How long do galvanized coatings last?

Introduction
Galvanized coatings have an unusual characteristic compared to other protective coatings in that they fail by weathering and oxidation from the surface. Paint coatings, once breached, deteriorate through under-film corrosion and can suffer rapid failure as a result.

Because of the electrochemical protection provided to steel by zinc (galvanized) coatings, no corrosion of the steel will occur while there is any zinc present, regardless of the thickness or condition of the galvanized coating.

Galvanized coatings, in atmospheric exposure conditions, corrode at an approximately linear rate. Once this rate has been established for a particular environment, the expected life of the coating can be defined by relating the rate of corrosion to the thickness of the coating.
Factors affecting galvanized coating life

The durability of galvanized coatings depends on a number of environmental factors. These include:

• Time of wetness
• Ambient temperature
• pH of moisture
• Chloride levels in atmosphere
• Sulfate levels in atmosphere
• Contact with other chemicals
• Contact with dissimilar metals
• Orientation of exposure (vertical, horizontal)
• Nature of exposure(sheltered, open)
• Ventilation conditions

Corrosion engineers take these factors into account when assessing the life-cycle performance of galvanized coatings. Organisations such as the CSIRO have developed environmental assessment techniques based on atmospheric computer models that facilitate the accurate assessment of metallic corrosion rates.

A number of international (ISO) standards have also been developed that use combinations of the parameters listed above to tabulate corrosion rate data for zinc (galvanizing) and other metals.

Classification of environments
Most standards and documents associated with coating performance use exposure classifications to define corrosivity of the atmosphere. For metallic coatings such as galvanizing, factors such as UV exposure do not impact on coating life, where with paint coatings, UV levels are an important factor in their durability.

For galvanized coatings, common Australia exposure classifications are arid/rural, mild/urban, industrial, marine and tropical. Much exposure testing has been done to obtain corrosion rate data in these environments, and this work is ongoing.

Testing done by Industrial Galvanizers in a number of long-term case studies has indicated that hot dip galvanized coatings in service may have lower corrosion rates than those of zinc coupon samples exposed in test facilities.

Reasons for this apparent lower rate of in-service corrosion have not been quantified, but are thought to be related to the quite different characteristics of a hot dip galvanized coating compared to pure zinc, typical of the samples used in exposure testing.

The hot dip galvanized coating contains alloys of iron, aluminium and sometimes nickel, each of which may modify the way the coating reacts with the environment.

The following table shows typical corrosion rates of hot dip galvanized coatings in the various environmental classifications.

Table 1
                                                Corrosion rate – microns per year
                                                Arid/rural                                               < 1
                                                Mild/urban*                                           1 - 3
                                                Industrial                                               3 - 5
                                                Marine**                                                5 - 15
                                                Tropical                                                1- 3
                                    * Metropolitan and urban areas within 25 km of the Australian
                                         coastline outside the ocean surf spray zone.
                                    ** Within the ocean surf spray zone from 0 – 1000 metres from
                                        ocean surf, depending on topography.

Coating thickness versus coating life
All continuously galvanized and after-fabrication galvanized steel products have coating thickness specified in various Australian, New Zealand and international standards. By relating this coating thickness to the corrosion rates in the table, an accurate estimate of galvanized coating life can be obtained.

Hot dip galvanized coatings that comply with AS/NZS 4680 – 2006 are those that will give the longest life, as they are typically 3 –5X the thickness of zinc coatings applied to continuously galvanized products.

On structural steel sections, 50 years life before first rust in other than marine or heavy industrial environments is a reasonable expectation. Case history studies of existing installations in tropical and industrial environments indicate that 100-year life is achievable with galvanized coatings applied after fabrication.

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