Corrosion

1 The Corrosion Process
In broad terms corrosion has been defined as the destructive alteration of a substance (usually a metal) because of a reaction with its environment. The corrosion process is very complex and has many aspects, all of which lead to the same destructive result. In the design of anchors and fasteners the most common types of corrosion are direct chemical attack and electrochemical reaction.

2 Direct Chemical Attack
Corrosion by direct chemical attack occurs when the base material is soluble in the corroding medium. One solution for this type of corrosion is to select an anchor or fastener material which is not susceptible to attack by the corroding chemical. Many books present compatibility tables which provide a guide for selecting the proper materials.

When selecting a base metal which is compatible with the corroding medium is not possible or economical, another solution is to provide a coating which is impervious to the corroding medium. These might include metallic coatings such as zinc or cadmium or organic coatings such as epoxies or fluorocarbons.

3 Electro-Chemical Corrosion
All metals have an electrical potential relative to each other and have been ranked, accordingly, to form the "electromotive force series" or "galvanic series" of metals. When metals of different potential contact in the presence of an electrolyte, the more active metal (more negative potential) becomes the anode and corrodes, while the other metal becomes the cathode and is galvanically protected.

The severity and rate of attack will be influenced by the relative position of the contacting metals in the galvanic series, the relative area of the contacting metals, and the conductivity of the electrolyte.

For anchoring and fastening applications, galvanic corrosion can be reduced by:

1. Using similar metals or metals close together in the electromotive force series;
2. Separating dissimilar metals with non-conductive gaskets, plastic washers or paint;
3. Selecting materials so that the anchor or fastener is the cathode;
4. Providing drainage to prevent entrapment of the electrolyte.

4 Corrosion Protection
The most common type of corrosion protection for carbon steel fasteners and anchors is zinc. Zinc coatings can be uniformly applied by a variety of methods to achieve a wide range of coating thicknesses. As a rule, thicker coatings provide a higher level of protection.

Based on research by ASTM and other organizations the estimated mean corrosion rate for zinc coatings in various atmospheres is shown in the table. These values are for reference only, due to the large variances in the research findings.

Zinc coatings can be applied to anchors and fasteners by different methods. Applicable ASTM specifications are as follows:

5 Hilti Anchors
Most Hilti metal anchors are available in carbon steel with an electrodeposited zinc coating of at least 5 µm with chromate passivation.

Hilti standard and super HAS threaded rods in ⁷/8" diameter size are zinc coated to at least 45 µm by the hot-dip process for use on highway applications.

Where the long-term integrity of a fastening is important, and there is risk of corrosion of a carbon steel anchor, then stainless steel should be specified. Under certain conditions, even stainless steel anchors will corrode and a suitable protective coating should be specified by the designer.

It is not feasible to hot-dip galvanize expansion anchors. Zinc is relatively soft, and thick coatings tend to act as a lubricant to reduce the anchor tension capacity.