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Corrosion Test of Friction Bolts Part 1

At your request we tested friction stabilizers and plates made of galvanized steel just as they were received from Skema Industries (PTY) Ltd. along with identical bolts and plates that had been installed at the NORCAT Underground Centre and then pulled out as well as plain steel bolts and plates from a local manufacturer.  The Test was done in accordance with ASTM B 117-73, Standard Method of Salt Spray (Fog) Testing. The bolt and plate combinations were assembled and the sets held in contact using glass rods and rubber bands.  They were exposed to a salt fog with neutral acidity at 35.5°C for 11 days. All of the specimens corroded uniformly over all of their surfaces except for a few areas which were on the underside and protected from the fog.

There was no indication of any more severe, localized attack. All the sets developed voluminous corrosion products: typical reddish rust in the case of the plain steel and a white zinc compound in the case of the galvanized products. By the 10th day the galvanizing in a few localized areas was starting to fail and was showing red stains in the white corrosion product, particularly on the plates. The amount of this staining was minimal and did not seem to correlate with damage to the galvanized coating caused by installing and pulling the bolts. The bolts and plates were weighed before and after the test. The tests were done in triplicate and the average weight loss as a percentage of initial weight is reported in Table I.

The results are not exact because, although the loose corrosion products were removed with a wire brush after the test, some variable amount of adherent material remained. Nevertheless, the individual test results tabulated in the Details section show a reasonable consistency and seem reliable.

Table I: Average Percent Loss in Weight
Material Loss in Weight %
  Bolt Plate Bolt Plate
1 Galvanized, New Galvanized, New 1.3% 0.6%
2 Galvanized, Installed & Pulled Galvanized, Installed & Pulled 1.2% 0.7%
3 Plain Steel Plain Steel 2.4% 0.7%

The galvanized friction stabilizers and plates showed one-half the weight loss of the steel components.  In fact, judging by the sparse rusty stains, most of the weight loss of the galvanized materials would have been due to chemical attack on the protective zinc coating and not to corrosion of the underlying steel. Therefore one would expect little loss in strength in the galvanized samples as compared to the unprotected steel.

So, even though weight loss was one-half of the plain steel, one would expect that the loss in strength would be much less.  the surface of the steel was obviously pitted as well and these pits would probably contribute to loss in strength.

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