Passivating type inhibitors

Stem (77) has recently discussed the mechanism of passivating-type inhibitors on the basis of the theory presented here. He points out that passivity may be accounted for operationally by anodic polarization phenomena irrespective of whether films are assumed to be responsible (79). This approach was also suggested by Mears (80). Tomasfcov (76) has proposed the use of the ratio of the extent of anodic polarization, - Ej p to the extent of the cathodic polarization,... 

PRECIPITATING (CATHODIC) INHIBITORS - The are chemicals which form insoluble precipitates that are able to coat and protect surfaces. They are less durable then the passivating type inhibitors. 

The use of potentiostatic curves has also facilitated the study of the rdle of oxidising agents and inhibitors in corrosion processes. Sterndiscussed the rdle of passivating-type inhibitors and used potentiostatic curves to explain their action. Poseyused the potentiostatic technique for determining the reduction of cupric ion on stainless steel. Both of these references... 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

There are two types of passivating inhibitors oxidizing anions such as chromate, nitrite, and nitrate, which can passivate steel in the absence of oxygen, and the nonoxidizing ions such as phosphate, tungstate, and molybdate, which require the presence of oxygen to passivate steel. Examples of passivators (anodic inhibitors) include chromate, nitrite, and orthophosphate (Dihua et al. 1999). 

The lower temperature is set by fluid composition. For wet corrosion to occur at any temperature there must exist either a discrete aqueous phase or sufficient water dissolved in a liquid phase to impart electrical conducting or ionic properties to a liquid such as a hydrocarbon, which does not possess these properties in the absence of water. Wet corrosion is an electrochemical process. It may be controlled by the use of passivating, neutralizing, or adsorption-type inhibitors, the use of which will be summarized below. 

Three types of anodic protection can be distinguished (1) impressed current, (2) formation of local cathodes on the material surface and (3) application of passivating inhibitors. For impressed current methods, the protection potential ranges must be determined by experiment (see information in Section 2.3). Anodic protection with impressed current has many applications. It fails if there is restricted current access (e.g., in wet gas spaces) with a lack of electrolyte and/or in the... 

There are basically three main types of inorganic inhibitors used in industry anodic passivating inhibitors, cathodic inhibitors and cathodic precipitators. 

When the boiler is placed back online, certain types of anodic inhibitors (which are generally employed to act as polishing treatments in the maintenance program) also may prove beneficial in further strengthening the passive film. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

A possible mechanism of formation of a sidewall passivation film in the case of silicon etching in a HCI/O2/BCI3 plasma is shown in Fig. 18 [77]. SiCf Hv-type byproducts are sputtered away by ion bombardment from the bottom of the trench. A portion of the sputtered flux strikes and sticks on the sidewalls on the trench. Oxygenation of the byproducts on the sidewalls results in a silicon dioxide type of film that resists etching. The sidewalls do not receive any appreciable ion bombardment and hence, depending on conditions, a rather thick inhibitor film may be formed. 

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