How to Pass a 100-Hour Salt Spray Test for Electroless Nickel

Q. We are a Nadcap-approved plating facility located in the U.S. One of our most recent requirements is to pass a 100-hour salt spray test with our current medium phosphorus electroless nickel process. Our present EN does not seem to pass this test on a consistent basis. How we can pass this test?

A. As most people know, EN coatings have been recommended for many years as an excellent choice for a corrosion resistant coating for many applications. Salt spray testing happens to be one of many tests selected to determine if the coating is acceptable or not. Salt spray testing is utilized as a form of a corrosion test for many coatings, but in most EN applications, it represents more of a porosity test than anything else. Meeting or surpassing 100 hours for medium Phos EN is certainly at its upper range of capability. However, under the right conditions, plating shops throughout the U.S. have achieved this on a consistent basis.

Here are some suggestions that will help you understand EN, and how EN can pass this rigid requirement:

Phosphorus in Deposit. EN coatings are an alloy of nickel and phosphorus. The amount of phosphorus in the deposit dictates the crystallinity of the deposit. Generally, the higher the phosphorus content, the higher the amorphous nature of the coating and hence corrosion resistance. Typically, mid phos EN chemistries have a %P range of 4-9 percent. Select the higher ranges, if possible, to achieve the more amorphous deposit.

Many of the RoHS-compliant EN chemistries available today tend to be in the lower phosphorus range. Keeping pHs at the lower recommended ranges can raise %P levels higher, thus providing a less crystalline deposit.

Heavy Metal Brighteners/Stabilizers. Many EN chemistries also use heavy metals for stabilizers and brighteners. Their presence can be deleterious to the deposit. Micro-pitting and porosity of the deposit have been found to be directly attributable to the presence of certain stabilizers. The elimination or reduction of their usage can be advantageous.

Age of EN Chemistry. As EN baths age, the stress of the EN deposit generally becomes more tensile. High tensile stress values can lead to more brittle deposits and sometimes cracking, which can lead to premature salt spray failures or bad adhesion.

Deposit Thickness. The minimum thickness that will give consistent values in salt spray tests for EN is 0.5 mils. EN deposits have an inherent degree of porosity below that value. Coatings of 1.0 -2.0 mils will generally give better results but may not be practical or cost effective.

Substrate Quality. The incoming surface quality of the part is paramount to produce consistent results. Generally, parts or panels with less sharp edges and smoother surface texture will give better results.
Pretreatment. Good surface preparation is also critical for consistent, reliable results. Excellent alkaline cleaning and uniform acid pickling are critical in creating a uniform active surface for the EN chemistry to initiate upon. Over-etching the substrate with strong alkaline cleaners (aluminum) or strong acidic solutions (steel) will set up premature failure. Initial uniform crystal growth is mandatory to create a uniform deposit with little or no porosity.

Duplex Coatings. Use of a high phosphorus EN overlayer or underlayer will absolutely help improve the salt spray values of the overall coating. But make sure this is acceptable to the manufacturer of the part before introducing this modification—many times it is not an option.

Chrome Rinse. It has also been proven that the addition of a final chrome rinse can be advantageous in sealing porosity and increasing salt spray results. Unfortunately for RoHS applications, this type of rinse is out of favor due to the potential presence of hexavalent chrome.

Excellent EN Control. Finally, maintaining pH, temperature, nickel and hypo-concentrations is paramount to providing consistent quality deposits. Low temperatures, pHs, or nickel or hypo-concentrations can and will lead to inconsistent results.

In conclusion, it is possible to meet and exceed 100-hour salt spray with mid phosphorus EN. The suggestions should result in not only achieving salt spray success but also fewer rejects and overall positive results. As more engineers recommend EN coatings for aerospace, electronics and automotive applications, passing salt spray criteria is becoming increasingly important.

Michael is president of Metal Chem Inc. Visit metalchem-inc.com.