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Simple Chemistry of Stainless Steel

Stainless Steel is a low carbon type of steel which contains at least 10% chromium by weight. Elements include, depending on grade, iron, nickel, chromium, and manganese. Together they have the remarkable primary common characteristic of resistance to chemical attack or corrosion. It attains its unique stainless, corrosion-resisting properties through the addition of chromium. Additionally, stainless steel has a melting point 2647.4° F and a density of 555.61 lb/ft3.

The Passive Layer (fig 1.1 - 1.3)

The presence of chromium in the steel allows for the formation of a rough, adherent, invisible, corrosion-resisting chromium oxide film on the steel surface: the passive layer. This film is self-healing (provided that oxygen is present) if the steel were to be damaged mechanically or chemically. When there is an absence of oxygen and the film is destroyed by chemical or mechanical action, the steel becomes “depassivated” or “activated” and is susceptible to attack. Stainless steel is said to be passive when the oxide film is present and exhibits maximum corrosion resistance as opposed to when the film is destroyed and the steel is termed active.

The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen. Molybdenum additions increase the passivity potential of the grade and nickel additions in stainless steels result in further corrosion resistance.

As Related to the Grinding Process

There are two main reasons that stainless steel is ground commercially, appearance and corrosion resistance. The appearance of stainless steel is enhanced in the grinding process. This appearance can be related in numbered finishes and common terms.

Mill Finish

Term

Grit

 

Target RA Value (µ)

#1

Mill Finish

N/A

 

N/A

2D

Dull

N/A

 

N/A

2B

Bright annealed

N/A

 

N/A

#3

Rough

100

 

75 µ

#4

Brushed

240

 

32 µ

#6

Satin

320

 

25 µ

#7

Buffed

600

 

15 µ

#8

Mirror

Buffs and Compound

 

Under 5 µ

Simply put, architectural finishes are listed more commonly in terms that relate to appearance and feel of the metal’s surface, such as brushed, mirror or satin. These terms can easily be related to grit finishes in the chart above. Conversely, food grade dairy, chemical and pharmaceutical finishing is performed for a different set of reasons. The reduction of impurities and the production of a consistent grain structure is critical in establishing the formation of the passive layer. The passive layer is the layer created by the interaction of chromium contained in the steel and oxygen in the environment as shown in the pictures to the right. Deeper scratches on the surface will become a potential corrosion site, as contaminates can collect in these areas and prevent the chromium oxygen reaction mentioned above. This is also critical in welded areas, where the area has been heated resulting in a weaker grain structure. This condition is called sensitation. Sensitized areas are more vulnerable to  corrosion and must be ground to establish a consistent grain structure to prevent corrosion as well. The most common finish in the chart above is a number 4b finish. This is an RA value under 35 and is accepted by the FDA as a limit for all food and chemical processing equipment.

 

Types of Corrosion

Just because steel is considered “stainless”, this does not mean that it is not susceptible to corrosive conditions in different environments. Stainless steel can only resist corrosive damage if its surface constantly remains passive. If the passive layer is attacked or penetrated, chemical destruction can occur very quickly. Therefore, when considering a stainless steel for a given application, it is imperative to have an understanding of the common types of corrosion and their causes.

Pitting occurs when an environment deprived of oxygen penetrates the passive layer film in just a few areas but not on the overall surface. This penetration creates “pits” at several different points along the surface. The cause of pitting is when objects become embedded in or attached to the surface of stainless steel during fabrication. Therefore, steps must be taken immediately after the fabrication process to effectively clean the stainless steel surfaces and prevent pitting. Also, ensuring that a sufficient amount of oxygen will be present in the design and fabrication of stainless steel will help to combat one of the most common and damaging forms of corrosion.

Stress Corrosion Cracking is one of the most severe types of damage that occurs in stainless steel. It occurs when the material is put under a great amount of stress, created by either a heat treatment or induced during a cold working operation. The tensile nature of the stress, combined with a corrosive environment (usually containing high amounts of chloride) is the main cause of stress corrosion cracking.

This stress results in cracking that spreads very fast through the grain of the steel and can result in absolute failure of the part.

Galvanic

When two unlike metals are brought into contact with one another in a medium capable of conducting an electric current, the process of galvanic corrosion occurs. An electric current is generated between the two metals which speeds up the corrosion of one while lessening or stopping corrosion in the other. The rate of corrosion thus depends on the difference in electrical potential between the dissimilar metals and the strength of the generated current.

Multiple steps can be taken to prevent galvanic corrosion. Insulation of the two metals from each other is a common way as is keeping the metals dry and/or protected from ionic compounds (like salts, acids or bases). The cathodic protection method uses one or more “sacrificial” anodes that are more active than the protected metal to act as a distracter.

Chloride Stress is due to a presence of oxygen, chloride ions, and a high temperature in an environment. It mainly occurs in cooling applications where bleach is used to kill bacteria that has occurred through bio fouling. Stainless steel can be coated with a protective film or insulated with a special chloride free insulation to combat chloride stress corrosion.

It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels. 316SS and 304SS are particularly susceptible to this type of stress corrosion. Chloride stress corrosion involves selective attack of the metal along grain boundaries. In the formation of the steel, a chromium-rich carbide precipitates at the grain boundaries leaving these areas low in protective chromium, and thereby, susceptible to attack.  It has been found that this is closely associated with certain heat treatments resulting from welding.  This can be minimized considerably by proper annealing processes.

 

Production of Stainless Steel

The production of stainless steel involves a series of processes, including: melting and casting, forming, heat treatment, descaling, finishing, and ultimately, manufacturing done by the end user.

First, the raw materials are melted together in an electric furnace (usually 8 to 12 hours of intense heat), and then it is cast into semi-finished forms. These forms include blooms (rectangular), billets (round or square shapes), slabs, rods, and tube rounds. Next, the semi-finished material goes through forming procedures, beginning with hot rolling, in which the steel is heated and passed through large rolls. After the stainless steel is formed, most types must next go through an annealing step.

Annealing is a heat treatment in which the steel is heated and cooled under controlled conditions. This is done to relieve internal stresses and soften the metal. Heat treatment known as age hardening, which provides greater strength, requires careful control, for even small adjustments from the recommended temperature, time, or cooling rate can seriously affect the properties. Lower aging temperatures produce high strength with low toughness, while higher-temperature aging produces a lower strength, tougher material. Which type of heat treatment to use depends on whether the steel is austenitic, ferritic, or martensitic. Austenitic steels are heated to above 1900 degrees Fahrenheit (1037 degrees Celsius) for a time depending on the thickness. Water quenching is used for thick sections, whereas air cooling or air blasting is used for thin sections.

Annealing causes a scale or build-up to form on the steel. However, the scale is removed using various processes. Pickling, which uses a nitric-hydrofluoric acid bath to descale the steel, is one of the most common of these processes that is used. Another method, electrocleaning, occurs when an electric current is applied to the surface using a cathode and phosphoric acid, and the scale is removed. The annealing and descaling steps occur at different stages depending on the type of steel being worked. Some, such as the original blooms and billets, go through further forming steps (more hot rolling or forging) after the initial hot rolling before they can be annealed and descaled. In contrast, what used to be slabs go through an initial annealing and descaling step immediately after hot rolling. After cold rolling (passing through rolls at a relatively low temperature), which produces a further reduction in thickness, they are annealed and descaled again to a final size, thickness, and shape.

Hot Rolling consists of three main types of rolls: strip rolls; sheet rolls; and rod rolls. This method is used to reduce the size of wrought-iron ingots or bars. The Bessemer process was made mandatory in the rolling technique so manufacturers could keep pace with the high demand of these materials needed for the market. Hot rolling is used for larger or thicker amounts of metal. The machines used for this application are very similar to cold rolling, but the requirements are not as important - such as keeping a precise thickness or exact edge dimensions. Hot rolling is mainly used with non-ferrous metals during the beginning stages of the breakdown of alloys.

Cold Rolling steel is generally used at the end of a metal forming process. The following functions are applied to obtain and enable accurate dimensions in the finished product. Cold rolling also ensures a smooth clean appearance or finish and the straightness of the edges of the product. Cold rolling is to help obtain a degree of hardness known otherwise as temper in alloys. Once the desired temper is obtained it cannot be re-heated or it will lose properties intended for specific jobs.

Cold rolling is applied during the end process of production for strip and section rolled metals, such as foil. The cold roll mills are to obtain the desired thickness and also to limit and prevent any excess thinning of the already thin material. When rolling these alloys they are made long enough to make sure there are no creases or tears in the metal. The end result after the process of cold rolling will acquire the metal thickness and temper which is needed for each type of work.

 

Common Types of Stainless Steel

Stainless steels are divided into groups or classes according to their composition and metallurgical characteristics. These main groups are: Martensitic, Ferritic, Austentic, and Duplex Stainless Steels. Within each group there are specific standard grades which have similarities as well as unique properties and capabilities for specific applications.

Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production (type 304 is the most commonly specified grade by far). The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. This enhances their corrosion resistance and modifies the structure from ferritic to austenitic. Super austenitic grades have enhanced pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due the further additions of chromium, molybdenum and nitrogen to these grades.

304 (L), 316 and 316 (L), 301, 302

Ferritic stainless steels are plain chromium stainless steels with a chromium content varying between 10.5 and 18% and a low carbon content. They are magnetic and not hardenable by heat treatment. Ferritic alloys have good ductility and formability but a relatively poor high temperature strength compared to that of austenitic grades. Ferritic stainless steel has a better resistance to high temperature corrosion than the martensitic group.

405, 409, 430

Martensitic stainless steels were the first stainless steels that were commercially developed (as cutlery) and have a relatively high carbon content compared to other stainless steels. They are plain chromium steels containing between 12 and 18% chromium. They are magnetic and hardenable by quenching and tempering like plain carbon steels and find their main application in cutlery, aerospace and general engineering. A relatively new group of martensitic stainless steels are the supermartensitic stainless steels. The supermartensitic grades combine high strength and low-temperature toughness with acceptable corrosion resistance in many applications.

403, 410, 414

 

Finishes

Stainless Steel Finishes

The surface finish is an important step because of the need to protect the oxide film that gives the steel its corrosion resistance and to create an aesthetically appealing exterior. Certain surface finishes also make stainless steel easier to clean, which is essential for sanitary applications. A smooth surface as obtained by polishing also provides better corrosion resistance. On the other hand, rough finishes are often required for lubrication applications.

The following are summaries of the different types of finishes that are most commonly used for stainless steel applications:

No. 0 Finish - Used for thicker plates, this finish is also referred to as Hot Rolled Annealed (HRA). The plate is hot rolled to a desired thickness, and then annealed. No pickling or passivation operations are required, resulting in a scaled back finish. This finish does not fully develop the corrosion resistant film on the steel and is generally unsuitable for most end uses, except for certain high temperature heat resisting applications.

No. 1 Finish - The result of this finish is a dull, slightly rough surface, which is suitable for applications which generally involve the range of plate thickness. No. 1 finish involves the plate being hot rolled, annealed, pickled, and then passivated. Typical uses include industrial applications that require high temperature steel.

No. 2D Finish - Produced by cold rolling to a specified thickness, and then annealed and descaled. Produces a dull finish that is the result of the descaling or pickling. This dull finish is favorable for applications that involve deep drawing because it retains lubricants very well.

No. 2B Finish - The 2B finish is produced in the same manner as the 2D finish, except that the annealed and descaled sheet receives a final light cold-rolled pass on polished rolls. This is done to produce a more reflective finish, similar to a cloudy mirror and is, like 2D, generally used in deep drawing applications.

No. 2BA Finish - More commonly referred to as a Bright Annealed (BA) finish, this bright, highly reflective finish is produced by annealing in a controlled atmosphere furnace. This prevents any scaling or oxidation that would occur during annealing. As the material is rolled to thinner and thinner sizes, the smoothness and reflective qualities improve.

No. 3 Finish - The No. 3 finish produces short, relatively coarse, parallel polishing lines that extend along the length of the coil. Abrasives are used to mechanically polish the surface and achieve the desired finish that is generally put to use in kitchen and food processing equipment.

No. 4 Finish - Widely used for restaurant, kitchen, and food processing equipment, a No. 4 finish has the same short, parallel polishing lines as a No. 3 finish. The difference is that a No. 4 finish is polished using more fine abrasives, higher in grit than those used in No. 3 finishes.

No. 4B Dairy Finish - A No. 4B finish is distinguished by a slightly frosted appearance and a very smooth surface. It is produced by rolling sheets of stainless steel and its is primary application is in the medical and food industry. This is why it is essential that sanitary standards are followed to ensure the highest degree of purity and cleanliness. 4B Dairy finishes involve removing surface defects from the metal using a 180-240 grit belt or wheel finish and then softening with a fine non-woven abrasive belt or pad.

No. 6 Finish - Also referred to as a Satin finish, this finish is produced by further reducing a #4 scratch pattern using finer grit coated and non woven abrasives. It has a lower reflectivity than No. 4 finishes and is characterized by a dull, silver-white finish with short, linear polishing lines. Mainly used for architectural applications.

No. 7 Finish - Characterized by a high degree of reflectivity, No. 7 finishes are produced by buffing a finely ground surface (like No. 4) but the grit lines are not removed.

No. 8 Finish - A No. 8 finish is the most reflective surface finish that is commonly produced. It is created in basically the same way as a No. 7 finish, using fine abrasives to polish and then buffing extensively with fine buffing rouges.

TR Finish - A Temper Rolled (TR) finish is desirable when higher than normal mechanical properties are needed. It produces a fairly smooth, reflective finish, which is the result of cold-working an annealed and descaled product or a bright-annealed product.

Special Finishes - Besides the finishes listed above, there are several stainless steel finishes that have been developed for aesthetic applications and are termed “special finishes”. Operations such as grinding, polishing, buffing, blasting, colorizing, and etching can be used to create a variety of special finishes. These special finishes include: rolled, colored, graphic, and mechanical.

1.1 - 1.3 - Passive Layer
Passive Layer 1
As seen here, the passive layer is already formed between oxygen in the atmosphere and chromium from the stainless steel.

 

Passive Layer 2
The passive layer can be scratched away but will reform in the presence of oxygen. If the scratch fills with contaminants which inhibit the oxygen molecules reaching the metal surface the passive layer will not be allowed to form and the scratch could become a site for corrosion.

 

Passive Layer 3
The passive layer has reformed between the oxygen and the chromium.

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