General etch corrosion

Uniform rates of corrosion such as general etch corrosion seldom occur in steam-water circuits. Rather, pitting, tuberculation, and other complex types of corrosion tend to predominate. These forms of corrosion often result directly or indirectly from reactions occurring in particular areas of the system where fouling and deposition may be present. 

As corrosion products develop, so the rate of 02 diffusion reduces and the rate of general etch corrosion slows down. But in practice, the presence of surface deposits tends to promote various forms of localized corrosion such as tuberculation, pitting corrosion, and stress corrosion, and consequently the rate of corrosion continues unabated. 

Certain Inconel alloys have been used with varying degrees of success. Inconel 600 (70% Ni, 15% Cr, 15% Fe) has been employed because of its improved resistance to chloride-induced, SCC, but it has failed on occasions because of intergranular attack and general etch corrosion. 

Hydrogen can be produced as a result of the breakdown of hydrazine or amines, general etch corrosion, or localized corrosion that may, in turn, lead to the development of hydrogen embrittlement corrosion... 

General Etch Oxygen Corrosion (Uniform Rate Corrosion/General Wastage Corrosion)... 

Oxygen corrosion, occurring as General etch corrosionAmiform rate corrosion (less common form) Localized corrosion (takes several forms and common where waterside conditions are less than ideal) Rare in correctly treated boilers but can affect drum waterline and tubes. More common in idle and low-load boilers, where water chemistry is unbalanced, under high MU conditions and after poor chemical cleaning. Also in peak-load boilers, especially where deposition can occur. Localized corrosion can be very serious, causing metal failure. 

Steel adherends are also subject to corrosion in moist environments. Unfortunately, no general etch or anodization treatment has been developed that provides superior bond durability [12,18,46]. In part, this is due to the lack of a coherent, adherent stable oxide— iron oxides do not protect the underlying substrate from the environment. Equally important, the different steel metallurgies react to chemical treatments differently—a procedure that may give good results for one steel alloy may give very poor results for another, similar steel alloy. 

Other means of identification sometimes used satisfactorily involve chemical etching of the surface (not to be generally recommended), or the formation of letters or numbers by means of a vibrating stylus. The former is advantageous in studies of stress-corrosion cracking in which stamped symbols could lead to regions of stress concentration. 

To protect the aluminum joint from the effects of the environment, especially water and corrosion, an artificially thickened oxide layer is generally formed on the surface. Historically, chemical etching as a surface preparation has provided the surest way of obtaining durable adhesive bonds with aluminum. 

The general sequence of surface preparation for ferrous surfaces such as iron, steel, and stainless steel consists of the following methods degreasing, acid etch or alkaline clean, rinse, dry, chemical surface treatment, and priming. The chemical surface treatment step is not considered a standard procedure, but it is sometimes used when optimum quality joints are required. It consists of the formation of a corrosion-preventing film of controlled chemical composition and thickness. These films are a complex mixture of phosphates, fluorides, chromates, sulfates, nitrates, etc. The composition of the film may be the important factor that controls the strength of the bonded joint. 

Moderate resistance general corrosion rate varies with the type of acid, concentration and temperature High corrosion rates, used for pickling, etching Pd, Ru, Mo, Ni render the titanium to be more resistant to corrosion... 

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