Introduction

METALLIC NONELECTROLYTIC ALLOY COATINGS produced from aqueous solutions are commercially used in several industries, including electronics, aerospace, medical, oil and gas production, chemical processing, and automotive. These coatings provide unique material properties that expand the physical properties beyond those of pure metal coating systems. Table 1 lists many of the properties and the coatings capable of providing these properties.

Table 1 Features and types of metallic nonelectrolytic alloy coatings

Property or use

Alloy types

Thickness range, /'m

Corrosion protection

Ni-P, Ni-P-Mo, Ni-Sn-P, Co-P, Co-P-Mo, Ni-Cu-P

12-125

Wear resistance

Ni-B, Ni-B-Tl, Ni-B-Mo, Ni-B-Sn, Co-P, Co-P-W, Co-B

30-120

Ni-P + SiC, Ni-P + WC (dispersion)

30-125

Au-Ni, Au-Co

0.5-3

Magnetic

Ni-Co-P, Ni-Co-B, Ni-Fe-P

<0.1

Catalyzation

Pd-Rh

<0.05

Solderability

Sn-Pb, Ni-P

0.5-10

Bondability

Ni-P

2-5

High temperature

Co-W-B, Ni-Re-P

1-125

Diffusion barrier

Ni-P

12-30

Nonelectrolytic coating systems use two types of reactions to deposit metal onto a part: electroless and displacement. The electroless or autocatalytic process uses reducing agents to convert the metal from its metal salt. Displacement reactions use oxidation and reduction of the base metal to deposit a thin coating of metal.

The advantage of using nonelectrolytic alloy coating systems over electrolytic is the uniformity of deposit in alloy composition and thickness, the ability to perform bulk processing, and the ability to produce unique catalytic coatings. These coatings are produced from aqueous chemical processes by conventional aqueous pretreatment procedures.

Nonelectrolytic processes can be used to deposit binary or ternary alloy coatings. The chemistries of ternary alloy plating can be very complex and involve several chelation and secondary reactions. The typical ternary system operates by having the primary metal reaction at the surface produce an electropotential greater than the half-wave potential of the complexed secondary metal. This induced surface potential is called an overvoltage potential. This causes the secondary metal to be included with the primary metal along with the cation portion of the reducing agent. Examples of this type of ternary system are alloys of nickel-copper-phosphorus or nickel-thallium-boron.

Nonelectrolytic processes generally operate at slightly elevated temperatures up to the temperature of boiling. These processing solutions contain the primary element and may contain other alloying metals. The operating parameters for pH can range from strong acid to strong base, depending on the metal system and the type of chelation and reducing agent being used.

With some processes, a secondary reducer is used to increase the efficiency and quantity of alloying metal incorporated. The secondary metals that are being galvanically plated at their overvoltage may limit the deposition of the primary metal. The secondary metals will generally be consumed at a high rate and require replenishment. Reducing agents may also be consumed at a higher rate than the primary metal and require more frequent replenishment.

Electroless (Autocatalytic) Alloy Coatings. Electrolessly produced metallic alloy coatings use a reducing agent to cause the metal salts to be reduced onto an oxide-free part surface. This reduction reaction proceeds until the part is removed from the process. During the reaction, metal and reducing agents are depleted and require replacement to continue the processing. The process is called electroless because no external electrical power is applied. The term autocatalytic is the technical term for the process.

The electroless process can be complex and involve several simultaneous reactions producing hydrogen and electrical charge at the interface. Through experience and good operating techniques the plating operation can produce a uniform alloy with specific phase structures and compositions.

Electroless dispersion alloy coatings use the conventional electroless reduction reaction process with a suspension of particles to produce a deposit with unique macroproperties. These particles fall into two categories: lubricants or hard load bearing particles. The soft particles contain powders of polytetrafluoroethylene (PTFE), fluorinated carbons (CFX), and fluoride salts, whereas hard particles consist of carbides, ceramics, insoluble powders, and diamonds. Dispersion coatings contain particles in the concentration range of 5 to 30%, depending on the particle and wear system. These alloy coatings are used in applications in which significant wear is present or repair and service are difficult.

Displacement (immersion) alloy coatings use the corrosion (oxidation) of the base material to produce a galvanic (reduction) reaction producing a thin alloy coating. This process is self-limiting and will stop when the oxidation reaction of the base metal stops. Generally these coatings are very thin and contain elements of the base material. These coatings are used primarily in electronic applications.

Immersion alloy deposition processes are easy to control and require little metal replacement because of the very thin deposits produced. Displacement tin-lead solder processes are the exception, producing thicker deposits requiring more frequent additions.

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