Pitting rate

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. 

Rossum, J. R., Prediction of Pitting Rates in Ferrous Metals from Soil Parameters , J. Amer. 

In extended exposure periods of up to 16 years in tropical sea water, Southwell and Alexander obtained an average corrosion rate for steel of 0-18 mm/y in the first year, falling off to a constant rate after 4 years at 0-025 mm/y. They also quote pitting rates as 1 mm/y in the first year falling... 

It should be noted that it is extremely difficult to predict service lives of buried pipelines from the results of controlled trials with small specimens, whether in the laboratory or in the field. For example a study on the comparative corrosion resistances of ductile and grey iron pipes carried out jointly by European pipemakers in 1964-1973 indicated a mean pitting rate of 0 -35 mm/y for uncoated ductile iron pipe exposed in a typical heavy Essex clay of 500-900 ohm cm resistivity for 9 years. This is clearly at odds with the rate of 1 mm/y normally found on a corroded service pipe from such a soil. The discrepancy appears to be due to the use of specimens that were only a third of a pipe length each and were buried separately. It may reflect the contribution of the total surface area of the pipe as a cathode to the corrosion current at the anodic area at the pitting site. 

Figure 4 Metastable pitting rate (k) for 316 stainless steel in 1 M NaCl solution as a function of potential for untreated samples and samples passivated with 20% or 50% nitric acid for 1 hour. (From J. S. Noh, N. J. Laycock, W. Gao, D. B. Wells. Corrosion Sci. 42, 2069 (2000).)...

Discontinuities in the corrosion product film result in differential aeration cells, leading to pitting corrosion. A pitting rate of 0.25-0.38 mm/yr on bare steel and 0.5 mm/yr on steel with mill scale has been observed. Assuming the average corrosion rate of 0.125 mm/yr, the pitting factor works out to be 2 to 3 for... 

The typical corrosion rates in soils range from 0.003 to 0.063 mm/yr with an average value of 0.020 mm/yr. The pitting rates range from 0.033 to >0.45 mm/yr with an average... 

Conversely, for several aluminium alloys, pit initiation can be accepted under many circumstances. This is so because numerous pits are usually formed, and the oxide is insulating and has therefore low cathodic ability, so that the corrosion rate is under cathodic control. However, if the cathodic reaction can occur on a different metal because of a galvanic connection or for instance deposition of Cu on the aluminium surface, the pitting rate may be very high. Since we in other respects can accept pit initiation, the time dependence of pit growth and pit depths is important, and we shall consider this more quantitatively. 

Pitting rate decreases with time due to increased mass transfer resistance over time. 

Because the dimensions of all field test specimens were on the order of inches up to about 1ft (3-30cm), the reported pitting rates represent minimum rather than maximum values. Actual depth of pits in a given time is found to increase with size of test specimen, probably because cathodic area per pit increases (i.e.. 

The rate at which pits grow in the soil under a given set of conditions tends to decrease with time and follows a power-law equation P = kf, where P is the depth of the deepest pit in time t, and k and n are constants. It has been reported [10] that values of n for steels range from about 0.1 for a well-aerated soil, to 0.9 for a poorly aerated soil. The smaller the value of n, the greater the tendency for the pitting rate to fall off with time. As n approaches unity, the pitting rate approaches a constant value, or penetration is proportional to time. 

The rate of penetration of pits in aluminium has been shown to decrease rapidly with time. Aziz and Godard found that in field test coupons the pitting rate curve follows a cube root law [2.14] ... 

The pitting rate index (PRI) is defined as the number of weeks needed to obtain a maximum pit depth of 1 mm. The equation is shown below ... 

The pitting rate equation discussed in this chapter, although not accurate, gives a reasonable evaluation of the aggressiveness of the water quahty in fuel storage basins. It has been used to monitor the basin water cleanup activities at SRS, and improvements in water chemistry have been verified by corrosion coupon tests. Additional work is needed to improve this correlation. 

MetaUographic evaluation of the corrosion coupons should include determination of pitting density and maximum pit depth. With knowledge of the pitting depth and exposure time, the pitting rate may be calculated. Video and... 

Mears and Brown [10.9] studied the influence of temperature on pitting of aluminium alloys in chloride solutions. They found that as the temperature increased, the density of pits and the probability of pitting increased, while the pitting rate or average pit depth decreased. Consequently it has been observed that it is extremely important to maintain basin water temperatures as low as possible to avoid corrosion. 

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