Ε. K. Co. imbibition paper, soaked 1 0 minutes in 0 . 5 Μ lead acetate, dried and soaked 1 0 minutes in 1 Μ sodium carbonate.
4 . 5 volts, 1 2 0 seconds, 2 0 0 lb./sq. in.
B. Magnified section of print A, showing detail in brush track. ( X 1 0 ) C. Electrographs of sulfate corrosion on bronze plate.
Pad: 1. C.S. & S. # 5 7 5 veiling sheet
2. Barium rhodizonate paper (C.S. & S. # 5 7 5 ) 3. C.S. & S. # 6 0 1 backing sheet
Electrolyte: 0 . 2 Μ NaCl
6 0 seconds, 1 0 ma./cm.2, aluminum anode Photographed with green filter to give greater contrast.
ELECTROGRAPHY A N D ELECTRO-SPOT TESTING 201
3) Backing sheet 0.5 Μ NH4C2H3O2
10
Note 1:> Sulfide. Gelatin papers should be soaked 10 minutes in the lead acetate, blotted, and com
pletely dried and then immersed in the sodium carbonate for the same length of time. Use of a car
bonate solution of twice the lead molarity insures complete precipitation of the latter and provides the necessary free salt for the electrolyte. Good prints are obtained in 30 seconds with the current-density given. If estimates of film density are desired, it must be reduced completely and sufficient time must be allowed for this.
Note 2: Chloride. A " veiling" sheet of paper is placed between the specimen surface and the silver chromate paper to prevent cathodic reduction of the silver chromate to metallic silver. Magnesium acetate is used as electrolyte to buffer the hydroxyl ion at the cathode. High alkalinity would decom
pose the silver chromate.
Note S: Sulfate. A "veiling" sheet protects the print against the adhesion of sponge metal, dust, etc., which might mask the bleaching of the barium rhodozonate. Ammonium acetate as electrolyte prevents excess alkalinity at the cathode, which tends to bleach the barium rhodozonate.
202 Η. W. HERMANCE AND Η. V. WADLOW
as ore minerals. Table VI summarizes the conditions for anion print
ing (29).
4.2. Electrography—The Recording of Distributive Patterns Whenever a n electrolytically soluble surface contains areas which differ either in t h e kind or q u a n t i t y of ions yielded under a given impressed potential, it would seem reasonable t o expect t h a t prints m a y be obtained reproducing such areas. An alloy m a y consist of a solid solution, b u t more commonly it contains two or more distinct phases comprising units whose size, separation, a n d arrangement determines the structure of t h e specimen. On anodic dissolution, these structural units m a y each yield a different cation, producing contrasting colors in the final reaction products in t h e print. On t h e other hand, a common cation might be yielded b y all of t h e phases but, because of composition differences, t h e q u a n t i t y of t h a t cation might vary, producing varying densities of a single color in t h e final print, corresponding to t h e different surface units. T h e yield of a cation m a y be further influenced b y differ
ences in t h e solution potential of t h e structural units. Often small changes in composition produce significant differences of potential.
This is typified b y t h e components of steel. W i t h low current densities, when t h e over-all potential drop between the specimen surface a n d the electrolyte does not greatly exceed t h a t between t h e individual phases, these phase potential differences will exert a controlling influence over the yield of the common ion, the greater p a r t of t h e electrical energy being then used to drive this ion from t h e more anodic areas. E v e n in a single phase alloy, a section m a y show potential differences between its crystal units because of t h e different orientations of their lattices. Here t h e close relationship between electrography and metallographic etching should be apparent. B o t h depend on a nonuniform r a t e of solution governed b y differences in composition, solution potential, and crystal orientation of t h e structural units.
I n electrography there are, of course, practical limitations which m a y not be present in differential etching of t h e original specimen. T h e fineness of printable detail is limited b y control of lateral diffusion and the background structure of t h e printing medium. Suitable color reac
tions m u s t be available for the cations involved.
4.3. Special Applications
4.3.1. Structure of Steel. I n 1929, Glazunov (19) published t h e first account of t h e electrographic method as applied t o the s t u d y of t h e macro-structure of iron a n d steel. He used potassium ferrocyanide as elec
trolyte a n d color reagent a n d obtained prints which recorded
macro-ELECTROGRAPHY AND ELECTRO-SPOT TESTING 203 structural features in varying densities of t h e blue iron ferrocyanide.
I n these prints, t h e behavior of a ferrous specimen parallels generally t h a t observed in its metallography. Areas which are most readily affected b y etching reagents also dissolve most readily when m a d e anodic t o a ferrocyanide electrolyte, yielding t h e deepest colors. This is illus
t r a t e d in Fig. 11. Magnetic iron, largely ferrite, and a pearlitic steel were clamped together t o form a single
electrographic specimen, polished and printed with an impressed potential of 1.4 volts. T h e pearlitic area, which normally etches rapidly, printed a deep blue, while t h e ferrite, which etches slowly gave only a s p o t t y p a t t e r n , due largely t o impurities. is also influenced by conditions of strain.
T h u s a cold worked surface is rendered more electropositive a n d will render as a deeper blue in t h e electrograph.
W i t h specimens of cast iron a n d proc
essed steels, Glazunov (19-21, 22, 25, 26), A m m e r m a n n (1), H r u s k a (34), a n d Jimeno, Bernal, and Ibarz (35) h a v e obtained elec-trographs which give clear tracing of such gross structural irregularities as slag inclu
sions, blow holes, piped conditions, fibrosity, and flow structure, a n d in general, all t h e features observed in macrographic exami
nations. Jimeno, Bernal, a n d Ibarz also report t h e successful applica
tion of electrography t o alloy steels, especially t o chrome-nickel steels which are otherwise difficult t o s t u d y because of their resistance t o chemical etching agents. For an excellent account of t h e m e t h o d and a series of reproductions of electrographs, the reader is referred to the original paper (35).
Glazunov's prints were m a d e on unsized paper, moistened in t h e ferrocyanide a n d applied t o t h e specimen surface with a rubber roller t o exclude air bubbles. T h e cathode was an a l u n v n u m or a stainless steel plate and a potential between 1 a n d 2 volts was impressed for from 15 seconds t o several minutes. A m m e r m a n n as well as Jimeno, Bernal, and