Corrosion resistant carbon steel

Corrosion of conventional refinery steels due to sulfur bearing, carboxylic acid containing hydrocarbon materials is minimized by forming on the surface of the steel a fine grain iron sulfide film where at least the steel surface is substantially all of a pearlite microstructure.

FIELD OF THE INVENTION 
This invention relates to a composition and a method for handling corrosive 
hydrocarbon materials, such as crude oils and distillates. More 
particularly, this invention relates to a composition that minimizes the 
corrosion of carbon steels normally used in the handling of hydrocarbon 
containing materials. 
BACKGROUND OF THE INVENTION 
Carbon steels used in refinery service undergo corrosion by sulfur 
compounds and carboxylic acids, especially naphthenic acids, present in 
crudes and distillate fractions. Many newly available crudes have high 
concentrations of these corrosive species, and as a result, carry a lower 
cost on a per barrel basis. Consequently, there is an economic incentive 
in processing these crudes, provided that the material costs, because of 
corrosion, can be minimized. Highly alloyed steels, such as 316 or 317 
stainless steel, are an approach, but these materials are so expensive as 
to make the processing of acidic, sulfur beating crudes too costly. 
Therefore, a real incentive exists for processing crudes and distillates 
containing sulfur and naphthenic acids with relatively inexpensive 
materials. 
SUMMARY OF THE INVENTION 
In accordance with this invention, hydrocarbon materials, e.g., crudes and 
distillate fractions, containing carboxylic acids, such as naphthenic 
acids, or sulfur compounds or both can be readily processed over carbon 
steel, plate or pipe, having a FeS (pearlite) surface layer of at least 
about 300 Angstroms. This fine-grained FeS film has a grain size of about 
500 Angstroms or less. Such an FeS film is formed by exposing carbon 
steel, at least the surface of which has a substantially pearlitic 
microstructure, to hydrocarbon materials containing sulfur preferably at a 
level of at least about 0.1 wt. pct. The pearlite/FeS composite surface 
layer acts to inhibit or substantially minimize corrosion by sulfur 
compounds or carboxylic acids or both in hydrocarbon materials containing 
either or both of the same. The thickness of the pearlitic region is 
preferably at least about 20 microns. The protective FeS film may also be 
formed by subjecting the steel having at least a substantially surface 
pearlitic structure to sulfur containing compounds such as hydrogen 
sulfide.

For the purposes of this invention, the term FeS (ferrite) will refer to a 
conventional steel which has a microstructure which is predominantly (70 
vol. pct. or more) ferrite, the balance being pearlite. The term FeS 
(pearlite) will refer to FeS formed on a steel microstructure that is at 
least about 70 vol %, preferably at least about 90 vol % pearlitic. 
Ferrite is a phase which has a body-centered cubic crystallographical 
structure, and is primarily iron with some dissolved carbon. Pearlite is a 
microstructural constituent that is made up of alternate layers of ferrite 
and cementite (Fe.sub.3 C). 
Steels used in refinery service are usually comprised of pearlitic and 
ferritic microstructural constituents, ferrite being the more predominant 
microstructure. Ferritic grains, however, are easily attacked by 
carboxylic acids in liquid hydrocarbons. The FeS layer that forms on the 
ferritic steel in the presence of carboxylic acid-containing liquid 
hydrocarbons is generally not very protective. Fe atoms from ferrite will 
migrate through such an FeS film and will be attacked by the hydrocarbon 
phase, leading to ultimate deterioration of the steel. On the other hand, 
the FeS film formed on a steel surface that is predominantly pearlitic is 
fine grained and compact. In such a fine grained sulfide film, Fe atom 
transport is suppressed because fine grains are generally free from 
defects that promote atomic migration. Therefore, iron loss from the steel 
is considerably minimized leading to high corrosion resistance for FeS 
(pearlite) films. 
The grain size of FeS grains when the steel is pearlitic is .ltoreq.500 
Angstroms, preferably .ltoreq.400 .ANG., Angstroms, and more preferably 
less than about 300 .ANG.; while the grain size of FeS grains when the 
steel is predominantly fenitic is .gtoreq.1 micron, e.g., 1-2 microns. 
(One 1! micron is equal to ten thousand 10,000! Angstroms.) 
The protective nature of the FeS (pearlite) film starts when the film is at 
least one grain in thickness, e.g., .gtoreq.300 Angstroms, more preferably 
where the film is .gtoreq.400 .ANG., still more preferably when the film 
is equal to or greater than about 500 Angstroms. More preferably, the 
protective FeS (pearlite) film is at least about 0.1 microns, still more 
preferably at least about 0.5 microns, and yet more preferably at least 
about 1 micron, for example, 1-2 microns, but generally no more than about 
2 microns. 
Preferred carbon steels used in this invention have carbon content at least 
at the surface and preferably throughout, of at least about 0.7 wt % 
carbon, more preferably .gtoreq.0.75 wt %, still more preferably about 
0.75-1.0 wt % carbon. Of course, other alloying agents may be present to 
provide other qualities useful in steels for refinery service. Other 
alloying agents may be Mn, Si, etc. A widely used refinery steel, for 
example, contains 0.14-0.20 wt %. C, 0.6-0.9 wt %. Mn, 0.035 wt %. P, 0.04 
wt %. S, the remainder being iron. A silicon containing grade, has in 
addition, 0.1-0.5 wt % Si. 
Conventional steels may be transformed from ferrite-pearlite microstructure 
to essentially all pearlite microstructure by known techniques. For 
example, a conventional ferritic-pearlitic steel with at least 0.7 wt % C 
may be heated to the austenitic recrystallization range, above about 
900.degree. C., for about an hour or more, after which the steel is 
transferred to an oven at about 675.degree. C. for at least 1/2 hour or 
more where pearlitic transformation begins to take place. The steel is 
allowed to cool in the oven, cooling is quite slow, the result being 
essentially complete transformation to pearlite. For the present 
invention, the pearlitic microstructure is preferably .gtoreq.90%, more 
preferably .gtoreq.95%, and still more preferably .gtoreq.99%, and most 
preferably 100% pearlite. 
Alternatively, only the surface e.g. at least about 1 micron thickness, of 
the steel need be of the pearlitic microstructure. Pearlitic surface 
layers can also be formed by well known methods in a carburizing 
environment; for example, heating the steel to .gtoreq.900.degree. C. in 
the presence of appropriate mixtures of methane and hydrogen which allows 
carbon to diffuse into the steel surface. Preferably, the carburized steel 
is heated to the austenitic recrystallization temperature and cooled, as 
described above, to allow the formation of a surface layer of pearlitic 
microstructure. 
In the case of forming a surface layer of pearlite, only the surface layer 
need contain carbon of .gtoreq.0.7 wt %, and preferably the carbon 
contents mentioned above, in order for the pearlitic microstructure to 
form on the surface. 
By virtue of this invention, hydrocarbon materials such as crudes, topped 
crudes, atmospheric resids, vacuum resids, etc. may be processed over a 
steel, at least the surface of which contains the desired pearlitic 
microstructure. The FeS (pearlite) film will form and act to protect the 
steel from sulfur or carboxylic acid corrosion. Generally, these 
hydrocarbon containing materials have at least about 0.1 wt % sulfur and 
up to about 4 wt % sulfur if the material is a whole or topped crude. 
Carboxylic acids, including naphthenic acids, when present in these 
materials are usually described by their total acid number (TAN). The TAN 
of a material is determined by the number of milligrams of potassium 
hydroxide (KOH) necessary to neutralize the acids in one gram of material. 
This invention can effectively accommodate materials having a TAN ranging 
from about 0.1-8; however, the incentives for using this invention 
increase as the TAN increases, and the invention is particularly 
applicable to materials having a TAN in excess of about 2.0, preferably in 
excess of about 3.0. 
The following examples will serve to illustrate, but shall not be construed 
as limiting, this invention: 
A conventionally used carbon steel material 1018 which is made up of a 
mixture of ferrite and pearlite phases was subjected to corrosion by a 
crude containing a relatively large concentration of naphthenic acid (TAN 
.about.8) and a relatively smaller concentration of organic sulfur 
molecules (.about.0.12%). The test was carried out in a stirred autoclave 
at a temperature of 600.degree. F. for a period of 20 hours. Two separate 
tests that were carried out show excellent reproducibility. The average 
corrosion rate was 130 mils per year (mpy). 
Carbon steel containing 0.75% carbon was purchased and appropriate heat 
treatment (consisting of heating the steel to 900.degree. C., maintaining 
at this temperature for 1/2 hour, quenching to 675.degree. C., holding at 
this temperature for 1/2 hour and furnace cooling to room temperature) 
carried out to form a fully pearlitic microstructure. Electron microscopy 
studies confirmed that a structure that is completely pearlite was formed. 
The steel was subjected to the same corrosion tests as the conventional 
carbon steel. Again, a set of two experiments was carried out. The results 
were reproducible and showed an average corrosion rate of 20 mpy. Thus the 
use of fully pearlitic carbon steel reduced the corrosion rate by a factor 
of 6.5. 
The above results are shown in FIGS. 1 and 2. In the conventional steel 
some sulfidation has occurred, but more importantly, grains have been 
pulled out by naphthenic acid attack. Also, the ferritic grains tend to 
dissolve whereas the pearlitic grains in the conventional steel remain 
resistant to further attack. The fully pearlitic steel microstructure 
shows low corrosion rate and remains intact after corrosion, only a thin 
sulfide film is evident on the surface. Corrosion by naphthenic acid has 
been significantly suppressed and that by sulfin species reduced.