Method and apparatus for controlling steel corrosion under thermal insulation (CUI)

Abstract

Galvanic anodes and barrier corrosion control for metal pipelines, tanks, and equipment under thermal insulation includes the use of pure aluminum and aluminum alloys in fresh water at high temperatures to protect steel elements in hot water without sufficient chlorides. Sacrificial aluminum and sacrificial zinc-aluminum powders in a mixed primer provide long-term, effective corrosion control of steel under thermal insulation can be achieved in all water temperature ranges regardless of the presence of sufficient chlorides. At temperatures below 75 C., the zinc particles in the primer act to protect the substrate steel. At temperatures above 75 C., the aluminum particles in the primer, and galvanic aluminum tape protect the substrate steel in chloride free neutral pH water and also reduce the consumption of zinc particles in the primer. When the electrolyte water for CUI is sufficiently contaminated with chlorides, the aluminum particles in the primer and the galvanic aluminum become active.

Claims

1. A system for controlling steel corrosion under insulation (CUI), comprising: a sacrificial zinc anode primer disposed on a steel surface to be protected; a sacrificial pure aluminum tape disposed over the sacrificial zinc anode primer; wherein the sacrificial pure aluminum tape is in direct electrical communication with the steel through an electrical connection that bypasses the sacrificial zinc anode primer; and a corrosion-resistant tape disposed over the sacrificial pure aluminum tape.

2. The CUI protection system of claim 1, wherein the sacrificial zinc anode primer also includes aluminum, or alloys thereof, to control the consumption rate of the sacrificial zinc anode primer at temperatures above a predetermined operating temperature.

3. The CUI protection system of claim 1, wherein the sacrificial zinc anode primer includes pure, elemental zinc particles.

4. The CUI protection system of claim 1, wherein the sacrificial zinc anode primer includes pure, elemental aluminum particles.

5. The CUI protection system of claim 1, wherein the sacrificial zinc anode primer includes aluminum alloy particles.

6. The CUI protection system of claim 1, wherein the sacrificial zinc anode primer includes zinc-aluminum alloy particles.

7. The CUI protection system of claim 1, wherein the corrosion-resistant layer is an aluminum alloy layer.

8. The CUI protection system of claim 7, wherein the aluminum alloy layer is a 3000 series aluminum alloy.

9. The CUI protection system of claim 1, wherein the corrosion-resistant layer is a dielectric layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a drawing that shows a triple corrosion protection system in accordance with the invention;

(2) FIG. 2 illustrates the protection mechanism below 75 C. at a tape damaged area;

(3) FIG. 3 illustrates the protection mechanism above 75 C. at a tape damaged area; and

(4) FIG. 4 shows the potential of pure zinc and pure aluminum in chloride free fresh water at various temperatures.

DETAILED DESCRIPTION OF THE INVENTION

(5) This invention resides in systems and methods for controlling the corrosion of steel structures under thermal insulation, whether operating at low, high or cyclic high-and-low temperatures. The invention is based on the discovery that when an aluminum passive film is disrupted above 75 C. in chloride-free fresh water, the potential of the pure aluminum or alloy shifts in a more negative direction, and can therefore be used as a galvanic anode in chloride-free fresh water above 75 C. By combining pure zinc and pure aluminum or aluminum alloys, the protection system and method provide effective protection of steel from CUI in all temperature ranges of water, from freezing to boiling.

(6) A basic system for controlling steel corrosion under insulation (CUI) comprises a sacrificial anode primer 5 disposed on a steel surface 6 to be protected, and a sacrificial aluminum or zinc layer 4 disposed over the sacrificial anode primer 5. The primer is a zinc-primer. When the pipeline is operated below 75 C., a zinc-only primer may be used. When the operating temperatures above 75 C., aluminum is added to the zinc-rich primer, with the Al to Zn ratio being dependent upon operating temperature. A typical inorganic zinc-rich primer contains zinc silicate as a binder (adhesive) which can be used up to 400 C. Water-based inorganic zinc-rich primers may alternatively be used.

(7) An optional corrosion-resistant layer 3 disposed over the sacrificial aluminum or zinc layer provides a third layer of protection. FIG. 1 is a drawing that shows a triple layered corrosion protection system in accordance with the invention. FIG. 2 illustrates the protection area 9 below 75 C. at a tape damaged area 8, and FIG. 3 illustrates the protection area 9 above 75 C. Water is shown at 2, and 7 is an electrical connection.

(8) When the steel temperature is below 75 C., the pure aluminum or aluminum alloys passivate. As a result, the aluminum cannot be used as a galvanic anode to protect steel when the electrolyte is not contaminated with sufficient chlorides. However, the pure zinc particles in the zinc-rich primer are active in chloride-free neutral pH water in all temperatures (0 to 100 C.). Although the zinc is consumed rapidly in higher temperatures and chloride contaminated water, by combining the zinc with more negative potential aluminum above 75 C., the zinc is partially protected by the aluminum. This can be achieved by adding aluminum powder into a inorganic zinc rich primer. Alternatively, this can be achieved with pure aluminum or galvanic aluminum tape covering the zinc primer.

(9) In steel pipes which include T-sections or other more complex configurations, tape coating s may be problematic. In such instances, aluminum powder loaded primers (like zinc rich primers) may be used. A zinc-rich primer or zinc-aluminum mixed primer may be applied initially, and then covered with a high content of aluminum primer.

(10) Thus, summarizing, at temperatures below 75 C., the zinc particles in the primer act to protect the substrate steel. In temperatures above 75 C., the aluminum particles in the primer, and galvanic aluminum tape protect the substrate steel in chloride free neutral pH water and also reduce the consumption of zinc particles in the primer. When the electrolyte water for CUI is sufficiently contaminated with chlorides, the aluminum particles in the primer and the galvanic aluminum tape become active in all temperatures and protect the steel any temperatures.

(11) Aluminum alloy 3000 and 6000 series are known to be corrosion resistant alloys. To extend the lives of the primer and the galvanic aluminum tape, corrosion resistant aluminum tapes composed of such alloys may be used to cover over the galvanic aluminum tape. Such an arrangement prevents the corrosion of the outer layer of the galvanic aluminum tape and the primer from the highly corrosive CUI electrolyte.

(12) Dielectric coatings may also be used to protect the sacrificial aluminum tape. When the corrosive electrolyte water gets under the barrier corrosion resistance aluminum tapes from the damaged areas or seams between the aluminum tapes, the aluminum-zinc mixed primer and the galvanic aluminum tape acts to protect the steel substrate from corrosion.

(13) The steel surface preparation for the primer is not critical because the steel surface is protected by galvanic current, not by coating. Steel corrosion productsrustis sufficiently ionically conductive to conduct the galvanic current to the substrate steel. Therefore, the steel surface preparing for the galvanic primer can be achieved by metal wire brushing, thereby significantly reducing the cost of surface preparation.

(14) Sacrificial magnesium anodes have traditionally been used in chloride-free water. However, the life of a magnesium anode in water is short due to the high percentage of self-corrosion. Indeed, the consumption rate of self-corrosion in chlorinated water is as high as 70 percent. By taking advantage of the behavior of pure aluminum or aluminum alloys which become active in neutral pH water above 75 C., aluminum can instead be used as a sacrificial anode, which has much higher electrical capacity than a magnesium anode.