Why does galvanized rebar always get a bad rap when it comes to salt-spray testing?

The goal of salt-spray testing is to predict how long a protective coating will last when subjected to corrosive substances, namely salts. However, the figures determined by the s test are not always indicative of real-world performance. This is especially true for salt-spray testing of galvanized steel.

The performance figures determined by salt-spray testing dont always do galvanized steel justice in that they can provide performance data which shows galvanized coatings have a much shorter service life than they do in real-world applications.

In salt-spray tests, the protective coatings on steel are exposed to a constantly wet environment containing salts, which prevents the zinc patina from forming since wet/dry cycling is necessary for the development of the patina. If the zinc patina never forms, the ability of the galvanized coating to protect the underlying steel is greatly diminished. Hence, the skewed performance figures when galvanized steel is subjected to salt-spray testing.

Having said that, according to Galvanized Steel Reinforcements in Concrete, galvanized rebar performed significantly better in accelerated corrosion tests when compared to black rebar. Here are a few points mentioned in the book.

• In salt-fog tests, corrosion of the coating initiated in 0.5 w/c OPC concrete at a chloride level 3.5 times higher than for black steel in similar PZ concrete at a level 2.9 times higher.
• In the most severe wetting and drying tests, corrosion of the galvanized coating initiated in 0.5 w/c OPC concrete at a chloride level 1.8 times higher than for black steel
• In the range of accelerated corrosion tests, the time to cracking of the cover concrete for galvanized bars was of the order 4-12 times longer than for black steel bars.
• The findings above were just a few discussed in Galvanized Steel Reinforcement in Concrete. Addition information from over 40 years of additional research is also discussed.

Even with the performance figures above showing the substantial protection offered by galvanized rebar when compared to black rebar, it is also useful to provide your customers and specifiers with real-world performance data. The AGA website offers several case studies how galvanized rebar has performed in real-world applications.