2. Related Art Diesel engines are widely utilized for providing power for trucks, ships and construction machines as well as for use in electrical power generation. In an effort to abate air pollutants such as nitrogen oxides (NOx) present in exhaust gases produced by diesel engines, government regulations may soon require the recirculation of exhaust gases in diesel engines. An exhaust gas recirculation system (EGR) may be utilized as a means of controlling NOx emissions, but also produces an environment within the engine that is characterized by increased corrosive potential compared to a non-EGR combustion system. Such an EGR system would likely introduce organic acids and other corrosive elements into the combustion chamber and could present a corrosive environment that would be harmful to conventional ferrous pistons.

U. S. Patent No. 5,450, 784 discloses a piston having an electro- deposited tin-based plating for wear resistance. There is no disclosure as to corrosion resistance to an EGR environment.

It is an object of the present invention to provide a ferrous piston which is resistant to attack by a corrosive EGR environment of a diesel engine.

SUMMARY OF THE INVENTION A piston for a diesel engine having exhaust gas recirculation (EGR) system comprises a piston body fabricated of ferrous metal and a coating of electroless nickel formed on at least a portion of the piston body providing a continuous, non- porous barrier to the EGR environment.

The piston of the present invention has the advantage of providing a continuous barrier layer or coating that protects the ferrous piston from the corrosive EGR environment.

The piston has the further advantage of providing a continuous barrier layer that does not require additional machining or finishing after the barrier layer has been applied. Because there is no additional machining, the coating remains continuous to protect the piston from the corrosive EGR environment.

The electroless coating can be applied in a uniform, thin layer such that the dimensional tolerances of the piston can be maintained without the use of further finishing operations after the coating has been applied. This arrangement allows for an economical corrosion resistant part to be produced without complex machining of the piston or the cylinder of a diesel engine to accommodate the thickness of a coating.

THE DRAWINGS These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: Figure 1 is a cross section of a piston constructed according to a first embodiment of the present invention.

Figure 2 is a cross section of a piston constructed according to a second embodiment of the present invention; and Figure 3 is a schematic sectional view of a diesel engine having the subject piston.

DETAILED DESCRIPTION With reference to Figure 1, there is shown a first embodiment of the corrosion resistant piston 5 for use in an exhaust gas recirculation (EGR) diesel engine.

The piston 5 includes a piston body 10 and a pair of pin bosses 15.

The piston body 10 includes a ring belt 13 having a plurality of ring grooves 20. The ring grooves 20 are separated by a plurality of ring lands 25.

The pin bosses 15 extend downwardly and are integrally formed as one piece with the piston body 10. The piston body 10 is fabricated of ferrous metal, such as steel which is prone to corrosive attack if exposed to the EGR environment of the diesel engine.

In a first embodiment of Figure 1, the piston body 10 includes an EGR coating 30 formed on a portion of the piston body, including the ring belt 13 and its associated ring grooves 20 and lands 25. The coating 30 comprises an electroless coating of nickel having a maximum thickness of about 8 micrometers. The coating 30 serves as a barrier layer which protects the coated portions of the steel piston body 10 from attack by the corrosive EGR environment.

Electroless nickel plating, also known as chemical or auto catalytic nickel plating, chemically deposits the nickel on the piston body 10 without the use of an external current source (i. e. , non-electroplated). The coating operation is based

upon the catalytic reduction of nickel ions on the surface being coated. electroless nickel deposition applies the EGR coating 30 at a uniform rate and depth and is relatively insensitive to surface geometry, unlike electrolytic plating which varies the current density and thus coating depth and is not capable of applying such a thin, uniform, continuous, non-porous coating as achieved with the electroless coating of the invention. The electroless nickel coating 30 preferably has a thickness ranging from about 3 to 8 micrometers and more preferably of about 5 micrometers. At a thickness of less than about 3 micrometers, the coating 30 does not provide adequate barrier protection to the piston body 10. At thickness greater than about 8 micrometers, the coating interferes with the dimensional tolerancing of the piston body 10 and would require machining or finishing operation after the coating has been applied.

After the coating 30 is applied, no further machining or finishing of the coated surface is conducted, at least in the areas where corrosion protection is desired, such that the coating 30 remains continuous and non-porous to maintain its protective qualities. The piston 5 is machined or finished to its final specifications before the electroless nickel coating 30 is applied. Because of the uniform nature of the electroless nickel coating 30, as well as the relatively thin character of the coating, no further machining or finishing of the coating 30 is required before the piston 5 is installed in an engine.

With reference to Figure 2, there is shown a second embodiment of the present invention in which the piston 5 is the same as that disclosed in the first embodiment but with the coating 30 applied to the entire piston body 10 including the

pin bosses 15.

Figure 3 shows a diesel engine in which the piston 5 is installed. A cylinder block 35 of the engine includes at least one cylinder in which the piston 5 is accommodated for reciprocation. The block 35 mounts a cylinder head which has intake and exhaust ports 40,45 communicating with the cylinder. Exhaust gases exiting the exhaust port are processed through the EGR system 55 which recirculates at least a portion of the combusted exhaust gases back into the cylinder for reducing combustion temperature. The EGR environment is corrosive to the ferrous piston 5 and is protected from attack by the coating 30. Other ferrous power cylinder components exposed to the EGR environment are also subject to attack and may be protected by the coating 30 in the same manner described above in connection with the piston 5.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.