Rust perforation

The removal of rust films or other corrosion products to facilitate inspection for perforation prior to termination of the exposure will change the natural performance of the material, and is therefore not tolerable. 

The permeable barrier was composed of a steel frame that was constructed to hold the SMZ and to allow for media replacement. The frame was constructed of 5-cm steel angle iron and 2.5-cm and 7.6-cm square steel tube. The frame had solid floor and end walls (1.3-cm-thick steel plates) to divide it into three distinct cells. Each cell had perforated metal walls (0.16-cm thick perforated steel sheets with 0.64-cm holes covering 50% of the surface area) transverse to the direction of flow. The perforated metal was installed on both the inside and the outside of the steel tube skeleton, resulting in a 7.6-cm-wide annulus between the inner and outer walls of the frame. The entire frame assembly was professionally painted with high-quality, rust-resistant paint. The barrier frame was placed in the pilot-test tank in three sections on top of a 1-m depth of aquifer sand that had been previously added to the tank in lifts. The physical and chemical properties of the sand are described later in this chapter. The three frame sections were bolted together after applying a silicone caulk (Sika-Flex ) for sealing. The end of the barrier in contact with the side of the tank was sealed to the... 

This cell consists of an iron tank acting as cathode, and from the cover is suspended a perforated anode box made of low-carbon steel to prevent the formation of spongy rust. The anode and cathode are separated by an asbestos sack suspended from the cover. 

Aqueous external corrosion results in rusting, pitting and ultimately perforation. Under certain conditions detinning may also occur. Perforation is of very serious concern because of loss of product, along with the more serious problem of contamination due to ingress of foreign material such as microbes in food cans. 

Some field data on corrosion defects such as (i) perforation (ii) surface rust (iii) blistering, and (iv) other defects are given in Table 4.37. 

Year Perforations (%) Surface Rust (%) Blistering (%) Any Defect (%)... 

The data given in the table show a decrease in perforation from 20% to 3%, surface rust from 78% to 50%, blistering from 60% to 38%, and other defects from 85% to 60% in the years from 1985 to 1993. 

The use of electrocoat or e-coat paints and primers is another technology that found extensive use in the 1980s. Electrocoated paint is applied on the part to be coated in a paint bath, and electrical current is applied. This method enables painting of the most intricate parts. The electrocoated automobiles were more corrosion resistant. The surface rust was approximately three times higher, and the number of perforations were also higher in vehicles that were treated in the standard conventional manner. Some of these differences may be because of zinc coating on the steel and phosphatizing. However, electroplated paint systems had a noticeable effect on corrosion resistance. 

Far more critical is plug-type dezinciflcation, which mainly arises in heat exchanger tubes upon deposition of sand grains, oil residues, rust, mussels, and algae with formation of aeration elements. Even thick tubes can be perforated in this way. Dezinciflcation can be impeded by adding small amount of arsenic or phosphorus. 

Also, corrosion on galvanized steel poles and fences in the coastal area of Simonstown and Muizenberg (Indian Ocean, Cape Province) is pronounced. At a distance of 10-20 m from the ocean spray, severe corrosion occurs after 3-5 years. Only duplex systems offer sufficiently long protection against rust. Unprotected steel is perforated within only a few years, but this knowledge led to successful trials of galvanized rails for the railway track. 

Galvanized perforated road strips, used by the army and other authorities to prevent sinking of trucks in weak soils and mud, have been in use for more than 26 years. They not only are exposed to the weather but also must withstand rough handling and severe impact. No rust has weakened the steel during this period. 

These products are also extensively used on road vehicles to reduce the severity of corrosion, particularly in cold-weather areas as roadways are coated with salt mixtures to break down snow and ice. There are plenty of locations in road vehicles where water can accumulate and rust form to cause premature failm e of the equipment or perforation of the sheet metal. The imderside of cars is especially vulnerable to rust and corrosion due to the exposme to high levels of moisture. 

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