Protective corrosive agents

The external environment experienced by plant can be more corrosive than the internal process stream. Any con-stmction material must be chosen to withstand or be able to be protected from external corrosive agents. 

Because of their generally poor resistance to solvents, acids, alkalis and other corrosive agents, paints are not normally used to protect plant internals handling anything... 

To prevent underground corrosion, lead is frequently protected with coatings of tar, bitumen, resin, etc., which are only effective if they completely insulate the metal from corrosive agents and stray currents. No coating is fully effective, but some give good protection ". The most successful method used is cathodic protection which for impressed currents, if correctly applied, can protect indefinitely (see Chapter 10). It is effective at a potential of E° = —0-8 V or about 0-1 V more negative than... 

This requirement clearly contradicts the other requiranent of corrosion protective coatings, namely to prevent the access of water as a corrosive agent to the mstal surface. Frequently binders used in practical corrosion protective coating systems scarcely svrell by water and are only slightly permeable to it. 

Fourier transform infrared reflection-absorption spectroscopy (FT-IFRAS) is applied to the study of corrosion protection of copper by an organic coating. Poly-N-vinyliroidazole (PVI(D) and poly-4(5)-vinylimidazole (PVI(4)) are demonstrated to be effective new polymeric anti-corrosion agents for copper at elevated temperatures. Oxidation of copper is suppressed even at 400° C. PVI(1) and PVI(4) are more effective anti-oxidants than the most commonly used corrosion inhibitors, benzotriazole and undecyllmldazole, at elevated temperatures. These new polymeric agents are water soluble and easy to treat the metal surface. 

There are several stakes in the management of first aid, depending on circumstances. The ideal is to be able to protect the eye by preventing any penetration of a corrosive agent. If action is taken after penetration has begun, the objective becomes to stop penetration and remove the chemical to be able to administer care according to the symptoms observed. 

Acidic soils are highly corrosive. Sulfur is a corrosive agent in automalive fuels and in the atmosphere (SO ) as well, and is frequently mentioned in connection with so-called acid rains. Sodium chloride in the air at locations near Ihe sea is strongly corrosive, especially at temperatures above 70 F (21.1 C). Copper, nickel, chromium, and zinc are among the more corrosion-resistant metals and are widely used as protective coatings for other metals. 

Because of the low surface energy, resistance to chemicals and corrosive agents, and resistance to weathering, fluorinated polyurethanes are very well suited for protective coatings. They can be deposited in a desired location and thickness with the added advantage of curing mostly at ambient temperatures. 

Metallic coatings are either more noble than the substrate, as in chromium plate on mild steel, or are base metals which corrode more easily than the substrate. In the former case, the underlying metal is protected by a continuous impervious film of the noble metal which is itself resistant to attack. This method is adequate provided the surface coating contains no holes or flaws and remains intact. Penetration of this layer by the corrosive agent leads to galvanic corrosion at the interface of the two metals. If the object is coated with a more base metal, then protection is by both the physical barrier of the metal film and by cathodic protection at any subsequent defects (Fig. 19). 

Chromium is a white, hard, lustrous, and brittle metal (mp 1903 10°C). It is extremely resistant to ordinary corrosive agents, which accounts for its extensive use as an electroplated protective coating. The metal dissolves fairly readily in nonoxidizing mineral acids, for example, hydrochloric and sulfuric acids, but not in cold aqua regia or nitric acid, either concentrated or dilute. The last two reagents passivate the metal in a manner that is not well understood. The electrode potentials of the metal are... 

While a copper-enriched surface has the implication of always causing accelerated electrochemical corrosion, replacing the native, hydroxylated, mixed Al-Mg oxide layer with a thin stable oxide layer seems to allow the plasma films to tightly adhere to the alloy surface. This adhesion, coupled with the barrier properties of the films, appears to provide additional protection of the oxide layer from contact with corrosive agents. 

Aluminum, a very active metal, reacts rapidly with O2 from the air to form a surface layer of aluminum oxide, AI2O3, that is so thin that it is transparent. This very tough, hard substance is inert to oxygen, water, and most other corrosive agents in the environment. In this way, objects made of aluminum form their own protective layers and need not be treated further to inhibit corrosion. 

The corrosion resistance of enamel is generally attributed to a surface layer of silica. Acid first dissolves the surface alkali oxide leaving a hydrated porous layer which is mainly silica, that acts as a barrier through which corrosive agents must difiltse for further attack to occur. As the affected layer deepens, the diffusion process slows down so that the enamel becomes protected by a skin of acid resisting silica. 

When an oil-bearing formation consists of loose, uncemented sand, a ceramic niter should be installed on the wellbottom (Fig. 64). The niter protects the bottomhole equipment from intensive wear that would be caused otherwise by the sand carried into the well with hot oil and gas. These niters are made for different permeabilities, they can withstand temperatures of up to 3,000°C, and they effectively stop sand penetration into the well. Because the surface of the niter is coarse-grained and has sharp unrounded edges, a turbulent Bow is created in the oil entering from the bed. For this reason the filter practically never plugs up. Moreover, the ceramic material is chemically inert therefore, the niter resists very well the action of low pH liquids and of other corrosive agents in the bottomhole. At the same time, the niter acts as a good heat insulator. 

The analysis of corrosion factors during polymer-metal pair wear has proved that the main path is electrochemical protection of the metal counterbody neutralization of corrosion agents formed in the friction zone and suppression of corrosion processes in the polymer-metal contact. These directions are realized by the means illustrated in Fig. 4.6 [37]. They are subdivided into two groups according to the use of special substances or physical fields and power effects. 

Protection of prestessing steel. The protection of prestressed tendons from external corrosive agents, in particular from the infiltration of de-icing salts, requires that they are completely surrounded with a protective barrier. 

In the strict sense, the passive mode implies that the newly formed phase is retarding the process, that is corrosion is slowed down with time protective scale), but there are cases in which the scale is nonprotective a scale with cracks, low viscosity, or foamy texture may not hinder the access of the corrosive agent to the substrate. 

A third basic law for passive corrosion comes from the analysis of the situation, where the reaction product formed on the material is completely blocking the further access of the corrosive agent. This law was derived from the study of metals at moderate temperatures. It is easily envisaged If every particle formed protects the substrate completely, we have a situation, in which the surface area available for reactions is diminished rapidly. The mathematical form following is the logarithmic law... 

The corrosion inhibitors appear to possess properties that impart to metals resistance to attack by a variety of corrosive agents, such as brines, weak inorganic acids, organic acids, COj, HjS, etc. The method of carrying out this process is relatively simple in principle the corrosion preventive reagent is dissolved in the liquid corrosive medium in small amounts and is thus kept in contact with the metal surface to be protected. Alternatively, the corrosion inhibitor may be applied first to the metal surface, either as it is or as a solution in some carrier liquid or paste. Continuous application, as in the corrosive solution, is the preferred method, however. The concentration of the corrosion inhibitors varies widely, but the preferable concentrations are 15-250 ppm. 

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