At no time have documentation and adherence to specifications been more pronounced than today. The business community has embraced the ISO 9000 theme and passed this down to all tiers of suppliers and end users. The first part of this article will discuss the more popular specifications currently used in the United States, including their strengths and weaknesses. The second part will highlight key issues that both the customer and the plater should take into consideration before any parts are plated with electroless nickel.
Current Specifications. The nickel-phosphorus specifications most frequently used in the United States are listed below.
- AMS 2404C, Electroless Nickel Plating
- AMS 2405B, Electroless Nickel Plating, Low Phosphorus
- ASTM B656, Standard Guide for Autocatalytic Nickel-Phosphorus
- Deposition on Metals for Engineering Use
- ASTM B733, Standard Specification for Autocatalytic
- Nickel-Phosphorus Coatings on Metal
Recently, military specification MIL-C-26074 was waived in favor of the AMS 2404. All the documents have strengths and weaknesses. The textile industry may be less demanding about the coating on a spool for a paper manufacturing application than the aircraft industry may be on a flight-critical landing strut. All things are relative to one's own environment.
Basic Overview of Current Specifications
AMS 2404C and AMS 2405B include basic information about what is expected of the electroless nickel deposit. Three categories of post testing are referenced: acceptance testing, periodic testing and pre-production testing.
- Acceptance: Visual quality without magnification for a smooth, continuous and uniform appearance free of blisters and other imperfections. Thickness validation by micrometer, microscopic method or equivalent method. These tests should be performed on every part lot.
- Periodic: The frequency of testing for this category is to be agreed upon between the buyer and the seller. Adhesion testing is performed by bending a plated sample or panel over a mandrel to a 180- degree angle and inspecting the affected area with 6x magnification for loss of adhesion. Cracking is allowed, but no evidence of lifting or peeling is allowed. A corrosion test conforming to ASTM B117, five pct neutral salt spray for 48 hours with no evidence of red rust is the minimum. The AMS 2405 also calls for a verification that the deposit is less than eight pct phosphorus.
- Preproduction: This category calls for all the preceding tests to be performed prior to or with the first order of production parts.
ASTM B656 is not a specification, but a guide for engineering use. It will be discussed later in the article.
ASTM B733 is a highly detailed specification that has the following breakdown: four service conditions; three test types; and five post treatment classes.
The specification details an elaborate coding system to denote all these categories. An example would be a part that requires 30 µm of a 10 pct phosphorus autocatalytic nickel deposited on a 4130 steel that will be used in an aggressive environment that requires certification of the alloy composition. (SC3, Type 111, Class 3 FeCrl/ NiP 10 30).
Service Condition. The severity of the environment is presented here.
Test Types. The test type defines which tests should be performed.
Post Treatment. Classes of post heat treatment are referenced:
General Comments of the Current Specifications
AMS 2404C and AMS 2405B. As with most specifications controlled by the Society of Automotive Engineers (SAE), these begin with the standard phraseology concerning preparation, procedure and post treatment. The same format is used in other AMS coating specifications. The documents present a basic outline of minimum quality requirements that are dated and time consuming. As an example, the need for a 48-hour salt spray test per ASTM B117 on a deposit of 0.001 inch (25 µm) thick is a waste of good salt. A plating of this thickness will far exceed this requirement. The use of terminology such as "preferably" or "as agreed upon by the purchaser and vendor," has no place in this kind of document. The specification assumes that the vendor (plater) has direct communication with the purchaser, who should be the end customer. Due to this loose approach, many of the tests referenced in the document that would help validate the worthiness of the plated parts are waived due to cost constraints and time. The classification system for the three different stages of post testing, acceptance, periodic and preproduction, is a realistic approach to daily quality call-outs.
ASTM B656 and ASTM B733 provide a powerful combination of information for both the end user and the plater. It is somewhat unfortunate that most users of the ASTM B733 are unaware of the ASTM B656 Guide. Many users often skim past the beginning of a specification to get to the "meat" of the document, which is unfortunate at times. The scope of B733 is presented in a clear and concise manner, with references to other ASTM documents, like the B656, if additional information is required for a particular application. There are many other positive aspects to these specifications, including the coding system for the thickness, application and post testing sections. An area of special note is section seven regarding the "Coating Requirements," in particular the part concerning appearance. It is one of the few documents that states that when an electroless nickel deposit is heat treated it may discolor, but this cosmetic condition is not cause for rejection.
An area that is not addressed very well is the phosphorus content of the deposit. A reference is made in section 7. 1.1 to a range of specific phosphorus contents, but the sections referenced, 3.5.3 and 5.9, do not exist in either B733 or B656. This coupled with the idea that the phosphorus content of the coating should be held to plus or minus one pct mass is difficult to achieve in many applications.
MIL-C-26074 was recently retired and replaced with AMS 2404. The logic behind this is that the government is putting the task of specification writing in the hands of people who have a greater command of their disciplines than the federal government. The new reference to the AMS specification is not necessarily a step in the right direction. Revisions to documents can be confusing at times. When a document is retired, as is the case of MIL-C-26074, and replaced by another document like the AMS, the confusion is expounded even more. Auditors see the MIL call-out on the print and want to see it on the certification. When you explain that it has been superseded by the AMS, confusion and time loss often follow.
What is a Specification? Although this may seem somewhat simplistic, the question may be better stated as, "What do you expect of a specification?" When determining what specification to use in a new application or interpreting one that is referenced on a print, the question to ask is, "What do they expect?" A specification should provide the necessary information to guide the user to the appropriate product within reasonable constraints and parameters. So again, the question has to be asked "what is expected?" AMS, ASTM and other private specifications are written to service the masses. These documents are provided to give the end users the minimum requirements for the end product. It is has been recommended that the plater develop his own working specification that further details the actual process and quality ramifications. This specification should be not only referenced but also listed on all quotations and certifications the plater issues. This action will provide a degree of support and guidance during any audit or review.
Specifications will never provide all the guidance needed for each application. Without them, the variety of coatings that would be on the market would surely cause major problems. Review the documents you work with and develop a list of questions to ask when starting work with a new account. Questions asked in the beginning will save much explaining in the future.