Corrosion evaluation testing method of coated metallic material and apparatus employed therefor

A method of testing coated metallic material for corrosion, comprising the steps of:

• (a) providing a metallic specimen plate having a film face coated by a film of paint and bringing the film face in contact with a corrosive test medium;
• (b) providing a non-film face, on the side of the plate reverse to said film face, and bringing the non-film face in contact with an alkaline solution;
• (c) potentiostatically electrolyzing said non-film face of said specimen plate, by a non-film face counter-electrode, at zero potential relative to the potential value of a non-film face reference electrode;
• and
• (d) detecting an electrolysis current flowing from said non-film face with the help of said second counter-electrode.

A corrosion evaluation testing method of coated metallic material which comprises the steps of:

• applying DC voltage, at a constant potential sweep speed, between a coated specimen plate having defects and a counter electrode with a corrosive medium being interposed, thereby effecting cathodic polarization, after said coated specimen plate having defects has been potentiostatically electrolyzed at its spontaneous electrode potential and then detecting the variation in the potential relating to said coated specimen plate with the help of a reference electrode together with the variation in the current flowing between said coated specimen plate and said counter electrode;
• detecting the existence of a peak portion in the cathodic polarization curve formed from said variation in the potential and said variation in the current;
• and
• detecting whether said spontaneous electrode potential of said coated specimen plate is anodic or cathodic with respect to the spontaneous electrode potential of substrate metal obtained under substantially the same electrochemical conditions, whereby the corrosion form is judged.

A corrosion evaluation testing apparatus which comprises:

• a pair of measuring cell vessels, which are so constructed that a coated metallic plate, at least one face of said plate being coated with film, is grasped by respective side edge portions of said paired measuring cell vessels at both its faces, a corrosive medium being filled in one of said paired measuring cell vessels on the film coated side of said coated metallic plate, a first reference electrode and a first counter electrode in said corrosive medium, the other one of said paired cell vessels, which is positioned on the uncoated side reverse to said film coated side, being filled with an alkaline solution, a second reference electrode and a second counter electrode being set in said alkaline solution;
• a first spontaneous electrode potential detecting means which is connected to said first reference electrode and said coated metallic plate, thereby to detect the electrode potential of the coated metallic plate in said corrosive medium;
• a first potentiostat means which is connected to said first counter electrode and said coated metallic plate thereby to potentiostatically electrolyze said coated metallic plate at its spontaneous electrode potential in response to a potential signal output from said first spontaneous electrode potential detecting means;
• a pulse potential applying means which is connected to said first counter electrode and said metallic plate thereby to apply cathodic-anodic pulsation potential between said first counter electrode and said coated metallic plate;
• a linear potential sweep applying means which is connected to said first counter electrode and said coated metallic plate thereby to apply sweep potential between said first counter electrode and said coated metallic plate, said sweep potential being arranged to be selectively varied in the anodic mode and in the cathodic mode;
• a pulse polarization current detecting means for detecting anodic and cathodic pulsation polarization current flowing between said first counter electrode and said coated metallic plate through application of said cathodic-anodic pulsation potential;
• a comparator means for comparing said anodic and cathodic pulsation polarization currents detected by said pulse polarization current detecting means;
• a polarization current detecting means for detecting polarization current flowing between said first counter electrode and said coated metallic plate through application of said sweep potential;
• a compensating means which is connected to said linear potential sweep applying means and said pulse polarization current detecting means thereby to subtract the polarization current portion specific to polarization effects caused by the electric resistance specific to said coated film from said anodic and cathodic pulsation polarization currents or from said polarization current caused by application of said sweep potential;
• an infinitesimally small current detecting means capable for detecting infinitesimally small current compensated by said compensating means;
• a second spontaneous electrode potential detecting means which is connected to said second reference electrode and said coated metallic plate thereby to detect the electrode potential of said coated metallic plate;
• a second potentiostat means which is connected to said second counter electrode and said coated metallic plate thereby to set said coated metallic plate at relative zero potential with respect to said second reference electrode in response to a potentional signal output from said second spontaneous electrode potential detecting means;
• an electrolysis current detecting means which is connected to said second counter electrode and said coated metallic plate thereby to detect the electrolysis current flowing between said second counter electrode and said coated metallic plate;
• control means connected to each of the above-said means, thereby to control them for the measurement relating to said film coated side in the sequence set forth, said control means actuating said first spontaneous electrode potential detecting means and said first potentiostat means so that said coated metallic plate is potentiostatically electrolyzed at its spontaneous electrode potential, said control means actuating said pulse potential applying means, said pulse polarization current detecting means and said comparator means, said control means actuating said linear potential sweep applying means and said polarization current detecting means wherein when the signal output from said comparator means is a signal carrying non-defects information thereon, said compensating means and said infinitesimally small current detecting means are simultaneously actuated, whereby said measurement relating to said film coated side is conducted;
• prior to or subsequent to said measurement relating to said film coated side, said control means first actuating said second spontaneous electrode potential detecting means and said second potentiostat means thereby to set said coated metallic plate to relative zero potential with respect to said second reference electrode and thereafter to actuate said electrolysis current detecting means;
• and
• a collecting, recording and calculating means which is connected to at least said pulse potential applying means, said pulse polarization current detecting means, said linear potential sweep applying means, said polarization current detecting means, said infinitesimally small current detecting means and said electrolysis current detecting means thereby to operate selectively said anodic or cathodic polarization curve and said infinitesimally small current-potential variation curve, and if necessary to the time variation of said electrolysis current and to at least store said operating results.

A corrosion evaluation testing apparatus comprises:

• a peak detecting means for differentiating the cathodic polarization curve specific to a coated metallic material having film defects therein;
• a first spontaneous electrode potential detecting means for detecting the spontaneous electrode potential of said coated metallic material in the corrosive medium;
• a means for inputting the spontaneous electrode potential of the substrate metal of said coated metallic material under the same corrosive conditions as can be found in said corrosive medium;
• a calculating means whose input side is connected with said peak detecting means, said first spontaneous electrode potential detecting means and said inputting means, said calculating means being arranged to judge whether or not peak exists in said cathodic polarization curve in response to the signal outputted from said peak detecting means as well as whether said spontaneous electrode potential of said coated metallic material is relatively more anodic or more cathodic with respect to said spontaneous electrode potential of said substrate metal, thereby to detect either pitting or crevice corrosion through combination of said peak-existing signal or peak-absent signal and a resultant signal resultantly obtained from said comparison concerning the spontaneous electrode potential;
• and
• at least one displaying means.