You have verified that the parts are manufactured to specification. Now make sure you take steps to protect these parts in the environment your customers expect. #base
Passivation remains an important step in maximizing the corrosion resistance of parts and assemblies machined from stainless steel. This can make the difference between satisfactory performance and premature failure. Incorrect passivation can cause corrosion.
Passivation is a post-fabrication technique that maximizes the inherent corrosion resistance of the stainless steel alloys from which the workpiece is made. This is not descaling or painting.
There is no consensus on the exact mechanism by which passivation works. But it is known for sure that there is a protective oxide film on the surface of passivated stainless steel. This invisible film is said to be extremely thin, less than 0.0000001 inch thick, which is about 1/100,000th the thickness of a human hair!
A clean, freshly machined, polished, or pickled stainless steel part will automatically acquire this oxide film due to exposure to atmospheric oxygen. Under ideal conditions, this protective oxide layer completely covers all surfaces of the part.
In practice, however, contaminants such as factory dirt or iron particles from cutting tools can get on the surface of stainless steel parts during processing. If not removed, these foreign bodies may reduce the effectiveness of the original protective film.
During machining, traces of free iron can be removed from the tool and transferred to the surface of the stainless steel workpiece. In some cases, a thin layer of rust may appear on the part. In fact, this is the corrosion of the tool steel, not the base metal. Sometimes cracks from embedded steel particles from cutting tools or their corrosion products can erode the part itself.
Similarly, small particles of ferrous metallurgical dirt can adhere to the surface of the part. Although the metal may appear lustrous in its finished state, after exposure to air, invisible particles of free iron can cause surface rust.
Exposed sulfides can also be a problem. They are made by adding sulfur to stainless steel to improve machinability. Sulfides increase the ability of the alloy to form chips during machining, which can be completely removed from the cutting tool. If parts are not properly passivated, sulfides can become the starting point for surface corrosion of industrial products.
In both cases, passivation is required to maximize the natural corrosion resistance of the stainless steel. It removes surface contaminants such as iron particles and iron particles in cutting tools that can form rust or become the starting point for corrosion. Passivation also removes sulphides found on the surface of open cut stainless steel alloys.
A two-step procedure provides the best corrosion resistance: 1. Cleaning, the main procedure, but sometimes neglected 2. Acid bath or passivation.
Cleaning should always be a priority. Surfaces must be thoroughly cleaned of grease, coolant or other debris to ensure optimum corrosion resistance. Machining debris or other factory dirt can be gently wiped off the part. Commercial degreasers or cleaners can be used to remove process oils or coolants. Foreign matter such as thermal oxides may need to be removed by methods such as grinding or pickling.
Sometimes the machine operator may skip basic cleaning, mistakenly believing that cleaning and passivation will occur at the same time, simply by immersing the oiled part in an acid bath. It will not happen. Conversely, contaminated grease reacts with acid to form air bubbles. These bubbles collect on the workpiece surface and interfere with passivation.
Worse still, contamination of passivation solutions, which sometimes contain high concentrations of chlorides, can cause a “flash”. In contrast to producing the desired oxide film with a shiny, clean, corrosion-resistant surface, flash etching can result in severe etching or blackening of the surface—a deterioration in the surface that passivation is designed to optimize.
Martensitic stainless steel parts [magnetic, moderately corrosion resistant, yield strength up to about 280 thousand psi (1930 MPa)] are quenched at high temperatures and then tempered to provide the desired hardness and mechanical properties. Precipitation hardened alloys (which have better strength and corrosion resistance than martensitic grades) can be solution treated, partially machined, aged at lower temperatures, and then finished.
In this case, the part must be thoroughly cleaned with a degreaser or cleaner before heat treatment to remove any traces of cutting fluid. Otherwise, coolant remaining on the part may cause excessive oxidation. This condition can cause dents to form on smaller parts after descaling with acid or abrasive methods. If coolant is left on shiny hardened parts, such as in a vacuum furnace or in a protective atmosphere, surface carburization can occur, resulting in loss of corrosion resistance.
After thorough cleaning, stainless steel parts can be immersed in a passivating acid bath. Any of the three methods can be used – passivation with nitric acid, passivation with nitric acid with sodium dichromate, and passivation with citric acid. Which method to use depends on the grade of stainless steel and the specified acceptance criteria.
More corrosion resistant nickel chromium grades can be passivated in a 20% (v/v) nitric acid bath (Figure 1). As shown in the table, less resistant stainless steels can be passivated by adding sodium dichromate to a bath of nitric acid to make the solution more oxidizing and able to form a passivating film on the metal surface. Another option for replacing nitric acid with sodium chromate is to increase the concentration of nitric acid to 50% by volume. Both the addition of sodium dichromate and the higher concentration of nitric acid reduce the likelihood of an unwanted flash.
The passivation procedure for machinable stainless steels (also shown in Fig. 1) is slightly different from the procedure for non-machinable stainless steel grades. This is because during passivation in a nitric acid bath some or all of the machinable sulfur-containing sulfides are removed, creating microscopic inhomogeneities on the surface of the workpiece.
Even normally effective water washing can leave residual acid in these discontinuities after passivation. This acid will attack the surface of the part if not neutralized or removed.
For efficient passivation of easy-to-machine stainless steel, Carpenter has developed the AAA (Alkaline-Acid-Alkaline) process, which neutralizes residual acid. This passivation method can be completed in less than 2 hours. Here is the step by step process:
After degreasing, soak parts in 5% sodium hydroxide solution at 160°F to 180°F (71°C to 82°C) for 30 minutes. Then rinse the parts thoroughly in water. Then immerse the part for 30 minutes in a 20% (v/v) nitric acid solution containing 3 oz/gal (22 g/l) sodium dichromate at 120°F to 140°F (49°C) to 60°C. ) After removing the part from the bath, rinse it with water, then immerse it in a sodium hydroxide solution for 30 minutes. Rinse the part again with water and dry, completing the AAA method.
Citric acid passivation is becoming increasingly popular with manufacturers who want to avoid the use of mineral acids or solutions containing sodium dichromate, as well as disposal problems and increased safety concerns associated with their use. Citric acid is considered environmentally friendly in all respects.
While citric acid passivation offers attractive environmental benefits, stores that have had success with inorganic acid passivation and have no safety concerns may want to stay the course. If these users have a clean shop, the equipment is in good condition and clean, the coolant is free of factory ferrous deposits, and the process is producing good results, there may not be a real need for change.
Citric acid bath passivation has been found to be useful for a wide range of stainless steels, including several individual grades of stainless steel, as shown in Figure 2. For convenience, Figure 2. 1 includes the traditional method of passivation with nitric acid. Note that the old nitric acid formulations are expressed as percentages by volume, while the new citric acid concentrations are expressed as percentages by mass. It is important to note that when performing these procedures, a careful balance of soak time, bath temperature, and concentration is critical to avoid the “flashing” described above.
Passivation varies depending on the chromium content and processing characteristics of each variety. Notice the columns for either Process 1 or Process 2. As shown in Figure 3, Process 1 has fewer steps than Process 2.
Laboratory tests have shown that the citric acid passivation process is more prone to “boiling” than the nitric acid process. Factors contributing to this attack include too high bath temperature, too long soak time, and bath contamination. Citric acid-based products containing corrosion inhibitors and other additives such as wetting agents are commercially available and are reported to reduce “flash corrosion” susceptibility.
The final choice of passivation method will depend on the acceptance criteria set by the customer. See ASTM A967 for details. It can be accessed at www.astm.org.
Tests are often carried out to evaluate the surface of passivated parts. The question to be answered is “Does passivation remove free iron and optimize the corrosion resistance of alloys for automatic cutting?”
It is important that the test method matches the class being evaluated. Tests that are too strict will not pass absolutely good materials, while tests that are too weak will pass unsatisfactory parts.
PH and easy-machining 400 series stainless steels are best evaluated in a chamber capable of maintaining 100% humidity (sample wet) for 24 hours at 95°F (35°C). The cross section is often the most critical surface, especially for free cutting grades. One reason for this is that the sulphide is pulled in the machine direction across this surface.
Critical surfaces should be positioned upwards, but at an angle of 15 to 20 degrees from vertical, to allow for moisture loss. Properly passivated material will hardly rust, although small spots may appear on it.
Austenitic stainless steel grades can also be evaluated by moisture testing. In this test, drops of water should be present on the surface of the specimen, indicating free iron by the presence of any rust.
Passivation procedures for commonly used automatic and manual stainless steels in citric or nitric acid solutions require different processes. On fig. 3 below provides details on process selection.
(a) Adjust the pH with sodium hydroxide. (b) See fig. 3(c) Na2Cr2O7 is 3 oz/gal (22 g/L) sodium dichromate in 20% nitric acid. An alternative to this mixture is 50% nitric acid without sodium dichromate.
A faster approach is to use ASTM A380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems. The test includes wiping the part with a copper sulfate/sulfuric acid solution, keeping it wet for 6 minutes, and observing the copper plating. Alternatively, the part can be immersed in the solution for 6 minutes. If iron dissolves, copper plating occurs. This test does not apply to the surfaces of food processing parts. Also, it should not be used on 400 series martensitic steels or low chromium ferritic steels as false positive results may occur.
Historically, the 5% salt spray test at 95°F (35°C) has also been used to evaluate passivated samples. This test is too stringent for some cultivars and is generally not required to confirm the effectiveness of passivation.
Avoid using excess chlorides, which can cause dangerous flare-ups. Use only high quality water with less than 50 parts per million (ppm) chloride whenever possible. Tap water is usually sufficient, and in some cases it can withstand up to several hundred parts per million of chlorides.
It is important to replace the bath regularly so as not to lose the passivation potential, which can lead to lightning strikes and damage to parts. The bath must be maintained at the proper temperature, as uncontrolled temperatures can cause localized corrosion.
It is important to follow a very specific solution change schedule during large production runs to minimize the possibility of contamination. A control sample was used to test the effectiveness of the bath. If the specimen has been attacked, it’s time to replace the bath.
Please note that some machines only produce stainless steel; use the same preferred coolant for cutting stainless steel to the exclusion of all other metals.
The DO rack parts are machined separately to avoid metal to metal contact. This is especially important for free machining of stainless steel, as easy-flowing passivation and flushing solutions are required to diffuse sulfide corrosion products and prevent the formation of acid pockets.
Do not passivate carburized or nitrided stainless steel parts. The corrosion resistance of parts treated in this way can be reduced to such an extent that they can be damaged in the passivation bath.
Do not use ferrous metal tools in workshop conditions that are not particularly clean. Steel chips can be avoided by using carbide or ceramic tools.
Be aware that corrosion can occur in the passivation bath if the part has not been properly heat treated. Martensitic grades with high carbon and chromium content must be hardened for corrosion resistance.
Passivation is usually carried out after subsequent tempering at temperatures that maintain corrosion resistance.
Do not neglect the concentration of nitric acid in the passivation bath. Periodic checks should be made using the simple titration procedure suggested by Carpenter. Do not passivate more than one stainless steel at a time. This prevents costly confusion and prevents galvanic reactions.
About the Authors: Terry A. DeBold is a Stainless Steel Alloys R&D Specialist and James W. Martin is a Bar Metallurgy Specialist at Carpenter Technology Corp. (Reading, Pennsylvania).
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