Automobiles corrosion

Figure 1. Automobile corrosion milestones as a function of road salt usage.

Figure 2. Geographic distribution of acid rain and automobile corrosion in the United States.

Figure 2 shows that the northeastern United States suffers especially severe automotive corrosion. Annual automobile corrosion surveys conducted by Texas Instruments Inc. have indicated that Montreal, Canada is also highly corrosive. Therefore Boston, Detroit, and Montreal were chosen as ideal sampling sites that should have the greatest amounts of corrosive species for comparison, Dallas, Texas was chosen as typical of what should be a mildly corrosive area. 

Elliott, P. (1991). State of the art of automobile corrosivity up to 1980. Corrosion 1991. National Association of Corrosion Engineers, Cincinnati, Paper 378. 

Sealers play an important role in automobile corrosion protection by preventing water and contaminants from entering crevices and joints. They are usually rubber, vinyl, asphaltic, or hot melt or wax compounds [24,25]. The choice depends on the specific application such as at door hem flanges, brackets, fasteners, and body joints. Structural adhesives are also used to bond assemblies such as doors, deckUds, and hoods, thereby sealing crevices and eliminating welds. 

Fleet tests are extremely valuable in providing direct information on corrosion behavior in specific locations. Groups of cars that are typically used for these tests are taxicabs, rental cars, municipally owned cars, and sales fleet cars. Because the operating parameters can be recorded through fleet record keeping, this type of test is probably the most reliable of all for automobile corrosion performance. 

The potential use of calcium magnesium acetate (CMA) has been extensively researched in North America, and field trials have been conducted in several states and provinces. The CMA specification in terms of composition, particle size and shape, color, and density has evolved over time. CMA application rates have generally been higher than those for salt. The majority of trials conducted have indicated effectiveness similar to that of salt at temperatures down to -5°C, but slower performance than salt at lower temperatures. Unfortunately, costs are reportedly more than 10 times higher than those of road salt on a mass basis. If a higher application rate of 1.5 times that of salt is assumed, a cost factor increase of 45 has been reported. Cost issues surrounding the use of CMA are complex and include factors such as potential environmental benefits, reduced automobile corrosion, mass production technology, and alternative raw materials. 

The use of formate compounds as highway deicers was explored as early as 1965. Lower reaction rates of sodium formate with snow and ice have been reported in Canadian field trials. In the Canadian studies, commercial grades of sodium formate were found to be contaminated with chlorides. Concerns related to automobile corrosion and increased costs have been expressed, and little information is available concerning possible adverse effects on the environment. 

G. Haynes and R. Baboian, Laboratory and Field Corrosion Test Results on Aluminum-Transition-Steel Systems on Automobiles, Corrosion and Corrosion Control of Aluminum and Steel in Lightweight Automotive Applications, E.N. Soepenberg, Ed., National Association of Corrosion Engineers, 1985, p 383-1 to 383-13... 

Automobile motor part (VGInsight view) Corrosion of the inner part (circle). Massive steel is drawn semitransparent. 

The durabihty and versatility of steel are shown by its wide range of mechanical and physical properties. By the proper choice of carbon content and alloying elements, and by suitable heat treatment, steel can be made so soft and ductile that it can be cold-drawn into complex shapes such as automobile bodies. Conversely, steel can be made extremely hard for wear resistance, or tough enough to withstand enormous loads and shock without deforming or breaking. In addition, some steels are made to resist heat and corrosion by the atmosphere and by a wide variety of chemicals. 

Decorative chromium plating, 0.2—0.5 ]lni deposit thickness, is widely used for automobile body parts, appHances, plumbing fixtures, and many other products. It is customarily appHed over a nonferrous base in the plating of steel plates. To obtain the necessary corrosion resistance, the nature of the undercoat and the porosity and stresses of the chromium are all carefliUy controlled. Thus microcracked, microporous, crack-free, or conventional chromium may be plated over duplex and triplex nickel undercoats. 

Chromates are used to inhibit metal corrosion in recirculating water systems. When methanol was extensively used as an antifree2e, chromates could be successfully used as a corrosion inhibitor for cooling systems in locomotive diesels and automobiles (185). 

The CASS Test. In the copper-accelerated acetic acid salt spray (CASS) test (42), the positioning of the test surface is restricted to 15 2°, and the salt fog corrosivity is increased by increasing temperature and acidity, pH about 3.2, along with the addition of cupric chloride dihydrate. The CASS test is used extensively by the U.S. automobile industry for decorative nickel—chromium deposits, but is not common for other deposits or industries. Exposure cycle requirements are usually 22 hours, rarely more than 44 hours. Another corrosion test, now decreasing in use, for decorative nickel—chromium finishes is the Corrodkote test (43). This test utilizes a specific corrosive paste combined with a warm humidity cabinet test. Test cycles are usually 20 hours. 

The Electrolytic Corrosion Test. Also developed for use on nickel—chromium and copper—nickel—chromium decorative automobile parts is the electrolytic corrosion (EC) test (44). Plated specimens or parts are made anodic in a corrosive electrolyte under controlled conditions for 2 min, and then tested for penetration to the substrate. 

As corrosion proceeds, reaction by-products may form on the metal surfaces, creating a resistance to electrical exchanges at these surfaces. Consequently, the reaction rate diminishes correspondingly. If the corrosion reaction is stopped, a time-dependent recovery occurs if the reaction is restarted, the initial corrosion rate is reestablished. This effect is often observed in conventional dry cells and automobile batteries. 

Slides Corroded automobiles, fences, roofs stress-corrosion cracks, corrosion-fatigue cracks, pitting corrosion. 

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