Cuprous copper corrosion

Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond direcdy with cuprous oxide (CU2O) at the metal surface, forming a "chemisorbed" film. The plane of the triazole Hes parallel to the metal surface, thus each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A recent study supports the idea of an electronic stabilization mechanism. The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. 

Internal surfaces of all tubes were severely attacked (Fig. 4.29). A brown deposit layer consisting of magnetite, iron oxide hydroxide, and silica covered all surfaces. Deposition was thicker and more tenacious along the bottom of tubes. These deposits had a distinct greenish-blue cast caused by copper corrosion products beneath the deposit. Underlying corrosion products were ruby-red cuprous oxide crystals (Fig. 4.29). Areas not covered with deposits suffered only superficial attack, but below deposits wastage was severe. 

The ammonia production is less than in hydrazine, but there may be a perceived of copper and brass corrosion. In fact, any corrosion risk is small, provided that DEHA-treated boiler plants are subjected to the same requirements as hydrazine-treated units, namely, ensuring that all in-leakage of oxygen in the condensate system is fully eliminated. If this objective is achieved, the oxidation of cuprous oxide to cupric oxide tends not occur to any significant degree, and the susceptibility for copper corrosion in the presence of ammonia is equally low. 

When considering the second kind electrodes in Section 2.3, copper covered by CujO was referred to as a prominent example. Cuprous oxide layers developed in electrochemical systems are interesting from various points of view. In the first place, such layers are known to form during copper corrosion. Research in this field are very important and large in number (see, e.g.. Ref [1-12] and references therein). Extensive investigations of photoelectrochemical phenomena in these systems provide further insights into the corrosion mechanism and may be useful in improving copper corrosion resistance. However, these problems are not the main objective of the present review. 

As mentioned earUer, oxides can be formed on copper electrodes in various ways, and two main mechanisms are worthy of consideration. One of these is simply copper corrosion, which often occurs in Cu(ll)-free naturally aerated solutions. Intermediate Cu+ ions derived from copper oxidation are able to react with OH anions yielding unstable CuOH that, in turn, decomposes into cuprous oxide and water... 

For copper corrosion in hot H2SO4, Ross and Berry found that benzotriazole (BTA) inhibits the rate of copper dissolution flow rate and aeration change the inhibitor concentration profile, and a film composed of the cuprous salt of BTA is formed. Other effective inhibitors for copper in H2SO4 include 2,4-dinitrophenyl-hydrazine, benzimidazole, indazole, and quinoline. Various azoles appear to be effective corrosion inhibitors for brass, as well as copper. Finally, Subra-manyan et al. studied the corrosion inhibition mechanisms of the alkaloids quinine (C21H22N2O2) and strychnine (C21H22N2O2). Both compounds inhibit the corrosion of copper, in 1% H2SO4 at 86°F (30°C), relatively well, and both chemisorb and... 

Copper. Little work has been carried out on the mechanism of inhibition by anions of copper corrosion in neutral solutions. Inhibition occurs in solutions containing chromate, benzoate, or nitrite ions. Chloride and sulfide ions are aggressive, and there is some evidence that chloride ions can be taken up into the cuprous oxide film on copper to replace oxide ions and create cuprous ion vacancies that permit easier diffusion of cuprous ions through the film, thus increasing the corrosion rate. 

Bronze disease necessitates immediate action to halt the process and remove the cause. For a long time, stabilization was sought by removal of the cuprous chloride by immersing the object in a solution of sodium sesquicarbonate. This process was, however, extremely time-consuming, frequentiy unsuccesshil, and often the cause of unpleasant discolorations of the patina. Objects affected by bronze disease are mostiy treated by immersion in, or surface appHcation of, 1 H-henzotriazole [95-14-7] C H N, a corrosion inhibitor for copper. A localized treatment is the excavation of cuprous chloride from the affected area until bare metal is obtained, followed by appHcation of moist, freshly precipitated silver oxide which serves to stabilize the chloride by formation of silver chloride. Subsequent storage in very dry conditions is generally recommended to prevent recurrence. 

The main advantages of the Cosorb process over the older copper ammonium salt process are low corrosion rate, abiHty to work in carbon dioxide atmospheres, and low energy consumption. The active CuAlCl C H CH complex is considerably more stable than the cuprous ammonium salt, and solvent toluene losses are much lower than the ammonia losses of the older process (94). 

Calcium carbonate has normal pH and inverse temperature solubilities. Hence, such deposits readily form as pH and water temperature rise. Copper carbonate can form beneath deposit accumulations, producing a friable bluish-white corrosion product (Fig. 4.17). Beneath the carbonate, sparkling, ruby-red cuprous oxide crystals will often be found on copper alloys (Fig. 4.18). The cuprous oxide is friable, as these crystals are small and do not readily cling to one another or other surfaces (Fig. 4.19). If chloride concentrations are high, a white copper chloride corrosion product may be present beneath the cuprous oxide layer. However, experience shows that copper chloride accumulation is usually slight relative to other corrosion product masses in most natural waters. 

Copper is not ordinarily corroded in water unless dissolved oxygen is present. In nearly pure aerated water, a thin, protective layer of cuprous oxide and cupric hydroxide forms. Oxygen must diffuse through the film for corrosion to occur. 

Typically the internals of the coil show pits and pinholes and may even perforate. Corrosion debris is evident, usually containing green hydrated copper carbonate (CuC03 nH20) and red cuprous oxide (Cu20). 

Problems with heating coils Internal coil corrosion Note corrosion debris is green hydrated copper carbonate Cu[11IC03 nH20 red cuprous oxide Cu20 /ntemal coil deposition Acid corrosion from soft water. Pinhole corrosion from 02 and C02. Erosion corrosion over 6 ft/s flow. Hard water scale from hard water. 

Copper salts usually are the result of corrosion in the post-boiler section and may be present as red cuprous oxide (Cu20), black cupric oxide (CuO), or blue-green copper sulfate (CuSO ). Mostly, copper salts are mixed with hematite and magnetite and take on a black color. 

Most corrosion processes in copper and copper alloys generally start at the surface layer of the metal or alloy. When exposed to the atmosphere at ambient temperature, the surface reacts with oxygen, water, carbon dioxide, and air pollutants in buried objects the surface layer reacts with the components of the soil and with soil pollutants. In either case it gradually acquires a more or less thick patina under which the metallic core of an object may remain substantially unchanged. At particular sites, however, the corrosion processes may penetrate beyond the surface, and buried objects in particular may become severely corroded. At times, only extremely small remains of the original metal or alloy may be left underneath the corrosion layers. Very small amounts of active ions in the soil, such as chloride and nitrate under moist conditions, for example, may result, first in the corrosion of the surface layer and eventually, of the entire object. The process usually starts when surface atoms of the metal react with, say, chloride ions in the groundwater and form compounds of copper and chlorine, mainly cuprous chloride, cupric chloride, and/or hydrated cupric chloride. 

The bronze disease corrosion process usually starts when copper atoms in bronze react with chloride ions under humid conditions, forming cuprous chloride, a bright blue-green, very unstable compound of copper ... 

Excavated copper alloys are also susceptible to accelerated corrosion even a long time after exhumation due to the presence of chlorides coming from the soil that have attacked the alloy and formed a layer of cuprous chloride (nantokite) deep inside the corrosion layers, just within reach of the metallic core. Cuprous chloride is the stage of a series of reactions in the presence of oxygen and moisture that make it very unstable. The overall gross reaction may be written as [249] ... 

The cuprous-cupric electron transfer reaction is believed to be the rate-limiting step in the process of stress corrosion cracking in some engineering environments [60], Experimental studies of the temperature dependence of this rate at a copper electrode were carried out at Argonne. Two remarkable conclusions arise from the study reviewed here [69] (1) Unlike our previous study of the ferrous-ferric reaction [44], we find the cuprous-cupric electron transfer reaction to be adiabatic, and (2) the free energy barrier to the cuprous cupric reaction is dominated in our interpretation by the energy required to approach the electrode and not, as in the ferrous-ferric case, by solvent rearrangement. 

Two aspects of Table 1 are important. The standard conditions are 298 K and all reactants and products are at unity activity. The second key is the selection of the hydrogen reaction as having a standard reversible potential of 0.0 V. The table allows the first use of thermodynamics in corrosion. For a metal in a 1 M solution of its salt, the table allows one to predict the electrochemical potential below (i.e., more negative) which net dissolution is impossible. For example, at +0.337 V(NHE), copper will not dissolve to cuprous ion if the solution is 1 M in Cu2+. In fact, at more negative potentials, there will be a tendency at the metal/solution interface to reduce the cuprous ions to copper metal on the surface. 

Copper, being a noble metal, has good resistance to corrosion. A thin adherent film of cuprous oxide and cupric carbonate is formed due to corrosion. Passivation is not a prominent process. The dissolved copper in solution affects the electrode potential such that the increase in velocity of the solution in contact with the metal results in increasing attack of the metal. Thus cuprous oxide is produced under dynamic flow of the solution. The thickness of the oxide film is about 500 nm. 

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