Metal dissolution brasses

Inhibition of general corrosion by biofilms has been reported for mild steel, copper, aluminum and stainless steels, and brass. The mechanisms most frequently cited for the inhibition are formation of a diffusion barrier to corrosion products that stifles metal dissolution, consumption of oxygen by respiring aerobic microorganisms within the biofilm causing a diminution of that reactant at the metal surface, production of metabolic products that act as corrosion inhibitors (e.g., siderophores) or specific antibiotics that prevent proliferation... 

When dezincification occurs in service the brass dissolves anodically and this reaction is electrochemically balanced by the reduction of dissolved oxygen present in the water at the surface of the brass. Both the copper and zinc constituents of the brass dissolve, but the copper is not stable in solution at the potential of dezincifying brass and is rapidly reduced back to metallic copper. Once the attack becomes established, therefore, two cathodic sites exist —the first at the surface of the metal, at which dissolved oxygen is reduced, and a second situated close to the advancing front of the anodic attack where the copper ions produced during the anodic reaction are reduced to form the porous mass of copper which is characteristic of dezincification. The second cathodic reaction can only be sufficient to balance electrochemically the anodic dissolution of the copper of the brass, and without the support of the reduction of oxygen on the outer face (which balances dissolution of the zinc) the attack cannot continue. 

The initial step (eq. 10a) in the proposed mechanism requires adsorption/ binding of the alkyl halide to the metal surface—a process already previously discussed. Anything that might enhance this adsorption should affect the rate and extent of reaction. We have found that o)-bromo-l-alkenes enhanced copper dissolution from alloys (Table V). Comparative studies on brass foil confirmed these observations and also showed that the dissolution of zinc was affected only slightly (Table VI). The compound... 

Since friction is accompanied by heat generation, so micro-areas can be formed in which the polymer in the metal-polymer junction is in a viscous-flow state. Therefore, surface layers of the metal counterbody may dissolve and the metal can be transferred onto the mating polymer part. This dissolution of the metal is characteristic of both static (adhesive) and dynamic (frictional) contacts [114]. As the polymer melt contacts metal alloys, they selectively dissolve since their rates of dissolution are different. For instance, predominantly zinc is dissolved when brass contacts pentaplast melt and the surface layer of the brass sample becomes enriched with copper. As PE contacts bronze, mainly lead is dissolved. 

Mechanical stability of corrosion product layer Where the corrosion deposit has insufficient mechanical stability to maintain the thickness required to achieve dynamic equilibrium between the corrosion and dissolution rates, particles of the deposit will break off on an intermittent basis producing peaks of contamination. This type of breakdown would only be expected after prolonged ageing (>70 days for brasses) of the test coupon. Determining the difference between the total metal leached into the test solution and its filtered metal content could give an indication of the presence of this problem. 

Two types of indirect oxidation using H2O2 are used. The first involves reoxidation of an intermediate, or relay , metal which in turn acts as an oxidant in the recovery process - examples include Fe(III) oxidation of U(IV) and of thiourea in gold production. The second involves oxidation to assist acid dissolution of metals, chiefly in finishing of stainless steel, brass or copper. 

A less but still complicated situation is found for actively corroding alloys which are not covered by passive layers, particularly in acidic electrolytes. In many cases, the very different corrosion characteristics of the alloying elements lead to preferential dissolution of one metal component. The preferential oxidation or dissolution of the zinc of brasses and the related accumulation of copper at the surface is a technologically important example of dealloying of a metal surface. In this case zinc is much more... 

The research on corrosion, started in this institute in the 1950s, continued successfully further. The intergranular corrosion of steels was measured by an electrochemical potentiodynamic reactivation method [310-312]. Since the 1960s, the passivity of brass was further studied, the rates of corrosion were measured by polarization resistance, the effect of deformation on anodic dissolution of steels was followed, and the surface roughness of metals was measured other subjects of research were, e.g., the behavior of passive films on steel, the effect of compositirai and motion of electrolyte on corrosion of passivated aluminum, the cathodic protection of passive metals against corrosion, the anodes for cathodic protection of steels, etc.[313-316]. Measurements of polarization resistance in the system iron—concentrated sulfuric acid or boiling nitric acid, of corrosion and matter... 

Selective dissolution, where the less noble element dissolves selectively leaving behind a porous metal phase enriched in the more noble constituent, resulting in dealloying. The more common case is the selective dissolution of Zn (dezincification) from a-brass (Cu-Zn). Selective dissolution is in some instances associated with circumstances of stress corrosion cracking (SCC) [175]. 

---