O n lu h U o s i fo b W a te s i
A . W A CH TER
Shell D evelopm ent Company, Emeryville, Calif.
SODIUM nitrite is a good corrosion inhibitor for water and under many conditions can completely suppress the corrosion of steel.
The concentrations of nitrite needed for pronounced inhibition vary with the severity of conditions and the pH and composition of the water. Sodium nitrite can completely prevent corrosive attack on steel in sodium chloride solutions and in aqueous alcohol solu
tions. It is also effective on steel which is covered with rust scale. \ Corrosion of many of the common ferrous and nonferrous metals is -either inhibited or not adversely affected by the presence of nitrite.
S
O D IU M nitrite is a highly effective corrosion inhibitor capable of preventing corrosion of steel by w ater and oxygen under m any conditions. A lthough the m echanism by which sodium nitrite functions as an inhibitor is not know n w ith certainty, evidence (3) indicates th a t it acts as an oxidizing agent to produce a very thin tenacious film of ferric oxide on anodic areas, and does not remove oxygen in th e m anner of sodium sulfite. M any references to th is substance are found in patents, b u t there are rem arkably few discussions in scientific literature. An earlier paper (8) described characteristics of sodium n itrite as an in
hibitor w ith p articular regard to its application for preventing internal corrosion by w ater in pipe lines for gasoline and other petroleum products. T his application (1, 2) has proved singu
larly effective, and a t present such inhibition is being employed in over 32% of th e to ta l gasoline pipe line mileage in th e U nited States. Sodium nitrite, incorporated into a m ucilaginous-base
1 B a s e d on d iscu ssio n b y t h e a u th o r a t t h e S y m p o s iu m on C orrosion Inhibitors, p r e sen te d i n t h e p reced in g pages.
coating composition (4), also provides efficient protection against corrosion by m oisture in steel drum s containing gasoline. Since m any other practical applications are probably possible, it is considered w orth while to present d a ta here which m ay be of general interest.
INHIBITION O F A Q U E O U S SO DIUM CHLORIDE SOLUTIONS
Sodium chloride m ay be taken as a common corrosion-enhanc
ing constituent in w ater. Experim ents have been made to deter
m ine th e corrosion inhibiting behavior with respect to steel of sodium n itrite in sodium chloride solutions. The m ethod was described previously ( 3 ) . Figure 1 shows typical results for low chloride concentrations. Concentrations of nitrite needed to completely suppress corrosion under these conditions are ex
trem ely low; 0.005% suffices for distilled water, 0.03% for a 0.05% sodium chloride solution, and 0.06% for a w ater sample from a gasoline pipe line.
F igure 2 indicates the m inimum concentrations of sodium ni
trite needed to prevent corrosion of steel by solutions containing up to 10% of sodium chloride. T he corrosion rates for the so
dium chloride solutions w ith no added nitrite under these condi
tions were about th e same w ithin the lim its of experimental un
certainty, the average value being 6.7 mils per year with maxi
m um deviations of =*=0.4. N o a tte m p t was made to control the hydrogen ion concentration; however, pH measurements were m ade before and after each experiment. Initial pH values of the nitrite-chloride solutions were between 6.2 and 7.2, usually slightly below 7. F inal pH values were as high as 9 to 11 in
ex-Vol. 37, No. 8
Figure 1. Influence of Sodium Nitrite Concentration on Corrosion of Steel b y Some Waters
Figure 2. Minimum Concentrations of Sodium Nitrite for Pre
venting Corrosion of Steel b y Sodium Chloride Solutions Conditions for Figures 1 and 2: Sandblasted low-carbon steel strip/ 3/8 b y 5 1 / 2 inches, m 1 20-m l. b o ttle containing 2 0 ml. a q u e o u s solution of sodium nitrite and sodium ch lo rid e, 7 0 ml. gasoline, a n d 3 0 ml. air space. Bottle rotated en d over end at 6 0 r.p .m . for 1 4 d a y s at room tem perature (approxim ately 2 5 ° C.). Fresh air
intro d u ced into air space every second day .
perim ents w ith less th an completely inhibiting concentrations of n itrite b u t were only slightly over 7 when no corrosion occurred.
In these experim ents it was judged th a t no corrosion occurred when th e ra te of a tta c k was less th a n 0.1 mil per y ear; th e speci
m en appeared b rig h t and th e liquid was clear.
Sodium n itrite requirem ents increased m arkedly as sodium chloride concentration increased. F or example; although 0.005%
an d 0.2 % sodium n itrite sufficed to prev en t corrosion in distilled w ater and in 0.5% sodium chloride solution, respectively, a t least 4 % sodium n itrite was needed to p rev en t ru stin g under these conditions in 3 % sodium chloride. I t should be k e p t in m ind th a t less severe corrosive conditions w ould require lower concentrations of sodium n itrite and correspondingly m ore severe conditions would require higher concentrations.
INFLUENCE O F pH
O n several occasions it has been observed th a t sodium n itrite does n o t in h ib it corrosion in acidic w ater. F o r example, a p H of a t least 6 is required for 0.06% sodium n itrite to be effective for w ater in a p artic u la r gasoline pipe line (3). Figure 3 gives results
Table I. Efficacy of Inhibitors with Slightly Rusted Steel P r e c o n d itio n in g p e r io d A : S a n d b la s t e d S A E 1()15 s t e e l rods (*/* b y 5 in c h e s) w ere r u s te d e q u a l ly b y a lte r n a t e d ip p i n g in 0 .0 5 % N a C l s o lu tio n , im m e r sio n fo r a b o u t 2 0 h o u r s i n g a s o lin e , a n d h a n g i n g in a ir fo r a b o u t 8 h o u rs; t h e c y c le w a s r e p e a te d s e v e r a l t im e s o v e r a p e r io d of 5 d a y s .
P e r io d B : R u s t e d ro d p r e p a r e d as in A , f a s t e n e d in a 4 - o u n c e b o t t l e c o n t a in in g 2 0 m l. of in h ib it o r s o lu ti o n ( p H 9 .0 t o 9 .5 ) o r w a t e r , 7 0 m l. o f g a s o line, a n d 3 0 m l. o f air s p a c e . B o t t l e r o t a t e d e n d o v e r e n d a t 6 0 r .p .m . an d ro o m t e m p e r a t u r e . F r e s h a ir in tr o d u c e d i n t o a ir s p a c e e v e r y s e c o n d d a y . C o r r o sio n r a te s c a lc u la t e d fr o m w e i g h t lo ss o f c le a n e d s p e c im e n s .
C o r r o s io n R a te® , M i l s / Y e a r A q u e o u s S o ln .
W a ter 1 . 2 % N a 2Cr04 1 . 2 % N a N 0 2
7 - d a y expt.& 1 4 - d a y e x p t . c 8 . 8
4 . 2 0 . 7
0 . 5 0. 60.2d
9 . 1 3 . 0 0 . 9
0 . 3 0 . 3 0 . 3
° A v e r a g e v a l u e s d u r in g p e r io d B a n d a v e r a g e d e v i a t i o n f r o m m e a n . b E a c h s o lu ti o n t e s t e d i n tr ip lic a t e . F r o m 3 r o d s a v e r a g e w e i g h t o f r u s t fo rm e d d u r in g p e r io d A w a s 161 =*= 18 m g . a n d a v e r a g e w e i g h t lo ss o f s t e e l w a s 1 1 0 =*=12 m g ., c o r r e s p o n d in g 't o a co rro sio n r a te of 15 m i l s / y e a r .
c E a c h s o lu tio n t e s t e d in q u a d r u p lic a te . A v e r a g e w e i g h t lo ss o f 4 r o d s in p erio d A w a s 1 2 8 =*=11 m g.
d 1 .0 % N a 2C r0 4 s o lu ti o n u s e d in th is e x p e r im e n t .
Table II. Effects of Sodium Nitrite with Different Metals C o n d itio n s: M e t a l s tr ip , 8/ s b y 5 V 2 i n c h e s , f a s t e n e d in a 1 2 0 -m l. b o t t l e c o n t a in in g 25 m l. of a q u e o u s s o lu ti o n (in d i s t i ll e d w a t e r ) . B o t t l e r o ta te d e n d o v e r e n d a t 6 0 r .p .m . fo r 14 d a y s a t r o o m t e m p e r a t u r e ( a b o u t 2 3 ° C .).
Air in b o t t l e r e p le n is h e d e v e r y s e c o n d d a y . C o r r o s io n r a te s c a lc u l a t e d fro m w e ig h t lo ss o f c le a n e d s p e c im e n s .
C o r r o s io n R a t e , M i l s / Y e a r M e ta l
0 .0 5 % N a C l so ln .
0 .0 5 % N a C l 0 .2 % N a N O s i
L o w -c a r b o n s te e l 1 7 . 6 0 . 0
T in - p la t e d s te e l 5 . 0« 0.2&
13% c h r o m iu m ste e l 0 . 2 0 . 0
1 8 - 8 s ta in le s s s te e l 0 . 0 C 0.0<*
A lu m in u m (52-S) 5 . 5 0 . 2
M u n t z m eta l (6 0 C u , 4 0 Zn) 0 . 3 0 . 3
A d m ir a lt y bra ss (7 0 C u , 2 9 Z n , 1 Sn) 0 . 3 0 . 1
R e d b r a ss (85 C u , 15 Zn) 0 . 5 0 . 3
7 0 c o p p e r - 3 0 n ic k e l 0 . 1 0 . 1
M o n e l 0 . 0 0 . 0
N ic k e l 0 . 0 0 . 0
0 A t t a c k e d p r in c ip a lly o n s t e e l a t e x p o s e d edgeB.
b S lig h t e ffe c ts o n p la te . c S lig h t ta rn ish . d B r ig h t , u n a ffec te d .
5 6 7 8 9 10 11
INITIAL pH OF SOLUTIONS Figure 3. Influence of pH on Inhibition
with Sodium Nitrite Conditions: P olished low-carbon steel strip, :i/ : b y 3 inches, in 1 2 0 -m i. b o ttle containing 5 0 ml.
aq u e o u s solu tion, 2 0 ml. gasoline, and 5 0 ml. air space. Each a q u e o u s solution c o n ta in e d 0 . 0 2 % b y weight of a buffer mixture consisting o! H3P O1, H3B O3, C H3. C O O H , N a O H . Bottles rotated end over e n d at 6 0 (.p.m . (or 1 6 d a y s at room tem perature. Fresh air intro d u ced into air space
every s e co n d d a y .
from a series of experim ents w ith n itrite-co n tain in g w ater at dif
ferent p H values, obtained b y a low con cen tratio n of a buffering m ixture. T h e buffered w ater w ith o u t n itrite is only m ildly cor
rosive u nder th e te s t conditions; a corrosion ra te of 1.3 mils per year is found w ith w ater of p H 7. I t is ev id en t t h a t p H has a m arked influence on corrosion in th e range 6 to 10 when insuf
ficient n itrite is present, gives only slight inh ib itio n a t 6 to 8, an d provides com plete inhibition a t 9 an d 10. W hen am ple sodium n itrite w as present (0.036% for th is w ater), p H changes had little or no effect in th e range 6 to 10, all corrosion ra te s being
751 substantially zero. In general, m axim um effectiveness of the
sodium n itrite is achieved in definitely alkaline solutions.
EFFICACY WITH V A R I O U S METALS sodium nitrite is successfully used in gasoline pipe lines in contact w ith about two hundred thousand tim es its volume of gasoline. sodium nitrite protects several from corrosion by w ater. R esults w ith different solutions are sum m arized in T ables II, II I , and IV, and w ith an isopropanol solution in T able -V. Although n itrite had no m arked effect w ith various brasses a t room t^ n - perature, significant reduction of a tta ck occurred a t elevated tem peratures. R esults of th e same n atu re were observed w ith Monel. Sodium nitrite also achieved m arked reduction.of attack on alum inum. Although m easurem ents have n ot been m ade w ith zinc or galvanized iron, it is believed from observations in other experim ents th a t zinc would not be protected by sodium nitrite and th a t a tta ck m ight even be somewhat increased.
glycol antifreeze compositions are found, in p aten t literature.
T ables V and V I summarize results of experim ents w ith sodium nitrite in aqueous solutions of isopropyl and m ethyl alcohols.
Figure 4 shows th e appearance of the m etal specimens a t th e term ination of th e experim ent in isopropaaol solution. Sodium nitrite effectively inhibits corrosion of steel and also tends to pro
tec t other m etals in these solutions.
Table IV. Effects of Sodium Nitrite with Different Metals C o n d itio n s: P o lis h e d str ip s, */« b y 3 in c h e s, im m e r se d in 1 5 0 0 m l. of
Table V . Corrosion Inhibition of Isopropanol Solution C o n d itio n s: M e ta l strips, ea ch */< b y 3 in c h e s e x c e p t so ld er, im m e r se d
Table VI. Sodium Nitrite Inhibition of Methanol Solution C o n d itio n s: P o lis h e d S A E 1015 s te e l rod, >/< b y 6 in c h e s, f a ste n e d in a