Pitting rate index

Although the prediction of the corrosivity of natural and other waters from the values of specific compositional parameters has proven to be extremely difficult, Pathak and Godard developed an empirical relationship for predicting the corrosivity of natural fresh waters to aluminium in 1967 [2.24]. Using 67 natural waters where analyses were available and 3003 alloy, they conducted tests to determine the maximum pit depth as a function of exposure time. From the data, the time required to develop a 1 mm pit was extrapolated. 

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. 

At SRS and other basins storing US defence related spent nuclear fuel, extensive efforts are under way to improve fuel storage conditions. These efforts should result in extended storage capability. This interim wet storage, however, cannot be extended indefinitely, and provisions must be made for other disposition. The ultimate solution for this problem is processing the fuel to a more stable form using standard techniques readily available and utilized 

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The API, with help from gear manufacturers, has a 1977 standard (API 613) for rating gears. (See Chapter 4.) A common procedure for comparing and sizing gear is based on the tooth pitting index, the K factor... 

Several methods have been developed for quantifying dental fluorosis. The most commonly used method is Dean s index [49], which classifies fluorosis on a scale of 0 to 4 as follows class 0, no fluorosis class 1, very mild fluorosis (opaque white areas irregularly covering <25% of the tooth surface) class 2, mild fluorosis (white areas covering 25-50% of the tooth surface) class 3, moderate fluorosis (all surfaces affected, with some brown spots and marked wear on surfaces subject to attrition) and class 4, severe fluorosis (widespread brown stains and pitting). The average score of the two most severely affected teeth is used to derive the classification. Other commonly used methods to rate dental fluorosis include the Thylstrup-Fejerskov Index (TFI) [50] and the tooth surface... 

The benefits of LPRM techniques are that the corrosion rates are instantaneous and also provide a pitting index measurement. The capital costs of LPRM equipment can be quite high, especially if several probes and monitors are required, but the probes can be reused and thereafter easily fitted with replacement electrodes. 

Furthermore it is the rate of kink motion that controls the process. In Fig. 5 the steps around a pit are square at a certain concentration of Fe+, so they are composed almost entirely of stqps lying in < 001 > directions. When the concentration of Fe is increased the corners of the pits become rounded and the overall rate of motion of the steps is reduced. Since the rounded steps do not lie parallel to a low index direction in the surface, they necessarily contain high concentrations of kinks. The rate of kink nucleation is therefore not limiting the motion of the steps rather it is the rate of kink motion that does so. 

LPR Instantaneous Corrosion rate, pitting index Fast Conductive Uniform Simple... 

Corrosion rates obtained with LPR instruments provide rate data directly and within a few minutes. This instrument is well suited to applications where upsets or other accelerated corrosive conditions can be detected quickly so that remedial action can be taken. The corrosion rates obtained with the meter assume uniform corrosion with a tendency to predict pitting attack. Deposits on the electrodes can bias the pitting index value. If the index value is high, the probe should be removed and the electrode visually examined for debris. Bridging the electrodes with conductive deposits will affect both the general corrosion value and the pitting index. 

The advantage of the ER meters is their ability to measure corrosion in liquids, vapor phases, and in inaccessible locations. The liquid does not have to be conductive or have a minimum conductivity as with the LPR systems. However, corrosion rates are not instantaneously determined. The time frame required to determine corrosion rates is a function of probe element and metal loss. Also, the ER probes are suitable only for uniform corrosion. There is no pit index built into the instrument. Any pits developed on the element will cause rapid penetration and indicate excessively high corrosion rates. 

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