Corrosion Processes in Metal-Polymer Contacts

Polymer-metal friction pairs are also characterized by frictional transfer of material. This implies the material displacement from the polymer part friction surface onto that of the metal counterbody. Macrotransfer is realized either as a fatigue detachment of polymer particles sticking to the counterbody or as galling, i.e. sticking of the viscous-flow fragments from the polymer surface layer, their extension and movement in the friction direction. 

Local corrosion of metals occurs because of many factors, the most critical of which are inhomogeneity of the metal phase and difference in composition of the corrosive medium, i.e. electrochemical heterogeneity on different areas of the metal surface. 

The most widespread corrosion types are point corrosion or pitting, filiform, crevice, contact and intercrystalline corrosion. The highest danger for the metal-polymer system is presented by the crevice type of corrosion. 

Crevice corrosion is often observed in clearances between metals and in places of loose contact between the metal and dielectric (including corrosion resistant materials). Passivating materials and alloys, such as stainless chromium and nickel-chromium steels, aluminum and magnesium alloys are most inclined to crevice corrosion [2,12,13]. 

Crevice corrosion starts because of hampered access of electrolyte and oxygen in the space of the clearance. This changes the pH of the electrolyte solution in the clearance, spurs the anodic and retards the cathodic processes. As a result, the metal electrode potential shifts to the negative with respect to the potential on the open surface. These processes bring about electrochemical microelements of the slot-open-surface t3Tje, in which the metal in the clearance serves as an anode [14]. 

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Thermal-oxidative destruction of polymer materials is accompanied by the formation of corrosion-active components that may essentially change corrosion processes in the polymer-metal contact. 

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. 

Let us consider some peculiarities of local corrosion of metals in contact with polymers stimulated by changes (inhibition or acceleration) of corrosion processes at the metal-polymer interface. These changes can be attributed either to a limited velocity of movement of the substances participating in the corrosion process or to chemical and electrochemical effects of the polymer on the metal and their influence on the corrosive media activity. 

Despite the fact that the peculiarities of metal wearing in contact with polymers were determined more than 30 years ago [35], the serviceability of metal-polymer friction joints has resisted estimates for a long time. The origin of this corrosion wear mode was considered only for plastics processing equipment, in which metals are in contact with the moving polymer melt [36]. 

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Corrosion of the plates not only detracts from their mechanical properties but also gives rise to undesirable corrosion products, namely, heavy-metal ions, which, when depositing on the catalysts, strongly depress their activity. The corrosion processes also give rise to superficial oxide films on the metal parts, and these cause contact resistance of the surfaces. For a lower contact resistance, metallic bipolar plates sometimes have a surface layer of a more stable metal. Thus, in the first polymer electrolyte membrane fuel cell, developed by General Electric for the Gemini spacecraft, the bipolar plates consisted of niobium and tantalum coated with a thin layer of gold. A bipolar plate could also be coated with a layer of carbide or nitride. 

Acid scavengers are also addressed as acid absorbers, antacids, or still less precisely as costabilizers. The addition of acid scavengers is necessary because catalyst residues from processing and manufacture may contribute to imdesired properties. This does not necessarily cause a diminished stability of the polymeric base material. Instead, residues from Ziegler-Natta catalysts in poly(olefine)s may contain traces of halogen that could cause corrosion reactions to metals that are in contact with the polymer. 

The IBM group led by Brusic et al. [57,58] also studied the use of polyaniline derivatives for corrosion protection of copper as well as silver. The unsubstituted polyaniline, in neutral base form, provided good corrosion protection both at open-circuit potential and at high anodic potentials. The dissolution of metal (both Cu and Ag) was decreased by a factor of 100 when the metal surface was completely covered by the neutral polyaniline. However, polyaniline doped with dodecylbenzene-sulfonic acid (the conductive form of the polymer) increased the corrosion rate of Cu and Ag in water. The doped polymer in contact with the metal is spontaneously reduced at a rate faster than the oxygen reduction rate. The faster cathodic process in turn increases the overall rate of the anodic reaction, which is the dissolution of Cu and Ag, as opposed to the formation of a passive oxide layer. 

Thermowells typically are cylindrical metal tubes that are capped on one end and protrude into a process line or vessel to bring the TC or RTD into thermal contact with the process fluid. Thermowells provide a rugged, corrosion-resistant barrier between the process fluid and the sensor that allows for removal of the sensor while the process is stiU in operation. Thermowells that are coated with polymer or another adhering material can significantly increase the lag associated with the temperature measurement, i.e., sigiuficantly increase the response time of the sensor. 

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