Chemical Additions Drip by Drip

It used to be easy to spot the employees who made chemical additions to the cleaning and phosphating tanks at Lord Corporation. They were the ones wearing the holey clothes! However, all that has changed. In the interest of safety and process control, Lord Corporation in Cambridge Springs, Pennsylvania, installed a new system for adding chemicals to its process tanks.

"When I first started in process control for the chemical line, I found that the chemical levels in the baths ranged from extreme highs to ineffective lows, but never seemed to hold steady," noted Rocky Milligan, chemical process control team leader. "When we made an addition it would go to the top limit and then it would die off. When it reached the lower limit we would make a big chemical addition, and it would rise to the top limit again," he explained.

"I decided that this method was not working. But to figure out what would work, I needed to know what was going on in the baths," Mr. Milligan stated.

Bill Gardner from Texo suggested a micro addition system whereby an exact amount of chemical is added to the bath every minute. "With the micro addition system you are sure all the chemicals are used, unlike the "slug addition" method. Also," Mr. Milligan continued, "the chemicals we use are acids (except for the cleaner), which are heavy and sink to the bottom of the tank. We use the same chemicals with this system, however, you add chemicals a drop at a time to produce a homogenous bath."

The baths employing micro addition systems include Texolite 1510 IL Immersion cleaner; Globrite 102 ZP heavy zinc phosphate coating, Globrite 748 ADD grain refiner; and Sealtex 1523 non-chrome final sealer.

The immersion cleaner is an intensely alkaline compound designed for immersion cleaning of stainless steels and other ferrous metals. It operates at 130F.

The zinc phosphate converts the steel surface to a non-metallic coating. The zinc phosphate provides maximum corrosion protection. This tank operates at 165 to 180F. Parts are in the tank for approximately 10 min.

The internal grain refiner used within the zinc phosphate improves corrosion and humidity resistance and adhesion by lowering the coating weight and reducing the crystal size of the zinc phosphate deposit.

The final tank is a chromium-free sealer. It provides flash rust protection during drying and an increase in coating weight. The sealer also provides improved salt spray performance and humidity resistance. It operates at 140F, with a pH of 4.5 to 5.5.

In conjunction with the line, Mr. Milligan set up a computer program to monitor the system and track data. Each bath has tolerance limits. The tolerance for total acid in the phosphate bath ranges from 16 to 46. The target is 31.

When the system was first installed, the data from the bath was "all over the charts." Mr. Milligan expected this. However, the system statistically normalized the process and targeted the mean. "When you first charge the bath it goes through a seasoning process," explained Mr. Milligan. "All the chemical reactions say that your chemical levels are high, however they are not. Once the chemicals season out, you get a true reading."

Once "true" readings are plotted, bath performance can be determined. After making several titrations, Mr. Milligan and his chemical line team can determine if the bath's chemistry is stable and if so, how to keep it within a certain operating range.

Mr. Milligan explained, "CPK is a measure of how much of the tolerance is being used. A CPK of one means the chemical baths are using the entire tolerance if normally distributed. The industry standard is 1.33. A CPK of two means you have 50 pct tolerance on either side (high/low) of the target. A CPK of three means that I am not using any of the tolerance and everything is evenly distributed in a tight area about the mean of the tolerance. It also means that the probability of the bath being out of specification at any given time is in the ppm range, and I sleep well at night."

Lord Corporation keeps the baths on the cleaning and phosphating line tightly controlled. However, the metering system is not the only player in the game. The employees are the first string players. Mr. Milligan and the line operators review the statistical charts developed from titrations done on each bath. Together they stick to a game plan designed to keep the baths operating at peak performance levels. "It is a strong team effort," noted Mr. Milligan.

Although the cleaning and phosphating line is running smoothly now, it required several months to have everything statistically controlled. "The confidence in the system is what really took the most time," noted Mr. Milligan. "We are now doing only one titration a day. And we are running only two shifts instead of three."

Now that the bath chemistries are under control, Lord Corporation is looking to control phosphate coating weight. The phosphate system is calcium modified. This produces coatings with nodules rather than crystal structures. The nodules provide for better adherence to the rubber that is eventually attached to the parts.

The real test of the phosphate coating is when lab personnel try to separate the rubber from the metal part. The rubber can tear; the metal can bend; but the bond between the coating and the rubber must remain intact. A part for railroad cars was stretched to eight inches under 28,000 lbs of pressure. The rubber was torn and the metal warped, but the bond holding the rubber to the metal held fast.

The micro addition system has helped Lord Corporation improve safety and process control of its cleaning and phosphating line. No longer can you easily tell who makes chemical additions. But you can tell that the system is operating at peak performance.