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What are the differences between Continuous and Batch HDG Solar Infrastructure?

In order to protect solar investments and ensure they are a reliable power source, proper corrosion protection methods should be in place for the racking structures. To accomplish this, hot-dip galvanizing is often specified for corrosion protection because it is a durable and sustainable coating that can withstand the constant exposure to the unforgiving sun’s heat and UV rays for decades without maintenance. Specifically, pre-galvanized sheet products and/or batch hot-dip galvanized (HDG) steel are often utilized to construct the solar panel frames, mounts, and posts.

Continuous hot-dip galvanizing (pre-galvanized sheet products) and after-fabrication batch hot-dip galvanizing are often confused because they are both “hot-dip” products, but differ in terms of coating thickness, longevity, and available product that can be coated using each process. Continuous hot-dip galvanizing can be specified for products such as sheet, pipe, and tube galvanized on continuous production lines and then fabricated into final products, while batch hot-dip galvanizing is performed after-fabrication by immersing steel products (poles, beams, frames, trailers and other assemblies) into a bath of molten zinc.

There are three common options for specifying hot-dip galvanized steel for solar infrastructure:  continuous hot-dip galvanized components, batch hot-dip galvanized components, or a combination of the two (i.e. batch hot-dip galvanized posts and mounts with continuous galvanized sheet frames). To determine whether continuous galvanized products or batch hot-dip galvanizing is suitable for the solar racking structures, it is important to consider zinc coating thickness, environmental exposure (atmospheric and soil), and specified design life.

Tables 1 & 2 below provide the available coating grades of continuous and batch hot-dip galvanizing and their corresponding zinc coating thicknesses in mils, or thousandths of an inch. The coatings commonly encountered on continuous hot-dip galvanized products (i.e. G30, G60, and G90 coating grades found “off the shelf”) are significantly thinner than batch hot-dip galvanized coatings. Since the coating thickness is directly related to service life, greater longevity is achieved when specifying batch hot-dip galvanizing. Generally, a hot-dip galvanized coating thickness of at least 3.0 mils is suitable for long-term corrosion protection in exterior environments.

Table 1: Continuous Galvanizing (pre-galvanized sheet products 7 gauge [3/16"] to 32 gauge [0.102"] thick), Minimum Average Coating Thickness [mils]

Coating Grade ASTM A653

Avg. Minimum Coating Thickness

G30 0.26 mils
G60 0.51 mils
G90 0.77 mils
G115 0.98 mils
G140 1.20 mils
G165 1.40 mils
G185 1.57 mils
G210 1.79 mils
G235 2.00 mils
G300 2.55 mils
G360 3.06 mils
Table 2: Batch Hot-Dip Galvanizing (After-Fabrication), Minimum Average Coating Thickness [mils]
  Steel Thickness
Material Category
ASTM A123

<1/16
[5<1.6]

≥1/16 to <1/8
[≥1.6 to <3.2]

≥1/8 to 3/16
[≥3.2 to < 4.8]

≥3/16 to <1/4
[≥4.8 to <6.4]

≥1/4 to <5/8
[≥6.4 to <16.0]

≥5/8
[≥16.0]

Structural Shapes

1.8 mils

2.6 mils

3.0 mils

3.0 mils

3.9 mils

3.9 mils

Plate

1.8 mils

2.6 mils

3.0 mils

3.0 mils

3.0 mils

3.9 mils

Pipe & Tubing

1.8 mils

1.8 mils

3.0 mils

3.0 mils

3.0 mils

3.0 mils

For the portions of the solar panel design which experience atmospheric corrosion only, it is possible to determine the anticipated coating life by referring to the Time to First Maintenance chart (Figure 1) regardless of whether continuous galvanizing or batch hot-dip galvanizing is specified. Simply refer to the minimum average coating thickness that will be achieved, based on the type of galvanizing. You will notice that due to the thinner coatings achieved through continuous galvanizing, it may be required to extrapolate from the data in order to obtain a time to first maintenance for the common coating grades G30, G60, and G90.  Alternatively, the Zinc Coating Life Predictor (ZCLP) can estimate time to first maintenance for both hot-dip (zinc) coatings by imputing site-specific parameters for the environment such as yearly average weather and pollution data.

Time To First  Maintenance 2016  English
Figure 1: Time to First Maintenance for HDG Coatings

For solar mounting projects where galvanized posts will be embedded in soil, there are 3 corrosion assessment categories to consider:  atmospheric corrosion above-ground, corrosion at the transition zone, and corrosion in the soil. Typically, the portion of a solar panel post which is embedded in the soil will experience the most corrosive environment. To estimate the service life of galvanized steel articles in soil applications, refer to the AGA Soil Charts (Fig. 2) below to provide an estimate of how long the hot-dip galvanized steel will last maintenance-free based on three primary soil characteristics: chloride level, moisture content, and pH. In extremely aggressive soils, a duplex system, such as coal-tar epoxy applied over HDG, can be considered to provide increased longevity of the embedded solar posts (1.5 to 2.3 times the combined service life).

Soil  Chart 2016  English
Figure 2: Time to First Maintenance of Hot-dip Galvanized Coatings in Soil

For many solar panel projects, continuous galvanized sheet products that are common and readily available in the marketplace (i.e. G60, G90 coating grades) are not suitable for long term corrosion protection in moderate and harsh atmospheric environments, nor for embedment in corrosive soils. However, these products may be suitable for a short duration (10-15 years) in mild atmospheric exposures. On the other hand, batch hot-dip galvanized coatings with a minimum of 3.0 mils coating thickness can provide at least 55 years of maintenance-free atmospheric corrosion protection for solar mounting structures. Although both hot-dip coatings can provide unmatched benefits in terms of maintenance-free and durable corrosion protection, the increased longevity of batch hot-dip galvanizing provides a sustainable and economic design that perfectly complements the clean, renewable energy source it is protecting.

Examples of successful batch hot-dip galvanized solar panel projects in North America can be found within the AGA Project Gallery: