Mechanism of Filiform Corrosion 1%'. H. SLABAUGHl . ~ N DMORRIS GROTHEER Kansas State College, Manhattan, Kan.
ILIFORSI corrosion, sometimes termed "underfilm" corrosion, is a type of rusting vhich results in the formation of threadlike filaments of corrosion product. Named by Sharman (4)and adequately described by Van Loo and coworkers ( 5 ) , this type of corrosion occurs under various types of organic films and certain metal plate on ferrous: magnesium, and aluminum surfaces. Figure 1 shone a typical example of filiform corrosion produced in the laboratory on cold-rolled steel. The salient characteristics of filiform corrosion, described by Van Loo (C), are as follows: It occurs on steel a t room temperature in the relative humidity range of 65 to 957,, groxing with wider filaments a t the higher humidities. Filiform corrosion has been observed on aluminum and magnesium surfaces, although the humidity fact,or has not been specified. Growth is more
structure of the metal substrate. The growing filsment has a blue colored head which contains iron(I1) in a liquid solution. The remainder of the filament is rust colored and is a dry, stable corrosion product. THE PROBLEM
At present there is no adequate solution l o the problem of inhibiting filiform corrosion. The usual inhibitive pigments, organic inhibitors, and metal pretreatments such as phosphat,e coatings exert only slight, if any, influence. With an adequate theory as a guide, it is anticipated that a successful method of controlling filiform corrosion can be found. Thus, the present work, is primarily concerned with supplying a workable theory for the mechanism of filiform! then proposing t o control it by considering those postulat,es vhich are basic to t.he mechanism. EXPERIMENTAL RESULTS AND DISCUSSIOS
Cold-rolled steel panels, coated mith a clear urea-alkyd baking vehicle, were exposed a t room temperature to 85'% relative humidity (R.H.) rvhere t,hey developed filaments of corro5ion as shown in Figure 2. These filaments averaged 0.5 mm. in n-idth and grew a t the rate of 2 to 3 mm. per -reek. With a Type R Leeds and Northrup galvanometer which was calibrated to read in millivolts, various areas of these panels were Etudied with the aid of a microscope, in order to seek areafi that were anodic or cathodic. Two steel probes, sharpened to micro points, were successively inserted into the blue head, the rust' deposited in the body of the filament, and unaffected areas of the metal surface. Each observation v-as followed by reversing the probes; the pot,ential was identical in both cases, although reversed in polarity. I n every instance, the blue, groning head of the filiform corrosion was anodic to t,he base metal direct'ly bchind the head, the unaffected metal adjoining the filament, and unaffected metal a t remot'e parts of the panel. Electropotentials measured in this manner never exceeded 0.3 mv. and averaged 0.2 mv., as shown in Table I. At no time v a s it possible to detect anode and cathode areas Tithin the head of the filament. This indicates that the blue
TaBLE
ELECTROPOTENTL4LS OBSERVED I S F I L I F O R X
CORROSIOX
(Galvanometer calibration, 0,001 volt = 10-om. scale reading) Galvanornetcr Electrodes Deflection. Mln. -~ ~~. Ei a t El a t To left T o light Unaffected steel, Blue head I3 1 inch away Unaffected steel, Blue head 20 1 inch away Unaffected steel, 18 Blue head 1 inch away Unaffected steel, 1 inch away Blue head 15 Blue head Rust filament 2'1 Blue head Rust filament 20 2.5 Blue head Rust filament 20 Rust filament Blue bcad Blue head Unaffected steel, 1 nim. away 18 Blue head Unaffected steel, 2!1 1 mni. away Unaffected steel, 1 mm. away Blue head 30 Unaffected steel, 1 min. away Bluehead 18 Mean 20 = 0 . 2 I X T .
Figure 1. Filiform Corrosion on Cold-Rolled Steel Test Panel 5.75 X 2.73 inch p a n e l
rigorous under thlcker 01 ganic films than under thin films, although it is inhibited by films that are impermeable to moisture, such as paraifn. Filiform i d 1 grox- under organic films only when they are formulated below their critical pigment volume concentrations (Z), where the film serves as a semipermeable membrane. Filiform filaments are usually initiated by weak points or discontinuities in the hlm they never cross each other, and their grom-th is independent of light and of the metallurgical 1
I.
Present address, Oregon State College, Corvallis, Ore
1014
May 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
101s
diffused through the film by osmosis or capillary action ( I ) , the water vapor pressure on the atmosphere side, P I , must slightly exceed the vapor pressure on the liquid side, P1. Thus, for the cell to grow a t 65% relative humidity, P I
2 P1
the threshold humidity for filiform growth, the value of P I a t 25" C. is 0.65 (23.8 mm.) = 15.5 mm. At higher relative humiditips, the value of PI becomes greater. Since P, will be only slightly smaller than PI, the approximate mole fraction concentration of the cell fluid can be calculated from Raoult's lam. Assuming ideality, the minimum mole fraction of solute (iV2) in the cell fluid a t 65% relative humidity would be
Figure 2. Typical Filaments of Filiform Corrosion head of the filiform is exclusively anodic toward all other parts of the corroding system, and that cathode areas are randomly scattered over the remainder of the surface of the metal. There ia considerable indication that the cathode areas are concentrated in the stable part of the filament. The presence of hydroxyl ions in meaeurable quantities was shown to exist in the filiform cell by pressing a piece of moistened pHydrion paper onto a newly opened cell. I n this manner a pH of 12.0 has been observed. This pH value is actually exceeded in the cell, because in order to measure the cell pH i t is necessary to dilute it somewhat in order to permit it to come into contact with the indicator paper. The highest pH in the cell apparently exists a t or near the junction of the blue head with the rust filament. Analysis of the cell fluid for nonvolatile solids a t 105" C. showed that the blue head of the corrosion filament contained a solution of high solute content. The cells were opened and the cell fluid was immediately blotted onto a tared strip of filter paper which was quickly weighed, then placed in a drying oven. Among several cells thus examined, the cell fluid varied from 38 to 50% in dissolved solids. No attempt was made to determine the chemical composition of FILIFORM G R O W 1 these solids, except that the presence of iron(I1) was evidenced by a positive test with potassium ferricyanide, No detectable amount of iron(II1) in the cell fluid was indicated by similar wet chemical methods. The conversion of iron(I1) to iron(II1) may be electrochemical, but i t does not occur in the blue head. By microscopic examination, the color changes indicate that this conversion occurs a t the junction between the blue head and the filament of stable corrosion product. This junction is a fairly sharp boundary. On the basis of the fact that filiform corrosion occurs only above 65% relative humidity, it is suggested that the concentration in terms of mole fraction of solute.of the cell fluid must be high enough to cause a diffusion of moisture from the atmosphere, through A
where P is the vapor pressure of pure solvent (water) and n2is the number of moles of solvent. The chemical character of the solute in the cell fluid presents a point of interest. There are several iron(I1) organic salts, such as ferrous acetate, with high solubility. These organic substances may already be a t the metal-film interface, or may be extracted from the film by the moisture x hich diffuses through the film. That this portion of the organic solute is leached from the film is indicated by the fact that filiform grows more actively under thicker films. I n addition, a thicker film would provide a more efficient semipermeable membrane which is essential t o the present proposition. Further support for the osmotic factor in filiform action is the report of Faneuf ( 3 ) that a final rinse of phosphatized metal with demineralized water reduces the growth rate of filiform. The removal of soluble residues from the surface of a metal before it is coated inhibits the formation of blisters under the coating when it is exposed to high humidities. It is thus concluded that a significant feature of filiform corrosion is the osmotic action which supplies the anolyte present in the head of the filament. I n order to summarize the electrochemical and osmotic effects concerned in this study, Figure 3 reprecents a cross section parallel
+
the organic film, and into the corrosion cell. Because the water in the cell has
z 5 P
2
w z
U
8 I-"
OF CELL FLUID
2 v)
B
C
0 7.
DISTANCE ALONG FILIFORM CELL PARALLEL TO GROWTH
Figure 3. Osmotic and Electrochemical Action in Filiform Corrosion
1016
I N O U S T R I A L A N D ENGINEERING CHEMISTRY
to the filament and normal to the surface of the metal. It also s h o w schematically the change in differential vapor pressure as a function of the mole fraction concentration of the cell fluid. The different,ial vapor pressure determines the direction and ext,ent of osmosis, which is also indicated. The driving force of this type of corrosion is controlled by the osmotic action in the growing cell and the modification of the cell fluid which produces a significant boundary, B, between the blue head and the rust filament. Starting at A , the concentration of cell fluid is very high; hence the osmotic rate is high,
Val. 46, No. 5
the critical role of humidity and its effect in producing osmotic cells, the application of a film which does not serve as a semipermeable, osmotic membrane is warranted. Van Loo (5) ha> suggested the use of highly permeable, pigmented films form Ilated above their critical pigment volume concentration. In an opposite sense, an impermeable film would also inhibit filiform gron%h by serving as a barrier to moisture, thus preventing thr formation of osmotic cells under the film. Other preventive measures concern the elimination of ~ ~ a t e r soluble components or extractives in the coating. If little or no solute xere picked up by the moisture as it passes through thr film, the cell fluid would be very dilute with the result that ORmopis would proceed dowly and only a t very high relative humidit,y. The presence in the system of a suitable agent n-hich coilverts iron(I1) to iron(II1) as soon as it forms would alp0 inhihit filiform corrosion by advancing boundary B; a consequent reduction in the size and width of the head will reduce its growing rate and thus inhibit filiform corrosion. ACKNOWLEDGMENT
The authors extend their thanks to Maurice T’an Loo and his colleagues of The Sherwin-Williams Co. for supplying samples of the organic coating, the steel panels used in thip work, and the photograph in Figure 1. Figure 4. Cross Section of Growing Filiform Head
LITERATURE C I T E D
(1) d’Ans, J., Wekua, K., and Ulbrich, K., Farbe u. Lack, 58, 387,
because Pi is much greater than P?. The resulting osmotic pressure is sufficient to disengage the film from the metal and form a cell. .4s the cell grows, bot,h normal t o the surface of the metal and laterally, the cell fluid becomes diluted by additional moisture entering the cell through osmosis. As the osmotic rate diminishes, chemical oxidation converts iron(I1) to iron(II1). Because of the presence of sufficient hydroxyl ions and t,he greatly reduced solubility of ferric salt< compared to ferrous salts, t,he iron is precipitated a t B as a hydrated ferric oxide. This abrupt reduction in the concentration of the cell solution causes an irnmediate reversal in the vapor pressure differential, with Pz now exceeding PI. Beyond B the osmosis xi11 be reversed by a transfer of moisture from the cell to the atmosphere. The contents of the filament a t C are relatively dry, stable corrosion product. The filament continues to grow a t a constant width by virtue of the ability of the transition boundary, B , to keep up with the grolving head. When active filiform heads are subjected to excessively high relative humidity, they grow more rapidly than boundary B can advance; hence radial action produces wide heads or blisters. At subcritical relative humidity the growth of the head is arrested, boundary B moves in, and the entire blue head is converted to stable corrosion product. The V-shape of boundary B is explained as the result of the shape of the roof of the corrosion cell as described in Figure 4. Diffusion of equal quantities of atmospheric oxygen through various portions of the film which forms the roof of the corrosion cell will produce higher oxygen concentrations in the shallow periphery region of the advancing head than a t the cent’er. Thus, in Figure 4 the conversion of iron(I1) to iron(II1) will occur more rapidly a t D than a t E. Thus E will lag behind D in oxidation with the resultant V-shape a t B. Filaments never cross each other, because as the head approaches an old filament the advancing head encounters film LThich has previously participated in filiform. From this old film the water-soluble components have already been extracted; hence a new filiform head is discouraged from developing in this area.
431 (1952). (2) Asbeok, W. K., and Van Loo, AI., IND.EXG.CHEM..41, 1470 (1939). (3) Faneuf, Euclid, Finish,9, 29 (1952). (4) Sharman, C. F., A’ature, 153, 621 (1944) ; Chemisttry &: Induslry, 46, 1126 (1952). (5) Van Loo, M,, Laiderman, D. D., and Bruhn, R.R., Corroeion, 9, 277 (1953). RECEIVED for review September 29, 1953. ACCEPTED January 26, 1 9 5 4 Preiented before the Division of Industrial and Engineering Chemistry at the 124th lfeeting of the AMERICAX CHEMICAL SOCIETY, Chicago, Ill.
Heat Transfer Coefficients of BseudoPlastic Fluids-Correction In the article on “Heat Trandcr Coefficients of Pceudo-Plastic Fluids” [ I A DEAG.CIIEM. 45, 1686 (1933)] the following error“ occurred. Page 51 A (advertising section). The formula should read:
W d
-
@Le
flQ
At the end of Table 11-the sentence ‘Isas read from graph of data of hlorris and Whitman at corresponding Reynolds number” should follow the first equation, for J*. A t the top of the first column of page 1687 the evpression ihouid lead.
Equation 8 and Somenclature on page 1696 should read:
I\HIBITIVE COVTROL
On the basis of the proposed theorv, Yeveral recommendations may be suggested for controlling filiform corrosion. I n view of
At = temperature difference across fluid film.
JU CHIKCHU,K. F. BURRIDGE, FRANK BROW
