Corrosion topography

Measurements on the relatively lightly corroded HW series bars showed that the breadth of pits (that is, the circumferential dimension) averaged slightly less than twice the depth, confirming that the assumption that pits are circular in cross section is reasonable。 Section loss at a pit thus increases approximately in proportion to the square of its depth (or width)。

The maximum loss of section at a pit averaged approximately twice the average loss of section (Fig。 5), although there was a wide scatter。 The ratio of maximum pit depth to mean penetration was generally in the range of 10 to 50, with a tendency for the ratio of maximum pit depth to mean penetration to reduce as corrosion progresses (Fig。 6)。 In pitting corrosion, a relatively large cathode drives corrosion at a concentrated anode。 As the size of the pit/anode increases, the ratio of anodic area to cathodic area will reduce, hence reducing the rate of depletion at the pit。 It is therefore to be expected that the rate of penetration at the pit would slow in relation to the mean rate of penetration as corrosion progresses。 The ratio lies above that recommended in the CONTECVET Manual,2 probably due to the higher degree of corrosion covered in supporting investigations。

Two of the corroded test bars from Series UB are shown in Fig。 7(a)。 One bar suffered only light surface corrosion, and section loss did not exceed 2% in any length increment。 The second bar was severely corroded with approximately 72% of the cross section lost to corrosion at the most severely damaged section。 The variation in diameter and the in cross-sectional area along the second bar are plotted in Fig。 7(b); it illustrates the highly nonuniform nature of localized corrosion attack。 No meaningful conclusions can be drawn about the relationship between local and mean attack penetrations from these relatively short bar specimens, however。

Residual mechanical characteristics of corroded bars: HW series

Tests for mechanical properties were conducted following similar procedures to those used for bars containing machined defects, with the exception that a gauge length of 12。5 times bar diameter was used for HW series bars。 Figure 8(a) shows the stress-strain diagram for Bar F3, a representative reference (uncorroded) bar from the HW series。 The plot is characteristic of a mild steel, with a clearly defined yield point and yield plateau, followed by a strain-hardening portion。 The descending tail on the plot marks the unloading of the bar shortly after peak stress when the test was halted to avoid damage to the strain sensor if the test bar was to fracture。 Reference uncorroded test bars had a yield strength of 311 N/mm2, an ultimate-to-yield strength ratio of 1。46, and an elongation at fracture of 29%, exceeding the requirements of the relevant standard。6 Mean strain at maximum force was nearly 20%。

Even after corrosion, all test bars met the requirements of the standard, although two of the bars came close to the limit with an elongation at fracture of 22。5%, compared with the specified minimum of 22%。 Figure 8(b) plots yield and ultimate tensile strengths of bars, calculated on the residual cross section, against section loss at the largest pit。 Figure 8(b)

Fig。 5—Relationship between mean section loss and loss of cross section at pit measured in HW series。

shows there to be no loss in yield strength when strength is calculated in this way, while ultimate tensile strength shows a slight increase with increasing section loss。 By the use of linear regression analysis, it was found that the ultimate tensile strength increased by approximately 5。7% for a 7。0% loss in section at a pit。 In effect, this means that the ultimate force developed in the bar is reduced by only approximately 1% for a 7% loss in section。 If results are plotted using mean instead of maximum section loss, the change in peak strength with corrosion is not found to be significant。