Detection of active corrosion sites on steel plate from marine environment by chemical methods.

Filter paper impregnated with indicator solution is positioned over the surface of the wire brushed steel panel. A very small amount of distilled water is misted onto the paper. Soluble iron migrates from the active sites and causes the indicator to turn blue. Each blue dot on the paper represents an active corrosion site.

TEST MATRIX

General Discussion

NACE Publication 6G198 "Surface Preparation of Contaminated Steel Surfaces" covers the effects and detection of various contaminants on steel. The document was issued first in 1986, is under review, and provides an excellent overview. Tel: 281-228-6200 to order.

The Potassium Ferricyanide Test for Soluble Ferrous Iron Salts (Fe⁺⁺) is described. This is a sensitive test for active corrosion sites where the iron is dissolving. The Ferricyanide test was used routinely but the specific tests for chloride with a rubber sleeve has displaced the usage because the yellow test paper is light sensitive, people have become more concerned about the limits on chloride (ionic) contaminants, and ferricyanide is not quantitative. However, in Dr. Frenzel’s opinion, it is a very good test to see if the material in pits is an active source of iron dissolution. The trick is to provide enough moisture to wick the material from the crevices and deeper areas and to monitor the timing of the development of the blue spots. This detection is completed within a minute. If the test paper is left on the surface, it can develop a diffuse blue color over all the paper as the steel surface starts to rust in a general manner. With this procedure, we have detected active corrosion sits on apparently sound, Grade A, steel, blasted to SSPC- SP 5 "white metal".

FROM NACE Publication 6G186

" Contaminants in the form of colorless iron salts concentrate at the bottom of rust pits. Their continued presence after surface preparation cannot be immediately detected visually, but shows up after several hours as dark rust spots on the surface.

The purpose of the test is to detect the presence of these ferrous contaminants immediately after surface preparation. In this test, the ferrous salts dissolve in an applied film of distilled water and are drawn by capillary action onto yellow test paper [sic …prepared with potassium Ferricyanide], where they react with the potassium Ferricyanide to form a characteristic Prussian blue color. This test is applicable only after surface preparation, as a crust of rust over the pits prior to surface preparation can prevent the test paper from reaching the contaminants in the bottom of the pits."

"Appendix A

Potassium Ferricyanide Test for Soluble Iron Salts

Preparation of the Test Papers

Prepare a fresh solution of 4 parts by weight of Potassium Ferricyanide in 96 parts of distilled water. Prepare test paper by soaking a medium grade of filter paper ( No. 1, 9 cm²) (or filter paper for home coffee makers), in the Potassium Ferricyanide solution and hang to dry in air. When dry, store in a black envelope or dark bottle to exclude light.

Test Procedure

Select an area of the blast-cleaned surface that has previously shown heavy corrosion or pitting. Apply a thin film of distilled water using a clean … pad about 20-30 mm in diameter. When the distilled water has nearly evaporated, apply the test paper to the slightly damp surface and press for 15 seconds with the thumb or forefinger to obtain good contact. Examine the underside of the test paper. The presence of blue spots indicates that soluble ferrous salts remain on the black-cleaned surface, which should then be reblasted. TOP

CURRENT TEST DESCRIPTION

Detection of active corrosion sites on steel plate from marine environment by chemical methods. This continues the test A0002 using the same panel, cleaned with water jetting instead of wire brushing.

Filter paper impregnated with indicator solution is positioned over the surface of the water jetted steel panel (positioned in the same area as in test A0002). A very small amount of distilled water is misted onto the paper. Soluble iron migrates from the active sites and causes the indicator to turn blue. Each blue dot on the paper represents an active corrosion site.

We will see if the remains of burnished areas (bright speckles) leave active sites behind, even after waterjetting.

TEST MATRIX