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Comparing Paint Systems & Galvanizing -- Part I

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Q: In order to protect against corrosion, we are trying to decide between galvanizing and painting. Can you give us your take on the pros and cons of each?

– P.L.

A: P.L., when planning a new project, one factor that often doesn’t get enough advance consideration is long-term protection of the investment from corrosion. Geography, environment, equipment, and structural design all play a role in determining the best method of corrosion protection. Further, equipment location (ease of access), a facility’s maintenance capacity, expected service life, and the type of coating itself all play a role in the long-term cost to maintain the effectiveness and appearance of the selected corrosion protection system. In order to maximize ROI, all of these factors should be given careful consideration as early in the project as possible.

Aside from electroplating and specialized coating systems for smaller components, there are essentially two common methods of corrosion protection for industrial systems and structures:  painting and galvanizing. For ultimate protection and aesthetics, a duplex system may be used which involves an advanced paint system applied over galvanizing. Each of these has various drawbacks and benefits that should be considered when specifying a coating system, including expected service life, mechanical properties, and cost.

Paint Systems

Painting provides a protective barrier between the base metal and the environment, and paint system effectiveness is highly dependent upon adhesion and continuity. Paint peeling or chipping due to poor adhesion will lead to premature failure. Likewise, lightly coated areas which exist at scratches and gouges or at surface edges and hollow interiors may cause even the best coating to fail prematurely. Careful, uniform application along with a durable paint system suited to the environment are keys to success. Although uniform standards govern the application of various paint systems, the specifier should use caution to ensure the fabricator and/or painting company is qualified in the designated application and knowledgeable of the requirements involved. Unlike galvanizing, high-tech coatings can be applied in low-tech facilities with varying degrees of success. Unfortunately, the success or failure may not be apparent until a few months in the field.

There are numerous paint systems available to cover a variety of environmental conditions, durability requirements, and capital budgets. A paint system may be as simple as one or more coats of paint with little or no surface preparation, to a complicated three to four stage epoxy primer and top coat system that can be a costly surface preparation. Direct-To-Metal (DTM) paints have been formulated to serve as both a primer coat and finish coat applied in one application. Although only slightly more costly than paint alone, this is typically the least dependable paint system for most applications due to the low coating thickness and the minimal surface preparation commonly used in this process. A variety of more advanced industrial paint systems are on the market today, including acrylic waterborne, 2-part epoxy coatings, polyurethane, inorganic zinc, and zinc rich moisture curing urethanes, to name a few. As indicated previously, and regardless of the final coating, all paint systems are highly dependent on surface preparation for optimal performance, and the combination of surface prep and coating type result in a broad range of coating/application cost. A coatings engineer should be consulted on more advanced painting applications to ensure an adequate system is specified to meet the unique needs of the project.

Hot Dip Galvanizing Process

In general terms, hot dip galvanizing is the process of applying a protective zinc coating to a steel surface. This is done by dipping the article into a bath of at least 98% pure molten zinc with a temperature range of 815-850 degrees Fahrenheit. This process creates a metallurgical bond of the zinc layer to the base metal, which is extremely durable and corrosion-resistant. Hot dip galvanizing follows a precisely controlled application governed by ASTM standards. The process begins by suspending the article from overhead supports and trolleys to enable movement between process tanks. Custom fixtures are often made to facilitate this requirement. Next, the article goes through a precise cleaning regimen, including a degreasing/caustic cleaning and rinse, a pickling process and rinse, and a flux immersion to ensure optimal surface preparation. Once the 3-step cleaning process is complete, the article is finally dipped into the molten zinc. As the base metal approaches the bath temperature, the metallurgical reaction takes place. This reaction continues until the item is removed from the bath, drained (smaller parts are centrifuged), and cooled.

Due to the temperatures involved in hot dip galvanizing, various design considerations must be taken into account. Thinner materials and non-symmetrical sections may become warped. Flattening and straightening requirements often make galvanizing this type of structure impractical and cost prohibitive. Fabrications must not allow entrapment of cleaning agents or other liquids due to the risk of superheating and steam explosions in the galvanizing kettle. Air/moisture entrapment in sealed hollow structures poses a similar threat. Vent and weep holes must be installed in hollow fabrications to allow drainage and ventilation. Designs must consider fixture holes and hanging position to enable handling throughout the galvanizing process. Fabrication size must also be a consideration when planning a project. Often, the size of a galvanized structure will be limited to the maximum tank size available. "Half/double dipping" is possible, but often not practical. In addition, a noticeable overlap line will be seen on the galvanized surface at any portion of the article exposed to the molten zinc for a second time. Certain base metals and welding rod combinations will receive the zinc differently and will be apparent by discoloration in the weld area. Depending on the application, this may or may not be an issue. The properties of high carbon, high-strength steel and certain steel that has been subject to a high degree of cold-forming may be compromised by the galvanizing process (embrittlement). Further, steel with high amounts of silicone may produce undesirable results upon galvanizing. It is good practice to discuss the details of a project with the galvanizer early to determine any design factors that must be considered.

Hot Dip Vs. Galvanized Sheet

Galvanized sheet follows a similar process as hot dip galvanizing except it is more automated. Steel coils are unwound and fed through a series of rolls and tanks similar to hot dip galvanizing except in a continuous process. At the end of the process line, excess zinc is removed by high pressure air, and the sheet is recoiled for eventual distribution to manufacturers. Aside from the application process, the main difference in galvanized sheet is the reduced coating thickness compared to hot dip galvanizing resulting in a reduction in expected life span. The surface appearance of both types of galvanizing will be the same over time, although sheet typically has a much brighter finish when new. Both surfaces offer the same painting challenges due to slight oxidation (patina), which appears over time. Galvalume sheet is annealed to fully alloy the zinc coating thus making it paint-ready without significant surface preparation.

P.L., I hope this initial information has been helpful. In our next newsletter, we'll move on to compare both the service life and the coating performance and mechanical properties of both paint and galvanizing.

Chris Skidmore is a Project Manager for WL Port-Land Systems and is responsible for coordinating information and project materials among the engineering group, purchasing, subcontractors, field superintendents, and our clients to transform a design/build contract into a functional turnkey system.

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