This can be explained by the ability to passivate in many aggressive environments.

The corrosion resistance of aluminum depends on many factors: the purity of the metal, the corrosive environment, the concentration of aggressive impurities in the environment, temperature, etc. The pH of solutions has a strong influence. Aluminum oxide forms on the metal surface only in the pH range from 3 to 9!

The corrosion resistance of Al is greatly influenced by its purity. For the manufacture of chemical units and equipment, only high-purity metal (without impurities), for example, AB1 and AB2 aluminum, is used.

Corrosion of aluminum is not observed only in those environments where a protective oxide film is formed on the surface of the metal.

When heated, aluminum can react with some non-metals:

2Al + N2 → 2AlN – interaction of aluminum and nitrogen with the formation of aluminum nitride;

4Al + 3С → Al4С3 – reaction of interaction of aluminum with carbon with the formation of aluminum carbide;

2Al + 3S → Al2S3 – interaction of aluminum and sulfur with the formation of aluminum sulfide.

Corrosion of aluminum in air (atmospheric corrosion of aluminum)

Aluminum, when interacting with air, becomes passive. When pure metal comes into contact with air, a thin protective film of aluminum oxide instantly appears on the aluminum surface. Further, film growth slows down. The formula of aluminum oxide is Al2O3 or Al2O3•H2O.

The reaction of aluminum with oxygen:

4Al + 3O2 → 2Al2O3.

The thickness of this oxide film ranges from 5 to 100 nm (depending on operating conditions). Aluminum oxide has good adhesion to the surface and satisfies the condition of continuity of oxide films.

When stored in a warehouse, the thickness of aluminum oxide on the metal surface is about 0.01 - 0.02 microns. When interacting with dry oxygen – 0.02 – 0.04 microns.

When heat treating aluminum, the thickness of the oxide film can reach 0.1 microns.

Aluminum is quite resistant both in clean rural air and in an industrial atmosphere (containing sulfur vapor, hydrogen sulfide, ammonia gas, dry hydrogen chloride, etc.). Because sulfur compounds do not have any effect on the corrosion of aluminum in gas environments - it is used for the manufacture of sour crude oil processing plants and rubber vulcanization devices.

Corrosion of aluminum in water

The interaction of pure aluminum (not covered with an oxide film) with water can be described using the reaction equation:

2Al + 6H2O = 2Al(OH)3 + 3H2↑.

Corrosion of aluminum in acids

As the purity of aluminum increases, its resistance to acids increases.

Corrosion of aluminum in sulfuric acid

Sulfuric acid (has oxidizing properties) in medium concentrations is very dangerous for aluminum and its alloys. The reaction with dilute sulfuric acid is described by the equation:

2Al + 3H2SO4(dil) → Al2(SO4)3 + 3H2↑.

Concentrated cold sulfuric acid has no effect. And when heated, aluminum corrodes:

2Al + 6H2SO4(conc) → Al2(SO4)3 + 3SO2↑ + 6H2O.

In this case, a soluble salt is formed - aluminum sulfate.

Al is stable in oleum (fuming sulfuric acid) at temperatures up to 200 °C. Due to this, it is used for the production of chlorosulfonic acid (HSO3Cl) and oleum.

Corrosion of aluminum in hydrochloric acid

Aluminum or its alloys quickly dissolve in hydrochloric acid (especially when the temperature rises). Corrosion equation:

2Al + 6HCl → 2AlCl3 + 3H2↑.

Solutions of hydrobromic (HBr) and hydrofluoric (HF) acids act similarly.

Corrosion of aluminum in nitric acid

A concentrated solution of nitric acid has high oxidizing properties. Aluminum in nitric acid at normal temperatures is extremely resistant (resistance is higher than that of stainless steel 12Х18Н9). It is even used to produce concentrated nitric acid by direct synthesis.

When heated, corrosion of aluminum in nitric acid proceeds according to the reaction:

Al + 6HNO3(conc) → Al(NO3)3 + 3NO2↑ + 3H2O.

Corrosion of aluminum in acetic acid

Aluminum is quite resistant to acetic acid of any concentration, but only if the temperature does not exceed 65 °C. It is used to produce formaldehyde and acetic acid. At higher temperatures, aluminum dissolves (with the exception of acid concentrations of 98 - 99.8%).

Aluminum is stable in bromic and weak solutions of chromic (up to 10%), phosphoric (up to 1%) acids at room temperature.

Citric, butyric, malic, tartaric, propionic acids, wine, and fruit juices have a weak effect on aluminum and its alloys.

Oxalic, formic, and organochlorine acids destroy metal.

The corrosion resistance of aluminum is greatly influenced by vapor and liquid mercury. After a short contact, the metal and its alloys intensively corrode, forming amalgams.

Corrosion of aluminum in alkalis

Alkalis easily dissolve the protective oxide film on the surface of aluminum, it begins to react with water, as a result of which the metal dissolves with the release of hydrogen (aluminum corrosion with hydrogen depolarization).

2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2↑;

2(NaOH•H2O) + 2Al → 2NaAlO2 + 3H2↑.

Aluminates are formed.

Also, the oxide film is destroyed by mercury, copper and chlorine ions.

Passivation and care of stainless steel brewing equipment

Despite its excellent reputation as a beer-making metal, stainless steel can cause corrosion or rust. So this week we'll take a look at how and why stainless steel can corrode, as well as how you can passivate your stainless steel brewing equipment to protect it.

Stainless steel and rust

Steel is made from an alloy of iron and carbon, and carbon makes up only half or a little over a percent of its composition. In comparison, stainless steel is made from iron and chromium. Chromium contains approximately 10-30% of steel, and it is an important element that makes stainless steel resistant to corrosion.

The chromium in stainless steel reacts very quickly with oxygen, and actually forms a protective layer of chromium oxide on the surface of the steel. This chromium oxide prevents the formation of rust and corrosion. However, if the chromium layer is compromised for any reason, the iron in the steel can actually begin to corrode and rust.

Corrosion of aluminum in air (atmospheric corrosion of aluminum)

The reaction of aluminum with oxygen:

4Al + 3O2 → 2Al2O3.

The thickness of this oxide film ranges from 5 to 100 nm (depending on operating conditions). Aluminum oxide has good adhesion to the surface and satisfies the condition of continuity of oxide films. When stored in a warehouse, the thickness of aluminum oxide on the metal surface is about 0.01 - 0.02 microns. When interacting with dry oxygen – 0.02 – 0.04 microns. When heat treating aluminum, the thickness of the oxide film can reach 0.1 microns.

Metal passivation

One of the effective methods of protecting the metal surface from corrosion is surface treatment using special chemical solutions.

When they interact with metal, a chemical reaction occurs, as a result of which a neutral (passive) compound is formed on the surface that can resist the occurrence of corrosive processes. This treatment is called metal passivation.

After this process is completed, an oxide film forms on the surface. It has the chemical properties not to enter into an oxidation reaction and thereby prevents the destruction of not only the surface layer, but also the entire part.

This type of processing is most common for steel, aluminum, nickel, copper and their alloys. In addition to the tasks of protection against corrosion, passivation is used to carry out decorative treatment of the surface of the finished product and reduce the resistivity of contacts in electrical connections.

Description of technology

Passivation is based on the principles of chemical interaction of the surface layer of a metal with various solutions of other metals, as a result of which a surface layer with new physical and chemical properties is formed on the surface. The passivation process involves the creation of absorption (phase) layers that change the structure of the original metal. The passivation layer creates a reliable barrier that prevents the oxidation process and serves as reliable protection against corrosion.

To carry out such chemical reactions, various metals are used. This depends on the composition of the original metal from which the part is made. To give it specific properties, the following can be used: chromium, nickel, cobalt, manganese, molybdenum and other rare earth metals. Depending on their percentage, a passivation solution is prepared and the necessary equipment is selected.

When passivating stainless steel during its production, various alloying metals are added to its composition. They provide better interaction with the chemical elements included in the passivation solution.

For example, chromium oxide is used to create a reliable anti-corrosion film on the surface of steel. A chrome plating operation is performed. It completely changes the physical and chemical properties of the surface layer.

When the processing is carried out correctly, an even layer of equal density is obtained. Various acids are used to carry out passivation. Most often, a solution is created based on nitric acid.

It is the created salts based on this acid that create a protective film on the surface of the steel with high protective characteristics.

After completion of the technological process, the quality of the resulting layer is checked. This is necessary to evaluate the surface of the machined part. In practice, various verification methods are used. For example, a chemical method is used: the surface is treated with a solution of potassium ferrocyanide in nitric acid.

This impact allows you to visually identify areas of poor quality processing. In places where the layer is thin enough or absent, that is, there is a large amount of free iron, a characteristic blue color will appear. This method is used in factory laboratories.

They check selected parts from the finished batch.

A simpler, but more time-consuming method is to place the finished product in ordinary water. After a long stay in water, poorly treated areas become covered with rust.

The technology for passivation of non-ferrous metals is practically no different from the technology for processing steel. The main difference is the composition of the solutions used. For example, potassium and sodium chromates or chromic anhydride are used to process aluminum, copper, and nickel.

The processing process is accelerated by adding various salts and acids to the solution.

Passivation of copper is carried out in solutions of sulfuric acid, surface treatment of copper is carried out in a solution of phosphoric acid, zinc and cadmium in solutions of hydrochloric and nitric acid.

In some cases, the process of interaction of a solution with metal is used to solve other important technical problems. The process of metal decomposition under the influence of oxides is used for the manufacture of printed circuit boards in radio engineering. This procedure is called etching.

In this case, a pattern of future conductive strips and locations for placement of radio components is applied to the surface of the metallized textolite plate using paint.

Then the plate with the applied pattern is lowered into a bath of solution, under the influence of which the metal layer is chemically removed from the surface of the PCB. As a result of passivation, only metal protected by paint remains on the surface.

After this, the plate is washed in running water and the applied paint layer is removed using solvents. The result of such passivation (etching) is a finished printed circuit board for a specific radio-electronic device.

The technology of applying a decorative layer to the main layer of the product does not differ from the general passivation process. When creating jewelry, a thin layer of gold film is applied to the surface of a silver blank. It is formed in a similar way. Thus, a product with a characteristic golden color is obtained.

An important point in obtaining a high-quality film during passivation is finishing processing. In all cases, after removing the part from the solution bath, it is necessary to rinse it thoroughly. This is necessary in order to stop the passivation process.

If you leave part of the active solution or even its diluted components, the technology will be disrupted and the quality of the resulting film will be significantly reduced. After thorough washing, it is recommended to dry the finished part.

This can be done by natural drying or using special hair dryers. In production, drying chambers are used that provide a uniform flow of warm air.

High-quality surface preparation, compliance with all processing modes, observance of passivation time, high-quality washing and drying make it possible to obtain a high-quality, uniform protective layer that can last quite a long time.

Application of passivation

The main objectives of passivation include:

prevention of corrosion processes occurring in the upper layers of metal;
protection from destruction of newly created connections, for example, at the site of a weld (passivation of welds);
increased electrical conductivity at the point of electrical contact;
creation of printed circuit boards using prepared templates (etching);
processing of the finished product in order to impart new decorative and consumer properties.

The first problem is solved for a large number of metals and their alloys. One option for such protection is bluing. In the second case, to create a strong welded joint, passivation of the anodes and final processing of the resulting joint after welding are used. Carrying out passivation can significantly increase the tightness of the resulting connections.

This is especially important when laying pipelines. This treatment is very useful when welding difficult-to-weld metals such as aluminum. Passivation of copper or brass is carried out to create temporary protection against tarnishing of the surface of the product for a certain period of time (usually about a month).

Sometimes it is used as a temporary preservation of prepared parts for storage between further processing or assembly operations.

This type of processing is necessary when using metal products in the following cases:

the use of fasteners, especially in aggressive environments and high mechanical loads;
when assembling pipelines, especially in places of welds;
to protect boiler equipment;
machine parts and mechanisms in contact with sea water;
structural elements operating under changing temperature conditions;
individual elements of hand and mechanical tools;
finished products used in everyday life (door handles, furniture fittings, etc.);
decorative crafts for the interior;
in radio electronics to improve the quality of contacts;
Jewelry.

The problem of increasing electrical conductivity is solved by applying a thin layer of metal with increased electrical conductivity, such as gold or silver, to the surface of the manufactured contacts.

Types of passivation

The main and most well-developed types of passivation are:

chemical;
electrochemical.

Chemical

Chemical passivation involves the use of solutions of salts of various metals. The most effective passivation is carried out with nitric acid. In addition, sulfuric acid or citric acid is used to form a solution.

To improve the quality of the process, a small amount of sodium bichromate is added to the solution. Its amount does not exceed 6% of the total mass. The composition of the solution is selected individually and largely depends on the type of metal being processed.

For example, to passivate iron, metal salts dissolved in high concentration sulfuric acid are used.

The essence of chemical passivation is the active attraction of negative ions that are present in the solution to metal atoms. This occurs due to the presence of a positive charge. As a result of such diffusion, a surface layer is formed.

For passivation, it is necessary to carry out preliminary preparation of the surface of the product. It is thoroughly cleaned using mechanical and chemical methods. The final result and the reliability of the formed film depend on the quality of this procedure. This is of great importance when passivating non-ferrous metals: brass, copper, bronze.

Electrochemical

This type of passivation is based on the principles laid down in the technique of galvanic processing of products. Acceleration of processing is carried out due to the influence of direct current, which flows through the solution, accelerating the chemical reaction. This passivation is called electrochemical.

In addition to the bath in which the electrolyte is placed, such an installation uses a direct current source, connecting wires and one electrode. The second electrode is the part itself.

Another option for contacts is one electrode and a bath body (it must be made of metal that is resistant to electrolyte and electric current).

In practice, electrical installations with a relatively low voltage level are used. Its value does not exceed 12V.

In both cases, when the installation is turned on, an electric current is passed through the solution. It stimulates the passivation process on the surface of the workpiece. In practice, I distinguish between anodic and cathodic passivation.

With this passivation, a positive potential is applied to the workpiece, and a negative potential is applied to the bath body. When using the electrochemical method, the protective film is formed faster and is more even. But this technology is more expensive than chemical passivation, because

it uses more complex equipment and consumes electricity. Under its action, the protective film is uniform. This is how a film is formed on the surface of copper blanks. Current is passed through solutions with chromium salts dissolved in them.

It is in them that copper acquires the greatest resistance to corrosion.

Important parameters in this process are the passivation time, the density and composition of the electrolyte, and the critical value of the passivation current. These parameters are calculated for various metals and are given in special tables. Based on these data, the allowable processing time is calculated.

Properties of metal after processing

The main objective of passivation is to improve the physicochemical and mechanical characteristics of the surface layer of the material from which the part is made. The remaining characteristics of the deeper layers remain unchanged. Therefore, after passivation is completed, the following properties and characteristics change in the surface layer:

a layer with a new chemical composition appears;
anti-corrosion activity changes (it slows down significantly);
the physical characteristics of the material are improved (only the surface layer);
in some cases, the mechanical strength of the product increases;
the color of the part changes (it takes on a more aesthetic shape);
consumer properties increase and presentation improves.

Passivation of stainless steel can significantly improve anti-corrosion properties and give the finished part a completely different color. The use of chromium or nickel in the passivation solution produces a shiny metallic color.

Passivation of iron with chemical elements close to it makes it possible to create an outer layer that is sufficiently resistant to corrosion. In this way, the scope of application of such products expands. They can be used even in active and aggressive environments. In addition to various grades of steel, cast iron is subjected to passivation.

The main task is to create a protective film against corrosion. In some cases, when thickened sodium nitrate is used, the surface layer acquires some elasticity. In this case, the fragility of the entire part is reduced. One type of steel is the so-called blued finish.

The result of processing is a reliable outer layer of black color.

The properties of the surface layer of non-ferrous metals change in a similar way. As a result of passivation, adsorption or phase layers of a certain thickness are formed. Placing an aluminum workpiece stimulates the process of natural passivation of the surface layer of this metal. When exposed to acidic solutions, the protective properties of the surface layer of aluminum increase.

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Passivation of stainless steel to protect it

When stainless steel products are made, they are typically immersed in a nitric acid bath at the end of the manufacturing process to remove contaminants.

The acid also activates the oxidation process of chromium in the air called passivation, where a protective layer of chromium oxide is formed when oxygen reacts with chromium.

Passivation occurs very quickly - usually within 20 minutes.

Now some stainless brewing equipment, particularly lower cost stainless materials, were likely machined, stamped, pickled, polished and welded only after the stainless steel had been fabricated and acid washed.
As a result, it may contain oils, polishes, welding compounds and other contaminants that protect the steel but should be washed away the first time you clean your parts. Plus, you probably don't want to find these oils and compounds in your beer.