Sucker rod oil well pump

A method is shown for constructing portions of a sucker rod pump of the type having a steel plunger with an exterior cylindrical wall and a barrel with a cylindrical bore. A chromium case if formed on the cylindrical bore to a selected depth for receiving the plunger in sliding contact. The chromium case is formed by forming a base electrolyte bath including water, chromic acid, a sulfate component, an alkyl sulphonic acid, and an anion of molybdenum. The bore is then exposed to the aqueous electrolyte bath at a current density and at a plating temperature sufficient to form a chromium deposit of desired thickness on the bore.

BACKGROUND OF THE INVENTION 
1. Cross-reference to Related Applications 
The present application is related to the co-pending application of Thomas 
S. Wilmeth, Steven L. Wilmeth and Frances E. Foster entitled METHOD FOR 
MAKING SUCKER ROD OIL PUMP, U.S. patent application Ser. No. 07/383,019, 
filed Jul. 19, 1989. 
2. Field of the Invention 
This invention relates generally to sucker rod oil well pumps of the type 
having hardened metal layers on the plunger and barrel components thereof 
and, more specifically, to an improved chrome plating process for 
depositing a chromium deposit of desired thickness on the barrel of such a 
sucker rod pump. 
3. Description of the Prior Art 
Sucker rod well pumps have been in wide use for many years. A barrel is 
mounted to the tubing. The barrel has a smooth cylindrical bore. A plunger 
is positioned inside the tubing and connected to the sucker rod. The 
sucker rod is reciprocated up and down to move the plunger in the barrel. 
A stationary check valve is located at the bottom of the barrel. A 
traveling valve is located at the bottom of the plunger. The valves 
cooperate on the downstroke and upstroke to pump fluid from the well to 
the surface. 
The mating surfaces between the plunger and barrel provide a very close 
fit. In wells which produce abrasive particles, such as quartz or sand, 
these particles can quickly damage the mating surfaces. The wear due to 
the abrasive particles will cause leakage past the plunger. This 
necessitates the pump being pulled for replacement at fairly frequent 
intervals. 
Various techniques have been used in the past to increase wear resistance. 
In one technique, a chrome case is plated on either of the barrel or the 
plunger. Circumferential grooves have also been formed in the plunger. 
In the previously referenced application entitled METHOD FOR MAKING SUCKER 
ROD OIL WELL PUMP, U.S. patent application Ser. No. 07/383,019, filed Jul. 
19, 1989, a sucker rod pump is shown having hardened layers to increase 
wear resistance. The barrel has a chromium case plated on its bore and a 
plunger which has a boronized case. The plunger also has circumferential 
grooves spaced-apart from each other. The chromium case is plated in a 
conventional manner, except that it is about twice the thickness of the 
chromium cases placed in conventional barrels of sucker rod pumps. 
Preferably, the layer is from about 0.006 to 0.008 thickness on each side. 
Despite the previous advances, a need exists for a process for improving 
the wear resistance of the components of a sucker rod well pump in order 
to extend the useful life thereof. 
SUMMARY OF THE INVENTION 
In the present invention, a method is shown for chrome plating a workpiece, 
such as a component of a sucker rod pump, from an aqueous electrolyte 
bath. The method includes the steps of forming a base electrolyte bath by 
combining water, chromic acid, a sulfate component and an alkyl sulphonic 
acid. The method also includes the step of adding a molybdenum anion such 
as ammonium molybdate or any other suitable molybdenum compound to produce 
an anion to the base electrolyte bath. The workpiece is then exposed to 
the aqueous electrolyte bath at a current density and at a plating 
temperature sufficient to form a chromium deposit of desired thickness on 
the workpiece. 
In the preferred method of constructing portions of a sucker rod pump for a 
well, a steel plunger is provided having an exterior cylindrical wall. A 
barrel is provided having a cylindrical bore. A chromium case is formed on 
the cylindrical bore of the barrel to a selected depth for receiving the 
plunger in sliding contact. The chromium case is formed by forming a base 
electrolyte bath comprising water, chromic acid, a sulfate component, an 
alkyl sulphonic acid, a molybdenum anion, and by exposing the bore to the 
aqueous electrolyte bath a current density and at a plating temperature 
sufficient to form a chromium deposit of desired thickness on the bore. 
The alkyl sulphonic acid is preferably a saturated aliphatic sulphonic acid 
having a maximum of two carbon atoms and a maximum of six sulphonic acid 
groups, their salts or halogen derivatives thereon. Most preferably, the 
sulphonic acid is methylsulphonic acid. The bath temperature is maintained 
in the range from about 20.degree. to 70.degree. C. and the current 
density is maintained in the range from about 50 to 100 A/dm.sup.2. By 
using pulsed D.C. current and adding boric acid or a borate to the base 
electrolyte bath, the efficiency of the plating process can be further 
improved. 
Additional objects, features and advantages will be apparent in the written 
description which follows.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the sucker rod pump 11 includes a barrel 13. The 
barrel is a thin wall tubular member. It has a cylindrical bore 15. 
Normally, the barrel 13 is formed of steel. As will be discussed in 
greater detail, an improved case is plated on the bore 15. 
A standing valve 17 is located on the bottom of the barrel 13. A ball 19 is 
part of the standing valve 17. A hold-down 21 is located on the upper end 
of barrel 13 in the embodiment shown. The hold-down 21 is not shown in 
detail, however, it has latches and seals for sealing releasably in a 
seating nipple 23. The seating nipple 23 is connected into the string of 
tubing 25. Barrel 13 is thus located within the string of tubing 25. A 
cage 27 is located on the upper end of the hold-down 21. Cage 27 has ports 
29 to allow well fluid to flow into the tubing 25 and to the surface. 
A plunger 31 is reciprocally carried inside the barrel 13. The clearance 
between the plunger 31 and the bore 15 is very small. Plunger 31 is a 
tubular steel member. It has a traveling valve 33 located on the lower 
end. Traveling valve 33 includes a ball 35. The upper end of plunger 31 is 
connected to the lower end of a string of sucker rod 37 that extends to 
the surface. Sucker rod 37 passes through the cage 27. 
With reference to FIG. 2, the plunger 31 has a cylindrical wall 39. A 
plurality of parallel circumferential grooves 41 are preferably formed in 
the wall. Grooves 41 are not shown to scale in FIG. 2. Preferably, each 
groove 41 is about 1/8 inch in vertical dimension and 1/8 inch depth. 
Preferably there are two grooves 41 near the top of plunger 31 and two 
near the bottom of plunger 31. The grooves 41 in each pair are preferably 
about 3/4 inch apart. There may be other grooves 41 along the length of 
the plunger 31 between the pairs at the top and the bottom. Each of these 
other grooves 41 are preferably about 6 to 8 inches apart from each other. 
After the grooves 41 are formed and before any hardening, a boronized case 
is formed on the cylindrical wall 39. The boronized case is formed in a 
conventional manner by fluidized bed techniques. In this technique, as is 
known to those skilled in the art, the plunger 31 will be heated to about 
1,800.degree. F. For about four hours while immersed in a fluidized bed 
containing boronizing powder. This produces a boronized case that is about 
0.003 to 0.010 inch in thickness. The boronized case may be formed by 
other methods including packing the plunger in boronizing powder, by 
liquid boronizing or other known techniques. 
Then the plunger 31 is hardened to harden the steel supporting layer 
beneath the boronized case. The preferred technique is by using induction 
hardening, which is a known process. In induction hardening, the plunger 
31 is passed through a coil (not shown). The coil has high frequency 
alternating current passing through it. This heats a surface layer of the 
plunger 31. The rate at which the plunger 31 passes through the induction 
coil, and the power supplied to the induction coil, are controlled so that 
the temperature in a surface layer of the cylindrical wall 39 will be 
above about 1,800.degree. F. This layer extends about 0.030 to 0.050 inch 
deep measured from the exterior of the cylindrical wall 39 and supports 
the boronized case. 
A quenching ring (not shown) is located on the induction hardening 
apparatus immediately behind the coil. The quenching ring discharges water 
onto the plunger 31 to quench the heated layer and harden it. This results 
in a hardened layer of steel supporting the boronized case. The hardened 
layer is about 0.030 to 0.050 inches deep. 
The hardness of the hardened steel layer beneath the boronized case will be 
about 55 to 60 Rockwell "C." The hardness at the surface of the boron case 
will be 18 over 80 Rockwell "C" and is typically measured on the Knoop 
scale. On the Knoop scale, the hardness of the boron case will be within 
the range from about 1,500 to 1,650. 
As has been discussed, a chromium case is normally plated on the bore 15 of 
the barrel 13. In the past, the chromium case was plated in a convention 
manner, except that it was about twice the thickness of the chromium cases 
placed in conventional barrels of sucker rod pumps. The layer was from 
about 0.006 to 0.008 thickness on each side. This resulted in a hardness 
at the surface of approximately 68 to 70 Rockwell "C." 
In the method of the present invention, an improved chrome plating process 
is utilized to produce a workpiece, such as a component of a sucker rod 
pump, having improved wear resistance over workpieces plated according to 
the prior art processes. Within the scope of the present invention "metal 
alloy" particularly signifies steel (iron alloys) and aluminum alloys. 
Functional hexavalent chromium plating baths containing chromic acid and 
sulfate as a catalyst generally permit the deposition of chromium metal on 
the base metal at cathode efficiencies of between about 12% and 16% at 
temperatures between about 52.degree. C. to 68.degree. C. and at current 
densities from about 30 to about 50 A/dm.sup.2. Typical state-of-the-art 
chromium plating baths are described, for instance, in U.S. Pat. No. 
3,745,097, issued Jul. 10, 1973 and U.S. Pat. No. 4,588,481, issued May 
13, 1986. The functional chromium plating baths shown therein deal with 
regularly shaped articles where rapid plating at high current efficiency 
and at useful current densities are important. In addition to water, 
chromic acid and sulfate component, these baths include the addition of an 
alkyl sulphonic acid, such as methane sulphonic acid, to increase plating 
efficiency. 
In the process of the invention, hard chromium is deposited on workpieces 
of metal alloy from an aqueous electrolyte containing chromic acid and a 
sulfate component such as sulfuric acid, namely from the classical 
chromium bath with CrO.sub.3 content of about 150 to 400 grams per liter, 
preferably about 250 to 300 grams per liter, and an SO.sub.4 content of 
about 2 to 15 grams per liter, preferably about 2 to 4 grams per liter. 
The base electrolyte bath of the present invention also includes, as one 
component, an alkyl sulphonic acid. Preferably, the alkyl sulphonic acid 
is a saturated aliphatic sulphonic acid with a maximum of two carbon atoms 
and a maximum of six sulphonic acid groups or their salts or halogen 
derivatives. Members of the above class of organic compounds include 
methane sulphonic acid, ethane sulphonic acid, methane disulphonic acid, 
1,2-ethane disulphonic acid, salts of the above mentioned acids or halogen 
derivatives. Most preferably, the organic compound is methane sulphonic 
acid, present in the range from about 1 to 18 grams per liter, most 
preferably about 2 to 4 grams per liter. 
In addition to the above listed components of the base electrolyte bath, 
the method of the invention includes the addition of an anion of 
molybdenum such as ammonium molybdate to the base electrolyte bath in the 
range from about 10 to 100 grams per liter, most preferably about 25 to 50 
grams per liter. The addition of the molybdenum anion materially changes 
the fundamental character of the base electrolyte bath, providing a 
workpiece with improved wear resistance obtainable at high current 
efficiency and at a useful current density. 
In addition to the above listed components, the base electrolyte bath can 
also contain other enhancement additives. For instance, the base 
electrolyte bath can contain boric acid or borates in the range from about 
4 to 40 grams per liter, most preferably about 6 to 12 grams per liter 
boric acid. The addition of boric acid or borates has the effect of 
increasing the hardness and increasing the cracks per unit area from about 
500 cracks/cm.sup.2 to about 2,000 cracks/cm.sup.2 or more. Microcracks, 
instead of larger cracks, tend to increase the corrosion resistance of the 
chrome. 
The following example is intended to be illustrative of the invention 
without limiting the scope thereof: 
An electrolyte bath is prepared having the following composition: 
2-4 grams per liter methane sulphonic acid; 
2-4 grams per liter sulfuric acid; 
250-300 grams per liter chromic acid; and 
6-12 grams per liter boric acid; 
25-50 grams per liter ammonium molybdate or other molybdenum salt producing 
an anion. 
At a current density in the range from about 2 to 6 Amps/in.sup.2 and at a 
plating temperature of about 135.degree. F., a cathode efficiency of about 
18 to 20% is realized. Where about 10 to 100 grams per liter of ammonium 
molybdate, preferably about 25 to 50 grams per liter, are added to the 
plating bath, an alloy chrome is produced with about one half percent 
molybdenum and about twice the wear life of the prior art workpiece. If 
pulsed D.C. current is used, about one and one half percent molybdenum is 
deposited. 
FIG. 3 illustrates the Taber wear test data obtained from a workpiece 
prepared with the chrome plating process of the invention, the graph 
illustrating the milligrams of weight loss per 5,000 cycles on the 
vertical axis versus the amount of molybdenum added to the bath on the 
horizontal axis. The data was obtained in accordance with the procedure 
outlined in Mil-A-8625D Federal Test Method Standard 141 Method 6192. The 
optimum wear resistance appears to be obtained by adding about 25 to 50 
grams per liter ammonium molybdate. 
FIG. 4 is a graph of Knoop hardness data for a workpiece prepared according 
to the method of the invention illustrating hardness on the vertical scale 
and amount of ammonium molybdate added to the bath on the horizontal 
scale. The data was obtained in accordance with the procedure outlined in 
ASTM E-384. Again, the optimum results are obtained by adding about 25 to 
50 grams per liter of ammonium molybdate or other anion of molybdenum to 
the base electrolyte bath. 
In the preferred method of the invention, portions of a sucker rod pump for 
a well are chrome plated. Specifically, a steel plunger is provided with 
an exterior cylindrical wall. A barrel is provided with a cylindrical 
bore. A chromium case is formed on the cylindrical bore to a selected 
depth for receiving the plunger in sliding contact. The chromium case is 
formed by forming the base electrolyte bath previously described including 
water, chromic acid, a sulfate component and an alkyl sulphonic acid. The 
bath also includes ammonium molybdate or other anion of molybdenum in the 
preferred ranged specified. The barrel of the pump is exposed to the 
aqueous electrolyte bath at a current density and at a plating temperature 
sufficient to form a chromium deposit of desired thickness on the bore. 
In operation, the pump 11 is placed in the tubing 25 and secured by the 
hold-down 21 in the seating nipple 23. A pump-jack (not shown) at the well 
surface reciprocates the rod 37 and the plunger 31. As the plunger 31 
moves downward, well fluid contained in the barrel 13 will flow past the 
ball 35 into the interior of the plunger 31. On the downstroke, the ball 
19 will seat on the seat 17. On the upstroke, the well fluid contained 
inside the plunger 31 will be pushed upward through the ports 29 into the 
tubing 25. Suction created by the upward movement of the plunger 31 lifts 
the ball 19. This allows well fluid in the tubing 25 below the barrel 13 
to be drawn into the barrel 13. 
Abrasive particles tend to be trapped in the grooves 41. The grooves 41 
also help equalize hydrostatic pressure around the plunger 31. The boron 
case on the plunger 31 and the improved chromium case on the barrel 13 are 
both harder than most abrasive particles expected to be found in the well. 
An invention has been provided with several advantages. The boronized case 
and improved chromium case are harder than the particles found in the well 
and wear resistance of the pump components is greatly improved, thereby 
extending the useful life of the pump. The improved chromium plating 
process provides improved hardness and wear resistance for the pump 
component and improves the cathodic efficiency of the plating process at a 
useful current density. 
While the invention has been shown in only one of its forms, it is not thus 
limited but is susceptible to various changes and modifications without 
departing from the spirit thereof.