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
