Mechanical galvanizing

Another corrosion mechanism, galvanic corrosion, is possible when two dissimilar metals such as copper and steel are connected together, whether buried or submerged. An electrolytic corrosion reaction will occur due to the difference in electrical potential between the two metals in contact with each other. This difference in potential produces a driving force, causing corrosion activity on the less noble of the two metals. Such galvanic couples must be avoided or isolated so that the possibility of corrosion damage is minimized. 

Coating with a hot-dip or mechanical galvanizing process provides a cost-effective and maintenance-free corrosion protection system for most general applications. Hot-dip galvanizing should conform to ASTM A153/A153M or ASTM F2329 as appropriate. 

As an alternative to hot-dip zinc coating, mechanical galvanizing (electro-deposited zinc, an inorganic zinc-rich paint, or other coating system specifically selected for corrosion protection), can be used. Mechanical galvanizing should conform to ASTM B695. 

Bolts shall be mechanically galvanized to Class 50 of ASTM B695, or hot dip galvanized to ASTM A153. Nuts shall be hot dip galvanized to ASTM A153. 

Therefore, this type of coating is not sensitive to defects, pinlroles or mechanical damage during service. A typical example is galvanized steel (Zn layer on steel). 

Test Specimens In carrying out plant tests it is necessary to install the test specimens so that they wih not come into contact with other metals and alloys this avoids having their normal behavior disturbed by galvanic effects. It is also desirable to protect the specimens from possible mechanical damage. 

The basic mechanisms involved in graphitic corrosion are familiar and easily understood. Hence, remedial and preventive measures are relatively simple to implement. Although commonly categorized as a form of dealloying, graphitic corrosion has much in common with galvanic corrosion. 

CoiTosion may be uniform or be intensely localized, characterized by pitting. The mechanisms can be direct oxidation, e.g. when a metal is heated in an oxidizing environment, or electrochemical. Galvanic coiTosion may evolve sufficient hydrogen to cause a hazard, due to ... 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Fitzpatrick and Watts [57] also applied imaging TOF-SIMS to deteiTnine the failure mechanisms of adhesively bonded, phosphated hot-dipped galvanized steel... 

Primers for protective coatings may be divided into three broad classes based on the mechanism of substrate protection barrier primers that function by preventing the ingress of moisture and electrolytes, primers that protect the substrate galvanically in the presence of electrolytes, and primers that contain electrochemical inhibitors to passivate the substrate. Each of these approaches requires a distinct primer film structure due to the different mechanisms of protection. 

Most of the controlled corrosion studies on beryllium have been carried out in the USA in simulated reactor coolants. The latter have usually been water, aerated and de-aerated, containing small amounts of hydrogen peroxide and at temperatures up to 300-350°C. Many variables have been examined, covering surface condition, chemical composition, temperature, pH, galvanic effects and mechanical stress . 

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