Pinhole corrosion

Domestic heating coil internal corrosion. Where naturally soft or lean city water is supplied and the Langelier Saturation Index (LSI) is below -1.0, acid corrosion takes place as a result of the acidic nature of the water. This water often has a high dissolved gas content, which additionally leads to pinhole corrosion. Where water velocities are too high (say, over 6 ft/s 1.8 m/s) the protective oxide layer is stripped off and erosion corrosion takes place. 

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. 

Figure 11.4 Pinhole Corrosion of Electroplated Ni-Au/Kovar in Ultraviolet (UV)-Wet air...

As an additional antifoulant measure, SHEs have been coated with a phenolic hning. This provides some degree of corrosion protection as well, but this is not guaranteed due to pinholes in the lining process. 

Rupture discs are also used below relief valves to protect them from corrosion due to ves.sel fluids. The rupture disc bursts first and the relief valve immediately opens. The relief valve reseals, limiting flow when the pressure declines. When this configuration is used, it is necessary to monitor the pressure in the space between the rupture disk and the relief valve, either with a pressure indicator or a high pressure switch. Othei-wise, if a pinhole leak develops in the rupture disk, the pressure would equalize on both sides, and the rupture disk would not rupture at its set pressure because it works on differential pressure. 

Cable sheaths may be covered with paper and hessian wrappings impregnated with bituminous compounds or with extruded or taped plastics outer sheaths. At pinholes or discontinuities in protective coatings the sheath will be particularly liable to electrolytic corrosion in stray-current areas, and it is desirable to supplement this form of protection by drainage bonds or direct cathodic protection. 

The efficacy of polymers when used to protect metals from corrosive environments is influenced by their efficiency as barrier materials. When applied to metals by some techniques, such as fluidised bed coating, there is always the danger of macro-diffusion through pinholes which are gross imperfections in the surface and which do not have to be visible to be very much greater than the dimension of penetrating molecules. 

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). 

In the long term (20-40 years), the overall domestic HW system is subject to pinhole leaks stemming from the galvanic corrosion of steel riser pipes (usually located in inaccessible areas), with resultant flooding within the building. Galvanic corrosion develops because these systems typically contain a wide range of mixed metals such as black... 

When rupture discs are used before a spring-loaded relief, a pressure gauge is installed between the two devices. This telltale gauge is an indicator that shows when the disc ruptures. The failure can be the result of a pressure excursion or of a pinhole caused by corrosion. In either case the telltale gauge indicates that the disc needs to be replaced. 

To address the imperfections of the anode coating, new Pt coating methods are being investigated by the electrode suppliers to improve deposition and eliminate pinholes. Also, no evidence of corrosion was observed during GB treatment in Demo II tests with the 2-kW SILVER II pilot plant, which is constructed of glass (AEA, 2000). AEA has indicated that the ability... 

General corrosion in the top 5 -inch region of the upper liner after the 500-hour test was also observed. This relatively uniform corrosion is estimated to penetrate the 0.15-inch-thick liner to about 0.008 inch. Had it not been for the pinhole leaks, the liner would have had significant remaining service life. However, Foster Wheeler concluded that the test did indicate the upper... 

More often the passive layer is broken down locally and then the steel is said to be attacked by localized corrosion, the most important forms being pitting, crevice corrosion, and corrosion cracking. Most often the localized corrosion is caused by halogen ions such as chloride, bromide, and iodide. Pitting or pitting corrosion is seen as small pinholes on the surface of the steel. This section describes electrochemical instrumental methods to investigate and measure this form of corrosion attack. 

The nature of bulk defects is straightforward. Pinholes, cracks, or other surface imperfections result in rapid diffusion of saline into the implant resulting in corrosion of electrical conductors. This corrosion lowers the resistance of the original leakage path until normal operation of the electronic circuit is impared and the Implant falls. 

Major causes for coating failure are surface cracking and undetected pinholes or voids. These can be repaired and serious problems avoided. Coatings generally fail in different modes, these are chemical failure, abrasion failure, adhesive failure, cohesive failure and undercoat corrosion. For performance evaluation of coatings on experimental basis on these parameters various ASTM and BS specifications are presently being used. 

Mechanisms Symptoms Carbon corrosion (air-air start) Gas impurities (e.g., CO, H2S, Sp2, ) Contaminants (e.g. some transition metal cations, anions) Catalyst instability (pt sintering, dissolution, re-crystallization) GDL loss of wet-proof (flooding) Seal failure (gross leaking) Membrane failure (pinholing, and tear)... 

This example of corrosion provides a general reason why paint and coatings of various kinds are only a partial answer to the corrosion-prevention problem. They often tend to develop cracks or pinholes, and these exposed spots provide access to oxygen for the electronation reaction, which results in unseen corrosion of the surrounding areas. 

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