Sacrificial Anode Protection Of Weathering Steel

Sacrificial Anode Protection Of Weathering Steel

Sacrificial Anode Protection of Weathering Steel Two novel methods of providing corrosion protection for weathering steel are researched herein: metallizing and application of galvanic (zinc) tape to the steel surface. Metallizing is a method of applying a layer of molten metal to a surface. The...
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Sacrificial Anode Protection of Weathering Steel

Two novel methods of providing corrosion protection for weathering steel are researched herein: metallizing and application of galvanic (zinc) tape to the steel surface.

Metallizing is a method of applying a layer of molten metal to a surface. The metal that is being sprayed is intended to act as a sacrificial anode. In the civil infrastructure, metallizing appears to be more commonly used on concrete for protecting the steel reinforcement than structural steel, perhaps because structural steel is more likely to be coated by painting to protect it from corrosion

Matthes et al. [2003] tested three metallizing alloys – pure zinc, 85% zinc + 15% aluminum, and 12% zinc + 0.2% indium + balance aluminum. Zinc is a good cathodic protector of the substrate, while aluminum is more passive and functions primarily as a mechanical barrier. Small amounts of indium are introduced to improve galvanic efficiencies. Specimens of these alloys (flame-sprayed onto lexan panels) were boldly exposed in a rural and a marine environment and their runoff was measured for approximately 2.5 years. It was found that the zinc runoff was directly proportional to precipitation rate, but also to the amount of zinc in the alloy; interestingly, the higher chloride levels of the marine site did not have an effect. Another test was reported by Kuroda et al. [2005]. In this case, twelve steel pipes were coated with zinc, aluminum, and an 87% zinc + 13% aluminum alloy, and were set vertically into seawater at a port in Japan. This test lasted for 18 years; corrosion was estimated by measuring the change of thickness of the metallizing. Over the duration of the test, most of the metallized specimens performed very well. The amount the coatings increased in thickness due to corrosion depended on their location vis-à-vis the water. One noted problem was that where the aluminum coating was damaged, red rust appeared, which is likely a result of the fact that aluminum has little anodic capacity. On the other hand, the zinc-aluminum alloy appeared to corrode at a much slower rate than the pure zinc. Kuroda et al. compared their results with those of similar tests, and found that they are about as expected.

Galvanic (zinc) tape functions by the same fundamental mechanism as metallizing, insofar as it provides a sacrificial anode to protect the steel substrate. It is essentially a layer of sacrificial material with an adhesive backing; however, this adhesive must provide a mechanical and electrical connection to the substrate for the tape to be effective. Nevertheless, galvanic tape is a new material for this application, and no published research is available quantifying its performance.

Sacrificial Anode Protection of Weathering Steel

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