Corrosion Effects of acid velocity and galvanic couples on corrosion of aluminum and stainless steel in strong nitric acid are unusual b y Mars G. Fontana data for 18-88 stainless C steel and aluminum in nitric acid were sunlmarized in this column in ORROSION
@.
November 1952 and February 1953, respectively. Charts containing isocorrosion lines showed corrosion of these materials as a function of concentration and temperature. These charts show that 18-8s exhibits good corrosion resistance to nitric acid in all concentrations up to the atmospheric boiling points, except for hot concentrated acid, and also that aluminum is rapidly attacked by nitric acid a t concentrations below about 80%, but is much better than stainless steel in the hot concentrated acid. These charts are based primarily on data obtained in static or nonflowing acids. Studies on effects of velocity of flow of acid and galvanic coupling of aluminum and stainless steel by J. P. I-Iirth and J. F. Willging in the Corrosion Research Laboratories a t The Ohio State University show unusual and unexpected results. This information is of practical interest because flowing acids are usually handled and because stainless steel and aluminum are often in contact in systems involving concentrated or fuming nitric acids. Figure 1 is a photograph of equipment used for studying erosion-corrosion or, in this case, the effect of velocity on deterioration of metals and alloys by strong nitric acid. This apparatus is actually a small operating pilot plant, so the results obtained should be representative of what would be expected in plant service. The basic elements are a 17-gallon aluminum alloy tank, which contains 11 gallons of acid in operation, a Durimet 20 stainless alloy centrifugal pump, aluminum piping, an orifice plate, and stainless steel and alloy valves. The velocity of flow is controlled by the size of the orifice plates. All parts of the equipment are examined for corrosive attack-for example, the pump impeller and the valves. The test November 1953
"specimens," however, are the orifice plate itself, the elbow, the tee, and also a removable plate placed inside the tank. The acid returned to the tank is directed onto, or impinges on, this test plate. All of these test specimens are located on the discharge side of the pump. These parts or specimens could be made of any metal or alloy to be tested. All parts are insulated to avoid galvanic corrosion unless metal contact is specifically desired. Test
Figure 1 ,
Erosion-corrosion often increases attack because the protective films on the metal or alloy are disturbed. Figure 2 shows the effect of velocity of flow of white fuming nitric acid at 108" F. on the corrosion of 18-88 Cb (Type 347) stainless steel. The rapid decrease in corrosion as the velocity is increased is clearly indicated. The attack decreases from a very high rate to a fairly low rate a t 12 feet per second acid floa rate.
Erosion-Corrosion Equipment
coupons are also hung in the tank by means of glass cradles. All data here reported for this equipment were obtained at a temperature of 108" F. Generally speaking, erosion-corrosion usually causes increased attack as compared to quiet solutions. The major exceptions are perhaps solutions which cause pitting or where concentration cell effects accelerate corrosion.
Figure 3 shows the effect of velocity on corrosion of 3 s aluminum under the same conditions. Here the reverse of the picture in Figure 2 is observed. The attack increases from practically nil to 15 mils per year a t 12 feet per second acid velocity. Note that the ordinates in the two figures are on different scales. The aluminum still shows lower corrosion rates than the
INDUSTRIAL AND ENGINEERING CHEMISTRY
91 A
SOLVE YOUR MlXlN
srrosion
O
2
a
6
6
1
0
1
2
VILOCITY fEET/SECOND
Figure 2. Effect of Velocity on Corrosion of Type 347 Stainless Steel by White Fuming Nitric Acid at 108' F.
stainless steel at velocities below 1% feet per second. It is fortuitous that the corrosion rates for these two materials are essentially the same a t R velocity of 12 feet per second. Tests reveal that aluminum m a y provide protection for stainless steel
Very unusual and interesting results were obtained in tests on these metals in contact with each other. I n geiaeral, when two metals or alloys are in contact, the corrosion rate on the one showing less corrosion resistance by itself is increased. This is an unfortunate situation because for a given
0
1
3
I
6
I
Id
I2
VELOCITY FEETISECOND
Figure 3. Effect of Velocity on Corrosion of 35 Aluminum b y White Fuming Nitric Acid at 108" F.
Side Entering Mixer
INDUSTRIES, INC. Dept. B, Regent St., Norwalk, Conn.
92 A
INDUSTRIAL
system containing parts in contact, failure occurs in a shorter time because of accelerated atlack on the weakest link in the chain. However, the reverse of the general rule mas found in white fuming nitric acid at 108" F. As a result of contact with 3s aluminum, the corrosion rate at an acid velocity of 6.2 feet per second of Type 347 drops
AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
.
Versatility - for use in dynamic systems , . tested and proven on gasoline and diesel engines . jet engines . . rocket motors . . blast measurements . . high pressure, high temperature chemical reactions. .. hydraulic and pneumatic systems.
..
.
.
.
-
Full Scale Pressures 500, 1,000,2,000,3,000, 5,000, 10,000 p s i . . . response down to 0 psi absolute with 1%of full scale accuracy and temperature compensation. High Frequency Response-flatto20,OOOcps.. Flush Catenary Diaphragm to a minimum.
.natural frequency upto45,OOOcps.
- reduces changes in volume of
pressure chamber
A word about NORWOOD CONTROLS - This name stands for an expanding line of commercial instruments for the measurement of pressure, flow, temperature and weight. It represents a fresh concept of creative engineering which, combined with New England manufacturing skill, is establishing new frontiers in the field of instrumentation.
November 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
93 A
take
Q
closer look af
Corrosion
e e
~
from about 60 mils to less than 1 mil per year! The rate of attack on the 3s in this couple is not greatly increased, going from about 13 to 16 mils per year. These results present the interesting possibility of protection of stainless steel by means of aluminum as a sacrificial anode under these conditions of acid and temperature. This exception to the general rule becomes more spectacular as the temperature of the fuming acid is raised. as shown in Table I. Aluminum and 18-8 both shorn low corrosion rates at room temperature, so the effect here is small. At 160’ F., corrosion of the stainless steel drops from 210 t o 1 mil per year. At this temperature, corrosion rates of aluminum jump to very high values.
STEEL IPE SIZ
Don’t &e “penny wise and pound f ooIish” where stainless steel piping i s concerned. The most economical choice does not always imply the least initial cost as working pressure, methods of joining, installation costs and allowance for loss by corrosion are critical factors. This is particularly true where costs resulting from failures in service-replacement of equipment and lost production due to down time-may exceed the initial cost of the piping.
Table I.
Galvanic Corrosion in Fuming Nitric Acid
(Statio conditions) Corrosion Rate, 3Iils/Year 75’ F. 160’ F. NateUnUniial coupled Coupled coupled Coupled 2s AI 3s AI 616 A1 Type327
CORROSION RESISTANCE For tong service life it i s advisable to allow for some loss in wall thickness where stainless piping i s employed to combat severe corrosion. As shown in Figure A even a small loss means an appreciable percentage loss of wall thickness in the lighter weight schedules.
WORKING PRESSURES As shown in Figure 8, the heavier pipe schedules permit higher working pressures, thus perhaps permitting the use of smalIer diameter piping, or offering greater flexibility for subsequent changes in operational procedures.
Methods of Joining & Installation Casts From the standpoints of economy and ease of installation, it i s extremely impartant that attention be given to methods of joining, fittings, etc. because:
1. Schedule 40 IPS i s the lightest weight pipe specifically designed for threading. 2. Fittings which provide a good thread and also afford structural strength at the ioint are not commercially available for lighter weight pipe.
3. Field welding of thin wall pipe i s difiicult. 4. Misalignment of connections can cause high indallation costs.
COSTS Although the lighter schedules cost less,
as
shown in Figure C, you actually get more for your money with the heavier schedules because the ratio of increase in cost i s less than the ratio of increase of wall thickness (Figure D).
2 2 1 1
6
7 3 1
34 34 60 210
600 610 540
1
l\leclianisiiis to explain the above plienomena concerning velority and galvanic corrosion ale available, but space does not permit discussion here. The above considerations may be life savers when used in some practical application. Corrosion of dilute nitric acid at temperatures above boiling and corrosion of hot concentrated acids are quite severe on the common metals and alloys. Correspondence concernlug this column 5$111 be foiuarded promptly lf addressed to the author, % Editor. I V D U B T R I A ~ AYD EXQIVEERIW CHEWSTRY, 1155.- 16th St., X . W ~ Washington , 6 , D. C.
While various types of fittings are available for the lighter weight pipe schedules, they should be examined carefully as to initial cost, installation cost, working pressure permitted and ease of adaptation to existing lines. Whatever your stainless piping problems, Mr. Tubes-your B&W Tube representativecan provide valuable assistance. Consult him for advice on the stainless piping or tubing that will afford optimum cost-life ratio under your service conditions. TA-l709(G)
THE BABCOCK 8t WlLCOX COMPANY WBULAR PRODUCTS DIVISION Beaver Falls, Pa.-Seamless Tubing; Welded Stainless Steel Tubing, Alliance, Ohio-Welded Carbon Steel Tubing
94 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
tandards of quality and uniformity-properties
n make sure of highest purity and uniformity b y calling backed by years of research and production experience. For a dependable source of supply-for prompt delivery-call Heyden for Formaldehyde. Formaldehyde Solution N.F. is shipped in bottles, carboys, drums, tank trucks or tank cars. 37% methanol-free grade in tank trucks and tank cars. *
e
I1 Heyden also for
PARAFORMALDEHYDE U.S.P.X Powder, Flo Granules and Granules AMETHYLENETETRAMI N E
*
u
Murray Hill 2-7600
342 Madison Avenue, New York 1 1 , N.Y. CHICAGO
PHILADELPHIA SAN FRANCISCO
DETROIT
PROVIDENCE
BENZALDEHYDE BENZOATES BENZYL CHLORIDE BROMIDES CHLORINATED A R O M A T I C S CREOSOTES * FORMALDEHYDE * FORMIC ACID HEXAMETHYLENETETRAMINE MEDICINAL COLLOIDS M E T R L E N E DISALICYLIC ACID * NEOMYCIN PARAFORMALDEHYDE GLYCEROPHOSPHATES * GUAIACOLS PARAHYDROXYBENZOATES PENICILLIN * PENTAERYTHRITOLS PROPYL GALLATE RESORCINOL * SALICYLATES SALICYLIC AClO STREPTOMYCIN
November 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
95A
BEHMND THESE
M A N U F A C T U R E R S
A N D
E R E C T O R S
S I N C E
1 9 1 3
