Three Ways to Passivate Metals
Passivation is a process that makes certain coating materials passive, or less sensitive to the environment. Passivated materials are less affected by the environment, and are generally more corrosion-resistant and more ductile. There are several common ways to passivate metals, including nitric acid and cadmium plating. This article will discuss three of the most common methods for passivating metals. Here are a few benefits of each method:
Citric acid passivation
When you are preparing your parts for citric acid passivation, you should know that the process has several steps that must be followed. Before you start the process, you should thoroughly clean the parts, as shop debris may interact with the citric acid and form gas bubbles. You can use a degreaser or switch detergents to remove contaminants. If you are dealing with thermal oxide, you may have to pickle or grind the parts first.
Citric acid is an organic acid that is found naturally in citrus fruit. Despite its weak acidity, citric acid is widely used in many industries, including food and beverage manufacturing. Moreover, it is an environmentally friendly alternative to nitric acid. In fact, an updated version of the A-380 standard was created to address the concerns of environmentalists and other stakeholders. Using a citric acid blend for passivation can achieve the same Cr: Fe ratio as nitric acid.
For many applications, citric acid passivation is a safer alternative to nitric acid. Because citric acid does not emit toxic fumes, it is an environmentally-friendly alternative. In addition, citric acid passivation costs far less than nitric acid passivation. This means you’ll be saving money while still receiving great results. And it’s much easier to use. But how does this work?
A typical citric acid passivation treatment will significantly improve the hydrophilic and bactericidal properties of a surface. The process will also increase the wettability of a surface. The higher the concentration of citric acid, the more permeabilized it will be. Therefore, it’s best to choose a citric acid passivation solution based on your application requirements. In addition to these advantages, it also has some drawbacks.
A high-iron water source can leave traces of iron on the surface of SS parts, resulting in minor rusting. So, you should make sure that you have water filtration in place before applying citric acid. Alternatively, bring in clean water from somewhere else. This solution is a good alternative for some situations. But if you can’t afford to use a filtration system, you can use a water filtration system. Then, you can apply citric acid passivation to any SS parts for a much lower cost.
Stainless steel is a naturally corrosion-resistant metal, but the machining and forming operations can introduce free iron and other contaminants onto the surface, compromising its passive layer and creating an opportunity for corrosive attack on the base metal. Citric passivation removes these surface contaminants and maximizes the corrosion resistance of most stainless steel. It works on all 300/400 series stainless steels. Its benefits are relatively low, which makes it a great choice when compared to other corrosion-resistant processes.
Nitric acid passivation
A common form of passivation is using a bath of nitric acid at temperatures ranging from ambient to 160 degrees Fahrenheit for a specified duration. However, there are a few key benefits of citric acid passivation that should make it a better choice for certain applications. The compact MK series passivation system includes two stages of nitric acid with special pneumatic components. The result is a highly effective passive film that resists corrosion and enhances the corrosion-resistant properties of metals and alloys.
The most common application for nitric acid passivation is the enhancement of corrosion resistance on freshly machined surfaces of stainless steel. The process results in a film of uniform, transparent, tenacious, and passive properties. Passivity is a property normally associated with noble metals. Because of the passive film, stainless steel surfaces exhibit exceptional corrosion resistance. The process is best suited for applications that require a high level of corrosion resistance.
While passivation is a process that prevents corrosion, it is also a good option for corroding metals. Unlike conventional passivation, the process reduces reactivity in metals by selectively oxidation. For example, nitric acid removes iron from the surface but does not oxidize the nickel. The proper oxidizing agent is required for complete passivation.
The passivation process is hazardous to the environment and the workplace. It can damage the structure of a building or the people working in it. As a result, the hazard communication program should be updated to reflect these new exposures. Personnel working near the tanks should wear a protective visor and wear acid-resistant work gloves and overalls. A shower station with emergency eyewash is essential as well. A respiratory protection program must also be in place for those exposed to fumes.
Another method of passivation is nitric acid with sodium dichromate. This method is environmentally safe, provides better results, and is faster than the former. However, it is important to remember that the two methods are not interchangeable. In addition to nitric acid passivation, citric acid passivation has advantages and disadvantages. You should choose one depending on your application’s requirements. It will improve your Stainless Steel’s corrosion-resistant properties.
The wastewater generated during the passivation process is regulated by the U.S. EPA Metal Finishing Categorical Standards and Publicly Owned Treatment Works’ local limits. Depending on the state you are in, your discharge permit may also require pretreatment before use. As such, make sure to follow all state regulations. If you are using passivation to process your steel components, you must follow your local guidelines.
Electrochemical passivation
Electrochemical passivation can be performed using two methods: anodic oxidation and cathodic oxidation. Oxidation is the process of removing a layer of surface oxides. Oxidation takes place on the conductive surface of a component. The anodic oxidation process is usually performed using a constant current of 0.1 ma/cm2 for 15 minutes. Alternatively, extremely high currents may be applied for short periods of time. Both methods have many advantages.
X-ray photoelectron spectroscopy was used to study the initial stages of oxide growth on different iron alloys. The results showed that the oxidized film tended to be more enriched in chromium than in the conductive oxide film. The oxide solution interface tended to be dominated by hydroxide species, while the oxidized film exhibited a high chromium concentration.
Besides oxidation, electrochemical passivation can also be used for reducing the presence of oxide scales. The process parameters used in the oxidation process are important for corrosion resistance. High current densities may enhance corrosion resistance. The process parameters and loading density should be chosen properly in order to obtain the desired effect. There are further investigations underway to determine the role of these parameters in electrochemical passivation. This work shows the potential of electrochemical passivation to improve biomedical grade SS.
The benefits of electropolishing over passivation are numerous. Electropolishing removes ALL material from the surface, whereas passivation only reacts with the surface-level elements. This process is better for stainless steel than electropolishing because it increases the chromium-to-ferrous ratio. In addition, passivation is better at protecting against corrosive agents. It also prevents rusting of stainless steel.
Studies of Ni-Cr-Mo alloys have shown that the addition of Mo to the alloy increases the film’s resistance to corrosion and facilitates passivation. While the exact mechanism of Mo alloying is unknown, it has a significant effect on the passivation process. Passivation kinetics differ depending on the pH. A more acidic electrolyte causes early oxidation of the alloy, while an alkaline environment promotes it.
Aside from the passive layer, the material itself also exhibits different passivation behaviors. A casting alloy, for instance, can undergo passivation with lower Si content than a coating. This is attributed to the presence of highly iron-rich localized regions that form during the coating processing. This is the reason why the metal becomes corrosion resistant under abrasion. The study is furthermore helpful for biosensor applications. You may be interested in this topic.
When studying electrochemical passivation, it is important to understand the various steps involved. For instance, Fe dissolution and Fe hydroxide formation are two-stage reactions in a pH 8*4 buffer solution. Both stages produce effective protection for the metal. It has been studied by Diez-Perez et al. They observed that this process results in a two-stage process, which occurs over a large surface area. However, the presence of ammonia increases the risk of forsterite formation.