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The Gulf Coast of the United States has a multitude of petroleum refining and chemical production facilities. Many plants are expanding as demand for raw materials grows. In the Gulf Coast, expansion means structural steel-lots of it. Galvanizing along with application of coating systems is relied upon to protect the steel from corrosion. Over time, protective coatings begin to fail and refurbishment is needed. But one facility experienced corrosion problems while still under construction. Why? Let's find out.
BACKGROUND
A steel fabricator won a contract to supply structural steel and erect a new process unit as part of a PVC production facility expansion near the Gulf Coast. The requirements for the structural-steel columns included galvanizing the upper portion of each column where they would be exposed to the environment. The remaining portions of the columns and other interior structural members did not require galvanizing but were required to be painted (Figs. I and 2).
Structural steel painted at this fabricator's facility in the southeast is typically prepared by abrasive blast-cleaning in an automated blast booth. However, since the structural columns were partially galvanized prior to painting, it was decided to prepare all of the ungalvanized surfaces by power wire brush (SSPC-SP 3) followed by spray application of a rust-inhibitive gray alkyd shop primer. The galvanized upper sections were not painted. The product data sheetfor the shop primer recommended a dryfilm thickness of 2.0-4.0 mils. The data sheet also indicated that the coating performs well over power- or hand-tool-cleaned surfaces (SSPC-SP 2).
Following painting and drying, the columns were shipped to the PVC production facility and erected. Within a few days, it was reported that painted sections of the columns were rusting badly. The fabricator requested assistance to investigate the cause.
FIELD INVESTIGATION
A coatings consultant visited the PVC production facility to examine the coating and corrosion on the partially galvanized columns. The fabricator's field project manager was present during portions of the examination. The investigation steps included a visual examination; determination of coating-failure pattern; coating adhesion and coating-thickness measurements; and collection of field samples for soluble-saltsanalysis.
Visual Examination: Viewed from a distance, the light-gray galvanized portions and dark-gray painted portions of the steel columns were clearly distinct. However, a zone of red rust, rust streaks...