Corrosion rates/resistance atmospheric

Figure 4.17 Gecor 8 (developed from Gecor 6 Figure 4.13) has separate heads for corrosion rate of atmospherically and buried or submerged corrosion rate measurement, resistivity, and assessment of cathodically protected structures. Courtesy James Instruments and Geocisa.

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

Tables 2.3 through 2.5 give general corrosion-resistance ratings of different materials. Table 2.3 lists various metals and Table 2.4 gives ratings for various nonmetals. Table 2.5 gives typical corrosion rates of steel and zinc panels exposed to the atmosphere in various locations about the U.S. Figure 2.1 also illustrates relative corrosion rates of steel and zinc in major areas of the world.

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

Soluble corrosion products may increase corrosion rates in two ways. Firstly, they may increase the conductivity of the electrolyte solution and thereby decrease internal resistance of the corrosion cells. Secondly, they may act hygroscopically to form solutions at humidities at and above that in equilibrium with the saturated solution (Table 2.7). The fogging of nickel in SO2-containing atmospheres, due to the formation of hygroscopic nickel sulphate, exemplifies this type of behaviour. However, whether the corrosion products are soluble or insoluble, protective or non-protective, the... 

The addition of small amounts of nickel to iron improves its resistance to corrosion in industrial atmospheres due to the formation of a protective layer of corrosion products. Larger additions of nickel, c.g. 36% or 42%, are not quite so beneficial with respect to overall corrosion since the rust formed is powdery, loose and non-protective, leading to a linear rate of attack as measured by weight loss. Figure 3.37 of Pettibone illustrates the results obtained. 

Aluminium is a very reactive metal with a high affinity for oxygen. The metal is nevertheless highly resistant to most atmospheres and to a great variety of chemical agents. This resistance is due to the inert and protective character of the aluminium oxide film which forms on the metal surface (Section 1.5). In most environments, therefore, the rate of corrosion of aluminium decreases rapidly with time. In only a few cases, e.g. in caustic soda, does the corrosion rate approximate to the linear. A corrosion rate increasing with time is rarely encountered with aluminium, except in aqueous solutions at high temperatures and pressures. 

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

Both zinc and zinc alloys have excellent resistance to corrosion in the atmosphere and in most natural waters. The property which gives zinc this valuable corrosion resistance is its ability to form a protective layer consisting of zinc oxide and hydroxide, or of various basic salts, depending on the nature of the environment. When the protective layers have formed and completely cover the surface of the metal, the corrosion proceeds at a greatly reduced rate. 

In atmospheric exposure to industrial environments its corrosion rate is only about one-third that of zinc and the corrosion reaction is stifled by the tenacious oxide which is produced nevertheless it can frequently function as an anodic coating both for steel and for the less corrosion-resistant aluminium alloys. 

Guide for estimating the atmospheric corrosion resistance of low-alloy steels Practice for calculation of corrosion rates and related information from electrochemical measurements... 

Recent reports [30-31] on the use of atmospheric corrosion sensors based on changes in electrical resistance showed that when there were no contaminants [29], in tests of 100-110 h., corrosion rate was zero or insignificant. These sensors can determine changes in metal thickness lower than one nanometer. However, in the presence of 0.08 ppm of S02 or 20 pg/cm2 of NaCl in the system, changes in thickness where always detected over 75% of relative humidity. Corrosion rate was determined at temperatures of 20, 30 and 40°C and the Arrhenius equation was used to calculate the activation energy of the reactions. This method is very similar to the natural conditions. 

The corrosion rates of various metals in industrial, marine and rural atmospheres are given in Table 4.76. Zinc has higher corrosion resistance than cadmium and iron in all the three atmospheres. Zinc has higher corrosion resistance than copper in industrial... 

A global mean for the rate of net chemical denudation of the continental surface is about 14 mm 1000 yr-1 or 14 pm yr k In comparison to the corrosion rates of metals exposed to a range of environmental conditions, the global continental surface is less resistant to corrosion than zinc and copper, but it is considerably more resistant than iron exposed to coastal oceanic and industrial-area atmospheric conditions. 

As can be seen, the inhibited film materials are able to display a prolonged electrochemical and microbiological activity thanks to the gradual liberation of Cl. In addition, they can represent a diffusive and mechanical barrier for harsh media. The corrosion rate of hardware packed in these films is lowered by tens to hundreds of times. Their fusing capability and resistance to atmospheric aging make the film packages tight and reliable. A... 

T. Valente, Laboratory investigation of concrete resistivity and corrosion rate of reinforcement in atmospheric conditions , Proc. Rilem/... 

The relative humidity of an environment is a significant factor in corrosion of metals under atmospheric conditions. A thin layer film is formed on metals at a critical value of relative humidity depending on the nature of the metal being corroded. Dehri and Erbil [49] studied the effect of relative humidity on the atmospheric corrosion of defective polyester-coated galvanized mild steel using impedance spectroscopy. The measurements were taken at relative humidity values between 70% and 100%. The relative humidity of the atmosphere increases the corrosion rate of the underlying metal. Figure 10.11 shows the equivalent resistance circuit used to evaluate the corrosion parameters [49]. 

Copper and copper alloys are highly resistant to atmospheric corrosion because of surface films mainly composed of basic copper salts. The corrosion rate is below 2-3 pm/year [8.9]. Tin as well as nickel and nickel alloys also corrode at similar rates. Lead possesses excellent corrosion resistance in atmospheres due to surface-protecting films (insoluble sulphate, sulphide, carbonate and oxide). 

Corrosion-product films formed in the atmosphere tend to be protective that is, the corrosion rate decreases with time (Fig. 9.1) [3]. This is true to a lesser extent of pure iron, for which the rate is relatively high, compared to the copper-bearing or low-alloy steels, which are more resistant. Rust films on the latter steels tend... 

Zinc coatings are relatively resistant to rural atmospheres and also to marine atmospheres, except when seawater spray comes into direct contact with the surface. Table 14.1 lists the ranges of typical atmospheric corrosion rates in each of the three types of atmospheres, rural, marine, and urban/ industrial [18]. 

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