E V A L U A T I O N O F E N V I R O N M E N T A L L Y F R I E N D L Y C O R R O S I O N I N H I B I T O R S F O R S O U R S E R V I C E

G. Schmitt, A.O. Saleh

Laboratory for Corrosion Protection Iserlohn University of Applied Sciences

Frauenstuhlweg 31 D-58644 Iserlohn

Germany

A B S T R A C T

Corrosion inhibitors continue to play a vital role in controlling internal corrosion associated with sour oil and gas production. From the environmental point of view, research efforts are necessary to develop corrosion inhibitor products which on one hand are effective in corrosion preventing and on the other hand are more acceptable to the environment than conventional types of inhibitor compounds. Performance screening of derivatives of polysuccinimide and lactobionic acid for carbon steel inhibition has been carried out under severe sour gas conditions in brine and brine / hydrocarbon mixtures at 90 to 130 °C with the rotated cage method. The resuks are compared with the performance of a well established conventional sour gas inhibitor.

Keywords: Corrosion, inhibitors, polyaspartic, polysuccinimide, environmentally friendly, hydrogen sulphide, sour service, evaluation.

I N T R O D U C T I O N

The work over of sour oil and gas production wells is generally caused by localized corrosion such as pitting and/or flow induced localized corrosion (FILC) 1 It is well known 2 that FILC can be mitigated by using more corrosion resistance alloys (CRA) or by applying corrosion inhibitors. There are numerous examples in the literatures 3.4.5 which document good corrosion prevention by using corrosion inhibitors in sour environment.

Since some recent years concern has increased as to the environmental impact of chemicals used in off-shore production of oil and gas which may fred its way into the sea. Investigations had shown that many of the standard generic inhibitors exhibit toxic effects to marine life.

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According to the recommendation of the Paris Commission (PARCOM) 6 which restrict the use of many current products in North sea off-shore production, the scientific efforts have increased to develope new chemical products to meet the environmental challenge. According to PARCOM environmentally acceptible chemicals should have a high biodegradibility, low toxicity for marine life and should not enrich in sediments.

Many authors 7,8 used different natural products as corrosion inhibitors in many different applications. Javancicevic et al 9 used succinic acid, polyaspartic acid and polytartaric acid as a corrosion and scale inhibitors in cooling water application, Silverman et al s used polyaspartic acid as a corrosion inhibitor for steel in different types of water.

Darling et al ~0 discussed replacement of toxic aromatic amines by the less toxic aliphatic amines and reducing the toxicity of the aliphatic amines by reacting them with long chain acids to form salts and amides, e.g. 2-pyridinecarboxylic acid with 1-0ctadecenylamine and tall oil fatty acid with diethylenetriamine.

Banerjee et al 1~ used carbohydrate derivatives of alcohols, aldehydes, etc., as corrosion inhibitors for steel in neutral water.

The work in this paper aimed at screening the performance of derivatives of lactobionic acid and polysuccinimide in corrosion inhibition of carbon steels under severe sour gas flow conditions.

EXPERIMENTAL

Material The material tested in this work was a carbon-manganese low alloy steel (composition: C

0.39, Si 0.09, Mn 1.38, P 0.016, S 0.039, A1 0.017, Cu 0.04). Specimens of50xl0x2 mm were used in rotating cage autoclave experiments 17,~s Prior to the corrosion experiments the specimens were degreased successively in petroleum ether and acetone (5 rain. each in an ultrasonic bath). The coupons were then pickled for 5 rain. in 10% HC1, neutralized with alkali, rinsed with water and methanol, dried in a dessiccator and weighed.

Corrosion Environment The corrosion medium consisted of brine (150 g/L NaCI+140 g/L CaC12) and

brine/hydrocarbon mixture (90:10, 70:30 and 50:50 vol/vol). The hydrocarbon phase was composed of 90 vol-% 2,2,4 trimethylpentane and 10 vol-% of xylenes (isomeric mixture). The tested medium was pressurized with H2S yielding total pressures of 25-40 bar (2.5 to 4.0 Mpa) at the desired temperature.

Inhibitors tested To compare the performance of the selected environmentally friendly corrosion inhibitors

with conventional inhibitors, a comrnericaUy available condensation product from polyamines and higher molecular weight carboxylic acids (dissolved in high boiling hydrocarbon fraction) was used which is known for its high efficiency in sour gas service ( in the following called inhibitor A). The environmentally friendly inhibitors included the following substances which were used in a concentration of 1000 ppm unless otherwise stated.

-Sodium salt of polyaspartic acid (Na-PAA). is a commercially available polypeptide with high biodegradabi~i+ty 12,~3+. Na-PAA (Formula 1)

It is a water soluble dispersent and sequestriant agent with high complexing activity for Ca , Cu , Fe 2+, Fe 3+ and Cr 3÷. This makes it useful as scale inhibitor and complexing agent for heavy metals. Polyaspartates have also been shown to have corrosion inhibition activity 6,~4,z5, particularly for CO2 corrosion environments frequently found in oil field application.

o o /~\ /'c'\ / c \ --NH--~H NH.--~H NH--~H NH--

CH2 CH2 CH2 I I I

COONa COONa COONa

Poly (L-Aspartic acid sodium salt) (1)

-Polysuccinimide (PSI) PSI (Formula 2) is the primary polymerisation product produced as intermediate in PAA

synthesis from maleic acid derivatives. Hydrolysis of PSI yields PAA with c~- and 13- bonded aspartic acid units 12. PSI is also soluble in water and exhibits complexing and antiscalant properties as well 12,13

/ c \ CH I CH2

~H N ~ C H N I I I

o O O Polysuccinimide (2)

-Polysuccinic acid octadecylamide (1:1 and 3:1) (PSI/ODA) PSI-ODA (1:1 and 3:1) compounds (Formula 3) were prepared from PSI and octadecylamine

in molar ratios of 1:1 and 3:1, respectively, using a general procedure described in the literature 16. It is a solid, insoluble in water and soluble in the hydrocarbon.

9 Oll ~--NH- CH2- (CH2)iB-CH3

~H NH I NH ~ C H CH2

I ~ C H 2

I ° C NH- CH2- (CH2)le-CH3 II o

Polysuccinic acid octadecylamide (3)

-Polysuccinic acid dodecylamide (1:1 and 3:1) (PSUDDA) PSI-DDA (1:1 and 3:1) compounds (Formula 4) were prepared from PSI and dodecylamine in

a molar ratio of 1:1 and 3:1, respectively.

£ ~--NH- CH2- (CHz)~o-CH3

N H ~ H

NH ~ C H CH2 I CH2

I o i • NH- CH~ (CH2),o-CH3 O

Polysuccinic acid dodecylamide (4)

-Potassium lactobionate (K-LA) Potassium lactobionate (Formula 5) is the potassium salt of lactobionic acid (4-O-8- D-

galactonic pyranosile-D-gluconic acid). This acid's molecular structure includes gluconic acid and galactose, two essential constituents of human nutrition. These two components are also contained in the lactose itself in molecular form (4-0-8 -D-galactonic pyranosile-D-glucose). In the human metabolism, intestinal bacteria metabolize the molecule through fermentation in the small intestine, making potassium available for the body. K-LA is completly compatible with marine environments.

,

u = " i' OH H 3 ~- -~ !.o., Y'c H H

Potassium lactobionate (5)

-Lactobionic acid cocosamide (LCA) LCA (Formula 6) is a water soluble (30 g/100 ml H20) white powder. It is a reaction product

oflactobionic acid (LA) and cocos amine containing predominantly C]2 carbon chains.

CH2OH

H

Lactobionic acid cocosarnide (6)

-Lactobionic acid tailowamide (LTA) LTA (Formula 7) it is the reaction product of LA and taUowamine containing predominantly

C18 carbon chains. 3 g of LTA are soluble in 100 ml water.

CH2 OH

Lactobionic acid tallowamide (7)

-Lactobionic acid oleylamide (LOA) LOA (Formula 8) is the reaction product of LA and oleylamine which contains predominantly

an unsaturated C~s chain. Due to the double bond the solubility in water (20g/100 rnl water) is higher than the solubility of LTA. All lactobionic derivatives are easily biodegradable and highly environmentally friendly.

CH2OH HIo H oHH \/(CH2) ' OH

Lactobionic acid oleylamide (8)

Apparatus The rotating cage experiments were performed in a two liter Alloy C4

arrangement described earlier 17,~ s autoclave in an

Evaluation of experiments At the end of the exposure time the specimens were washed with distilled water, then dried by

methanol. The appearance of the specimens was documented photographically and the scale morphology was then investigated in the scanning electron microscope (SEM). The scale composition was analzyed qualitatively by EDX. The scale thickness was measured by microscope or by SEM depending on the scale thickness. After pickling of the specimens with inhibited l0 % HC1, the surface appearance was documented photographically and the integral corrosion rate was determined by weight loss measurements. The diameter and depth of pits were measured using a microscope.

RESULTS AND DISCUSSION

Performance in hydrocarbon free brine. Figures 1 and 2 summarize the performance of the inhibitors tested at 90 to 130 °C. All

substances except polysuccinimide reduced the material loss considerably, compared to the blank experiment. However, only some of them were able to prevent localized attack. All lactobionic acid derivatives could not suppress FILC in the concentration applied (1000 ppm) (Figure 1). The commercial inhibitor A exhibited the best performance both in reducing the corrosion rate and prevention of FILC (Figure 1). However, it has to be considered that the concentration of inhibitor A was 3000 ppm instead of 1000 ppm in all other cases.

Interesting results were obtained by modifying polysuccinimide with dodecylamine and octadecylamine. In all cases only uniform corrosion was found (Figure 2). The protectivity of PSI/DDA compounds increased with increasing PSI/DDA ratio. In case of PSI/ODA combinations the PSI/ODA ratio had less significant effect. The sodium salt of polyaspartic acid showed some inhibitive efficiency, however, seemed to stimulate localized attack (Figure 2).

The best performance in this series of experiments with 1000 ppm of inhibitor was found with PSI/DDA(3:I). Therefore, it was investigated how the efficiency would change if the concentration would be reduced. It appeared (Figure 3) that this modified PSI derivative also performed well at 250 ppm and effectively prevented FILC at this concentration.

The presence of the inhibitor affected the morphology of iron sulphide corrosion product scales considerably. In case of no or poor inhibition the scale was thick, rough and spongy with poor adherence to the surface. The appearance was graphite like gray. In case of good inhibition, the scale was thin, smooth and adherent to the surface. Figure 4 summarizes the dependence of the scale thickness on the temperature and the nature of inhibitor. The best performance, i.e. the smallest scale thickness and the best protectivity exhibited those the scales produced in the presence of inhibitor A.

Performance in the presence of hydrocarbon phase The presence of a hydrocarbon (HC) phase generally increased the efficiency of the inhibitors

(Figure 5). Only 10 vol-% HC was enough to show significant effects. However, higher hydrocarbon cuts did not promote the performance any further. Therefore, in further experiments with brine/hydrocarbon mixtures the HC content was held constant at 10 vol-%. For this kind of medium Figures 6 and 7 give the results at 90, 110 and 130 C. The benficial effect of HC increased with increasing carbon chain length of the amine moiety in lactobionic acid derivatives (Figure 6), both with regard to corrosion rate and localized attack. No FILC was observed with LOA, K-LA and inhibitor A (Figure 6).

Excellent performance was also observed with the modified PSI derivatives (Figure 7). They gave low general corrosion rates ( < 0.2 mm/y ) and prevented FILC. PSI/DDA (3:1) performed well also at a concentration of only 250 ppm.

The presence of hydrocarbon also affects the morphology and thickness of the sulphide scale (Figure 8). In the series with lactobionic acid derivatives the lowest thickness was obtained with LOA. As the long alkyl chain improves the solubility in the HC phase this result is in agreement with expectations.

The effect of temperature on the scale thickness in media with 10 vol-% HC cut is significant in the absence of inhibitor additive. The presence of inhibitor reduces the temperature dependence of the scale thickness (Figure 9).

CONCLUSIONS

Environmentally friendly (EF) inhibitors based on derivatives of lactobionic acid, polyaspartic acid and polysuccinimide can be effective in highly sour brine and brine/hydrocarbon media. The amide derivatives with C12 to C18 alkyl group proved to reduce the corrosion rate and the localized attack in a way comparable to the excellent performing inhibitor A, which, however, belongs not to the group ofEF inhibitors.

Thus, partial amidation of polysuccinirnide yielded excellent inhibitors which reduce the corrosion rate to 0.10 mm/y and prevent FILC up to 130 °C when used in a concentration of only 250 ppm.

It is expected that the amidation ofpolyaspartic acid will also produce EF inhibitors with good performance.

REFERENCES

. G. Schrnitt, C. Bosch, M. Mailer, "Modelling the Probability of Flow Induced Localized Corrosion from Critical Hydrodynamic Data and Fracture Mechanics Data of Scales from CO2 Corrosion of Steel", European Federation of Corrosion, Publication No. 26, "Advances in Corrosion Control and Materials in Oil and Gas Production", The Institute of Materials, London, 1999, 24-51.

.

.

.

.

am

7.

.

.

10.

11.

12. 13.

14. 15.

16.

17. 18.

G.Schmitt, "Hydrodynamic Limitations of Corrosion Inhibitors Performance", Proc. 8th European Syrup. Corrosion Inhibitors (SEIC), Ann. Univ. Ferrara, N.S., Sez. V, Suppl. N. 9, 1995, Vol. 2, 1075-1099. G.Schmitt, W. Bruckhoff, "Corrosion Inhibition in Sour Gas Wells", Proc. 5th European Symp. on Corrosion Inhibitors (SEIC), Ann. Univ. Ferrara, N.S., Sez. V, Suppl. N. 7, 1980, 323. G.Schmitt, G. Lohe, W. Bruckhoff, "Influence of Corrosion Products on the Effeciency of Inhibitors in Sour Gas", CORROSION'87, National Association of Corrosion Engineers, Houston, Texas, 1987, Paper No. 33.

G.Schmitt, W. Bruckhoff, "Inhibition of Low and High Alloy Steels in the System Brine/Elemental Sulfilr/H2S", CORROSION'89, National Association of Corrosion Engineers, Houston, Texas, 1989, Paper No. 620. J. McMahon, D. Harrop, " Green Corrosion Inhibitors: An Oil Company Prespective" CORROSION' 95, NACE International, Houston, Texas, 1995, Paper No. 32. W. P. Singh, J.O'M. Bockries, " Toxicity Issues of Organic Corrosion Inhibitors Application of QSAR Model." CORROSION'96, NACE International, Houston, Texas, 1996, Paper No. 225. D.C. Silverman, D.J. Kalota, F.S. Stover," Effect ofpH on Corrosion Inhibition of Steel by Polyaspartic Acid", CORROSION'95, NACE International, Houston, Texas,1995, Paper No. 34. V. Javancicevic, D. Hartwick, "Recent Development in Environmentally-Safe Corrosion Inhibitors, "CORROSION'96, NACE International, Houston, Texas, USA, 1996, Paper No. 226. D. Darling, R. Rakshapal, "Green Chemistry Application to Corrosion and Scale Inhibitors", CORROSION'98, NACE International, Houston, Texas,1998, Paper No. 207. G. Banerjee, A. Banerjee, C.S. Shah, "Environmentally Acceptable Corrosion Inhibitors for Carbon Steel '% CORROSION'99, NACE International, Houston, Texas,1999, Paper No. 100. M. Schwambom, Nachr. Chem. Tech. Lab. 44 (1996) 1167-1170. R.J. Ross, K.C. Low, J.E. Shannon, "Polyasparatate Scale Inhibitors-Biodegradibility

Alternatives to Polyacrylate", CORROSION'96, Houston, Texas, 1996, Paper No. 162. D. Kalota, D.C. Silverman, U.S. Patent 4,971,724. W. Hater,"Environmental Compatible Scale Inhibitor for the Mining Industry", CORROSION'98, Houston, Texas, 1998, Paper No. 213. H.N. Kovacs, J. Kovacs, M.A Pisano, B.A. Shidlovsky, J. Med. Chem. 10 (1967) 904- 908. G. Schmitt, W. Bruckhoff,K F~ssler, G.Bliimmel, Mater. Perform. 30 (1991) 85-90. G.Schmitt, W. Bruckhoff, "Relevance of Laboratory Experiments for Investigation and Mitigation of Flow Induced Corrosion in Gas Production", CORROSION' 88, National Association of Corrosion Engineers, Houston, Texas, 1988, Paper No. 357.

• 3.0

2.5

2 .0 =o ") 1.5

.~ 1.0

o.5 0.0

I

• B L a n k • LCA 'ql LAT

A L O A 0 A V K - L A i

Open symbols : No FILC

• ' Closed symbols : Lmax < 1000 p.m

.' m ~ m |

90 110 130 Temperature / °C

FIGURE 1 - Performance of inhibitors at 90 to 130 °C in hydrocarbon-free brine. ( Lmax: blank :130- 180 ~m; LCA: 60-1401.tm ; LOA:130-1000 pm K-LA 70-90 pm; inhbitor A" no FILC).

Blank ~ Na-PAA F-"-] PSI/ODA 3:1

U PS1 ~ PSI/DDA 1:1 ~ PSI/ODA 1:1 i i

• 3.0 - b

- 2 . 5 •

2 . 0 ' - t = !, t ~ 1.5 ~ i o

0 . 5 - , , ;

- 0 . O _ m a t u r e . . _ - - mm~l.m l.~,[p.rn] 180 p00 290 0 0 , 0 0 130160 9 0 1 0 0 0 0

temp./°C 110 130

FIGURE 2 - Performance of modified polysuccinimide in 100% brine at 110 and 130 °C.

4 . 0

3.5

3.0

2 .5

• 2 .0

1.5

1.0

0.5 "

0 .0

I PSI/DDA (3:1) [

I I I I

2 5 0 5 0 0

I I

-PSI Lmax= 100 ~m

1000 Inhibitor coneenlration (ppm)

FIGURE 3- Effect of concentration of modified PSI/DDA (3:1) in 100 % brine at 110°C. (The modified inhbitors prevented FILC completely).

m Blank ,~k LOA ~ LTA - - LCA 0 A ~ ' K-LA ~ P S I

. J

90 " Open symbols: No FILC ~ J V -.><i 8 0 " Closed symbols : Ltaax < 1000 p.m | • ?0 -

6 0 - 1

5 0 "

~ 2 0 ~' _ n

9 0 110 130 Temperature / °C

FIGURE 4- Effect of temperature on the scale thickness in 100 % brine.( Lm~" blank • 80- 130pm; LOA : 130-10001~m ; LCA" 60-140 ~m ; K-LA :70-90 l.tm; LTA: 40-60 ~m ; inhibitor A prevented FILC completely).

3.0

2.5

g 2.0

1 .5

I I Blank ,A LOA O L C A 0 A

Ol~m symbols: N FILC Closed symbols: IJr~,x < 290

.~ 1.0

~0.5 0.0

100 90:10 70:30 50:50 Brine/Hydrocarbon Ratio

FIGURE 5- Effect of brine/hydrocarbon ratio on the performance of the inhbitors at 110 °C.

• 3.0

2.5

2.0

1.5

1.0

~0 .5 ~ 0 . 0

~ Blank V K-LA gk LOA LCA O A <1 LTA

i i i i i i i

Open symbols: No FILC • Closed symbols: Ian~_180 pm i

• t

• " |

90 110 130 Temperature/°C

FIGURE 6- Effect of temperature on the inhibitors performance in 90: I 0 brine/hydrocarbon. (Lmax " blank • 60-180 lam; LCA 40-60; LOA, K-LA and A prevented FILC completely).

[ ~ Blank ~ PSI/DDA(I:I) ~ PSI/ODA(I:I) ! ~ PSI ~ PSI/ODA (3:1) I I PSI/ODA !3:1)

L

{2.5 ~2.0 i 1.5 ~ 1 0

'o.5 - = 0 . 0 . V / ~ J ~ , , , , , ..... , ,,,,, ~ ,

' 0 0 0 ' LmaxIPm] 60 O0 0 BO 60 0 ' 0 0 0 i

J i I I

Temp ./°C 110 130 , L

FIGURE 7- Performance of modified polysuccinimide inhibitors in 90:10 brine/hydrocarbon at110 °C and 130 °C (PSI/DDA 3:1 concentration only 250 ppm).

80

70

60 50

40

30

20

10

• Blank 0 A LCA ~ LOA

i v

Open symbol No: FILC Closed symbol: ~ , ~29o

- - m

I

100 90 :10 70 :30 50:50 Brine/Hydrocarbon Ratio

FIGURE 8 - Effect of brine/hydrocarbon ratio on the scale thickness.

I I I

| II Blank ~ L O A V K-LA

80 t Open symbol: No FILC ~ ~ 70 F c~o~ symbo,: t~,~_,s~

• 60 ]y n ~ !:0I ~°t z x ~ ~ - - - - ~ I

0 90 110 130

Temperature / °C

FIGURE 9 - Effect o f temperature on the scale thickness in 90:10 brine/hydrocarbon.