Passivation , in physical chemistry and engineering, refers to material that becomes "passive", that is, less affected or eroded by the future usage environment. Passivation involves the creation of an outer layer of shielding material applied as microcoating, created by a chemical reaction with a base material, or allowed to build from spontaneous oxidation in air. As a technique, passivation is the use of a light layer of a protective material, such as a metal oxide, to create a corrosion shell. Passivation can only occur under certain conditions, and is used in microelectronics to increase silicon. Passivation techniques strengthen and maintain metal appearance. In electrochemical treatment of water, passivation reduces the effectiveness of treatment by increasing circuit resistance, and active action is usually used to overcome this effect, the most common polarity reversal, resulting in limited rejection of the fouling layer. Another proprietary system to avoid passive electrodes, some discussed below, is the subject of ongoing research and development.
When exposed to air, many metals naturally form hard and relatively inert surfaces, such as silver stains. In the case of other metals, such as iron, a rather coarser porous layer is formed from loosely compliant corrosion products. In this case, large amounts of metal are removed, stored or dissolved in the environment. Corrosion coatings reduce corrosion rates by varying degrees, depending on the type of base metal and its environment, and especially slower in room temperature air for aluminum, chromium, zinc, titanium, and silicon (a metalloid); corrosion shells inhibit deeper corrosion, and operate as a form of passivation. The inert surface layer, called the '' native oxide layer '', is typically oxide or nitride, with monolayer thicknesses of 0.1-0.3 Ö (1-3Ã, ÃÆ'...) for precious metals such as platinum, about 1.5Ã , Nm (15 ÃÆ'â € | for silicon, and closer to 5Ã, nm (50 ÃÆ'â € | to aluminum after several years).
Video Passivation (chemistry)
Mekanisme
There is much interest in determining the mechanisms that regulate the increase in oxide layer thickness over time. Some important factors are the volume of the oxide relative to the volume of the parent metal, the mechanism of oxygen diffusion through the metal oxide to the parent metal, and the relative chemical potential of the oxide. The boundary between microscopes, if a crystalline oxide layer, forms an important pathway for oxygen to reach the unoxidized metal below. For this reason, a layer of vitreous oxide - which has no grain boundaries - can inhibit oxidation. The necessary conditions (but not enough) for the passive are recorded in the Pourbaix diagram. Some corrosion inhibitors help the formation of passivation layers on the metal surfaces used. Some compounds, soluble in solution (chromate, molybdate) form a non-reactive and low solubility film on the metal surface.
Maps Passivation (chemistry)
Discovery
In the mid-1800s, Christian Friedrich SchÃÆ'¶nbein discovered that when a piece of iron is placed in dilute nitric acid, it dissolves and produces hydrogen, but if the iron is placed in concentrated nitric acid and then returns to dilute nitric acid, little or no reaction will occur. SchÃÆ'¶nbein names the first condition as active and the second is passive. If the passive iron is touched by the active iron, it becomes active again. In 1920, Ralph S. Lillie measured the effect of an active piece of iron touching a passive iron wire and found that "activation waves sweep rapidly (at several hundred centimeters per second) across its length".
Specific material
Silicon
In the field of microelectronics, the formation of strong passivating oxide is essential for silicon performance.
In the field of photovoltaics, passive surface layers such as silicon nitride, silicon dioxide or titanium dioxide can reduce surface recombination - a significant loss mechanism in solar cells.
Aluminum
Pure aluminum naturally forms a thin layer of aluminum oxide in contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in many environments. Some aluminum alloys, however, do not form oxide layers well, and thus are not protected from corrosion. There is a method to increase the formation of oxide layers for certain alloys. For example, before storing hydrogen peroxide in an aluminum container, the container may be classified by rinsing it with a dilute solution of nitric acid and peroxide alternately with deionized water. Nitric acid and peroxide oxidize and dissolve any impurities on the inner surface of the container, and deionized water cleanses oxidized acids and impurities.
Generally, there are two main ways to passivate aluminum alloys (excluding plating, painting, and other barrier layers): chrome and anodizing conversion layers. Alclading, which metallurgically binds a thin layer of aluminum or pure alloy to a different base aluminum alloy, is not fully passivated from the base alloy. However, the coated aluminum layer is designed to spontaneously develop an oxide layer and thereby protect the base alloy.
The chromium conversion coating converts the aluminum surface into an aluminum chromate layer in the thickness range of 0.001-0.0004 inches. The aluminum chromate conversion coating is amorphous in a structure with a water-hydrated gel composition. Chromate conversion is a common way to validate not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys.
Anodization is an electrolysis process that forms a thicker oxide layer. The anodic layer comprises hydrated aluminum oxide and is considered resistant to corrosion and abrasion. The solution is stronger than any other process and also provides electrical isolation, which may not be possible by two other processes.
Iron
Iron, including steel, can be protected by promoting oxidation ("rust") and then converting oxidation to metallophosphate by using phosphoric acid and subsequently protected by surface coating. Since the non-coated surface is soluble in water, the preferred method is to form a manganese or zinc compound by a process known as a Parkerizing or phosphate conversion. The older electrochemical conversion layers, less effective but chemically similar include black oxidizers, historically known as bluing or browning. Ordinary steel forms a passive layer in an alkaline environment, like a reinforcing bar in a concrete.
Stainless steel
Stainless steels are naturally stainless, which may indicate that their pacification is not necessary. However, stainless steels are not fully resistant to rust. One common mode of corrosion in corrosion-resistant steel is when small spots on the surface begin to rust because grain boundaries or bits attached to foreign bodies (such as grinding) allow water molecules to oxidize some iron at these points despite chromium alloys. This is called rouging. Some grades of stainless steels are highly resistant to rouging; parts made from them may therefore forget the passivation step, depending on the technical decision.
Passive processes are generally controlled by industry standards, the most common among them today are ASTM A 967 and AMS 2700. These industry standards generally include some usable passivation processes, with the choice of specific methods left to customers and vendors. The "method" is a passive bath based on nitric acid, or a citric acid based bath. The various 'types' listed under each method refer to the temperature difference and acid bath concentration. Sodium dichromate is often required as an additive to increase oxidation in certain 'types' of nitric acid based baths.
Common among all the different specifications and types are the following steps: Before passivation, the object must be cleaned of contaminants and generally have to undergo validation tests to prove that the surface is 'clean'. The object is then placed in an acidic passive bath that meets the temperature and chemical requirements of the method and type determined between the customer and the vendor. (Temperature can range from ambient to 60 degrees C (140 degrees F)), while minimum passive time is usually 20 to 30 minutes). The parts are neutralized using aqueous sodium hydroxide bath, then rinsed with clean water and dried. Passive surfaces are validated using moisture, high temperature, influence agent (salt spray), or some combination of the three. However, an exclusive passivation process exists for martensitic stainless steels, which are difficult to pass, since microscopic discontinuities can form on the surface of the engine part during passivation in a typical nitric acid bath. The passivation process removes exogenous iron, creates/restores a passive oxide layer that prevents further oxidation (rust), and cleans parts of the dirt, scale, or other compounds produced by welding (eg oxides).
It is not uncommon for some aerospace manufacturers to have additional guidelines and regulations when passivating their products that exceed national standards. Often, these requirements will be downgraded using Nadcap or other accreditation systems. Various testing methods are available to determine the passivation (or passive state) of stainless steels. The most common method of validating passivity of parts is some combination of high humidity and heat for a period of time, which is intended to induce rust. Electro-chemical testers can also be used to verify passivation commercially.
Nickel
Source of the article : Wikipedia
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