Inhibition, corrosion compound precipitation

Properties Miscible with water but insoluble in organic solvents. It possesses sequestering, dispersing, and deflocculating properties and precipitates proteins. In very low concentration, it inhibits corrosion of steel and prevents the precipitation of slightly soluble, scale-forming compounds such as calcium carbonate and calcium sulfate. 

The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solu-tion interface. Three possible types of adsorption are associated with organic inhibitors n-bond orbital adsorption, electrostatic adsorption, and chemisorptions. A more simplistic view of the mechanism of corrosion inhibitors can be described as controlled precipitation of the inhibitor from its environment (water and hydrocarbons) onto metal surfaces. During the past decade, the primary improvements in inhibitor technology have been the refinement of formulations and the development of improved methods of applying inhibitors (Totlani and Athavale 2000 Farquhar et al. 1994). 

Zinc ions inhibit corrosion by a cathodic polarization mechanism based on the precipitation of a zinc hydroxide film at cathodic sites on the metal surface. Zinc in combination with phosphates will lead to a protective film containing zinc phosphate. Film formation is usually rapid due to the low solubility of the zinc compounds at an alkaline pH. The low solubility of zinc in alkaline solutions requires the incorporation of dispersants. The rate of film formation with cathodic inorganic inhibitors should be carefully controlled, as dangerous fouling may occur. Protective films caused by cathodic inhibition are macroscopic and often easily visible, whereas anodic inhibitors generally from very thin, hardly detectable passive films. 

While some cathodic poisons such as sulfides and selenides are adsorbed on the metal surface, compounds of arsenic, bismuth, and antimony are reduced at the cathode to deposit a layer of the respective metals. Sulfides and selenides generally are not useful inhibitors because they are not very soluble in acidic solutions, they precipitate many metal ions, and they are toxic. Arsenates are used to inhibit corrosion in strong acids, but in recent years the trend has been to rely more on organic inhibitors because of the toxicity of arsenic. 

Chromates are very effective inhibitors of Fe, Al, Cu, Zn corrosion. The unique chemical and electronic properties of the oxo-compounds of chromium give rise to a unique ability to inhibit corrosion in ferrous and nonferrous materials. They are both anodic and cathodic inhibitors due to their abilities to form precipitates with the dissolving metal ions... 

For primers with self-healing and corrosion inhibitive properties, it is desirable to have one component to inhibit corrosion of the underlying metal and a second component to repair the polymer coating. These individual components can be multifunctional or multistep. For the protection of metals, many paint systems consist of an inhibitive primer and a topcoat. In case the coating is mechanically damaged or local delaminations are present, the repair function is usually achieved via release of the inhibitors from the inhibitive primer, whereby the corrosion performance of the underlying metal surface benefits from autonomous surface recovery processes by the precipitation of stable inhibitor compounds at cathodic and/or anodic sites. 

A corrosion-inhibiting admixture is a chemical compound which, when added in small concentrations to concrete or mortar, effectively checks or retards corrosion. These admixtures can be grouped into three broad classes, anodic, cathodic and mixed, depending on whether they interfere with the corrosion reaction preferentially at the anodic or cathodic sites or whether both are involved [48]. Six types of mechanisms, viz. anodic (oxidizing passivators), anodic (non-oxidizing passivators), cathodic, precipitation... 

Polarization occurs because of ion concentration buildup near the anode and/or cathode. Once the ion concentration reaches saturation, corrosion essentially stops. Polarization can occur when (1) Hydrogen ions concentrate at an active cathode in the absence of a cathodic depolarizer. Dissolved oxygen acts as a cathodic depolarizer. (2) Metal ions saturate the electrolyte around an anode. (Soluble Fe++ may saturate the anode, perhaps as the result of the precipitation of an insoluble iron salt, inhibiting the diffusion of Fe++. For example, insoluble surface compounds such as carbonate scales in a fresh water often occur on carbon steel.)... 

Most effects of elevated temperatures are adverse to corrosion inhibition. High temperatures increase corrosion rates (about double for a 15°C rise at room temperature), and they decrease the tendency of inhibitors to adsorb on metal surfaces. Precipitate-forming inhibitors are less effective at elevated temperatures because of the greater solubility of the protective deposit. Thermal stability of corrosion inhibitors is an important consideration at high temperatures. Polyphosphates, for example, are hydrolyzed by hot water to form orthophosphates that have little inhibitive value. Most organic compounds are unstable above about 200°C (see Table 17.1) hence, they may provide only temporary inhibition at best. 

Inhibitors can also be classified on the basis of their functions. For instance, chromates and nitrates are called passivating inhibitors because of their tendency to passivate the metal surface. Some inhibitors, such as silicates, inhibit both the anodic and cathodic reactions. They also remove undesirable suspended particles from the system, such as iron particles, by precipitation. Certain types of inhibitors make the surrounding environment alkaline to prevent corrosion. Such inhibitors in the gas phase are called vapor phase inhibitors, and they consist of heterocyclic compounds, such as cyclohexylamine. These inhibitors are used within packing crates during transportation by sea. 

The various routes that have been examined to enhance the corrosion resistance of anodized aluminium using cerium compounds, such as nitrates or sulphates, include pre- and post-treatments and anodizing with cerium species added to the bath formulation (the addition of cerium to commercial alloys for corrosion protection has not been reported). Each of these approaches has revealed some benefits in short-term laboratory corrosion tests. However, the imderlying mechanisms of corrosion protection are imclear. Corrosion inhibition of aluminium alloys caused by cerium species is well-established, with cerium acting as a cathodic inhibitor at intermetallic particles (Amott et al, 1987). Cerium oxide/ Itydroxide precipitates in response to the increase of pH at cathodic particles as the alloy briefly corrodes following its immersion in the environment. The precipitates stifle further corrosion by hindering the reduction of oxygen and water. 

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