7 questions about zinc phosphating – each manufacturer should get answers to with number 6 being the most important

Zinc phosphating

Corrosion is a costly problem. It is a typical scene where you receive a neatly packaged metal component from the supplier or component manufacturer, but when you unpack, you see it- the dreaded rust!

Rust formation is unavoidable. If you deal with metals or manufacture metallic components- specifically auto-components, you need to have processes in place to counter the effects of rust or corrosion. Any metal part having exposure to corrosive chemicals should have a protective coating.

The coating process of the metallic component involves ‘phosphating.’ The phosphating process is known for over a century. This treatment primarily provides an inexpensive, non-toxic, reasonably hard, highly adherent, and electrically non-conducting phosphate coating.

Question 1- What is phosphating?

Phosphating is an electrochemical process.  When the steel surface comes in contact with a phosphoric acid-based solution, an electrochemical reaction takes place. This reaction forms a layer of metal-phosphate on the steel surface. A layer of phosphate coating typically includes iron, zinc, or manganese crystals. The characteristics of the formed layer are-

1. It is porous
2. Absorbent 
3. Suitable for subsequent powder coating or surface treatment

We can compare phosphating to applying lacquer, veneer, or polish to wooden parts. Apart from the fact that these processes improve the look and feel of the wooden parts, they also provide some additional benefits. Phosphating performs the same role- but for metallic surfaces. Manufacturers have to perform phosphating as a pre-treatment process before surface coating or painting. 

Chemical reactions in zinc phosphating- 

A zinc phosphate coating starts with oxidation at the steel surface when a Fe atom dissolves into the phosphoric acid solution. A Fe2+ ion leave behind two electrons on the surface. Simultaneously, reduction occurs in the phosphate bath. The two electrons residing on the steel surface combine with H+ in the bath to form hydrogen (H2).

A decrease in the pH adjacent to the steel surface establishes the conditions needed for the precipitation of zinc phosphate. In summary, acid gets consumed near the steel surface, causing a local decrease in pH. This decrease causes acid-soluble compounds to precipitate in the form of zinc phosphate coating on the steel surface.

Question 2- What are the different types of phosphating and their characteristics?

Zinc Phosphating-

Corrosion protection is the primary function of zinc phosphating. Zinc Phosphate is suitable for challenging weather applications and is most popular in the automotive industry. Zinc Phosphate increases lubricity as well as acts as a carrier for paint, oils, and waxes. 

Wire manufacturers, automotive component manufacturers, and appliance and electronics industries widely use zinc phosphating. Interestingly, armed services specify using zinc phosphating for equipment working in severe atmospheric conditions. 

Iron Phosphate

Iron Phosphating increases paint adherence and impact resistance as well as protects against oxidation. Iron phosphating offers the least corrosion resistance of the phosphating processes, but manufacturers often use it on indoor equipment because of its low cost.

 Manganese Phosphate

Like all phosphate processes, the primary application is as an undercoat for lubricants or paint. Manganese phosphating is perhaps the most absorbent of the three processes. Manufacturers often use it as a carrier for rust inhibiting oils and waxes. The use of this phosphating process is to prevent metal-on-metal contact. Manganese phosphate is generally more corrosion resistant than iron phosphate and is also more stable than zinc phosphate under extreme pressures and temperatures. It is the most wear-resistant of the three but not as corrosion-resistant as zinc phosphate. 

Question 3- What is the zinc phosphating process?

A typical zinc phosphate process involves the following steps-

1. Cleaning / Degreasing stage- 

During this stage, thorough cleaning of the component to be coated is carried out. This stage removes any dirt, oil, or grease sticking to the surface. The step is essential, and manufacturers need to pay extra care since zinc phosphate coating will not adhere properly to an oily or dirty surface. Defective degreasing or cleaning process proves very costly since quality issues emerge when the entire component goes through the whole process.

2. Water Rinse- 

After the degreasing or cleaning stage, the component is dipped or sprayed with water to remove any residual dirt or impurity. 

3. Activation- 

Surface Activation chemicals activate the metal to obtain fine crystalline structure during phosphate coating, increasing corrosion resistance and adhesion properties. Crystalline forms of less than 10 microns are always preferred. Without activation rinse, one can observe large phosphate crystals formation. Large crystals create voids in zinc phosphate coating. There is a high chance of corrosive forces penetrating these voids. 

4. Zinc Phosphating-

During this stage, we get the zinc phosphate layer formation on the component. There are two methods popularly used for zinc phosphating- 

– Immersion- In this, zinc and alkali metal phosphating systems do not significantly differ from one other.

-Spray- In spray phosphating, we find significant differences in the process sequences for zinc phosphating and alkali
metal phosphating systems. Usually, such kinds of systems have a passivation system at the end.

5. Water Rinse- 

Again, the entire surface goes through a rinsing process to remove any residual chemicals or sludge during the zinc phosphating step.

Question 4- What is zinc phosphate sludge, how is it formed?

The zinc phosphate coating process produces a solid byproduct called “sludge” in addition to the desired solid coating on the metal surface. While using the zinc phosphate solution, sludge eventually has to be removed from the bath and disposed of properly. Zinc phosphate sludge reduction and removal are of interest for manufacturers since discarding the sludge gets expensive as environmental norms get strict.

Chemically, sludge at the bottom of the phosphate tank is FePO4. Sludge is the byproduct that remains insoluble in the zinc phosphate bath after the zinc phosphate coating reaction is complete. This insoluble compound forms since the ions displaced from the metal surface during coating need neutralization for the process to achieve equilibrium. 

Sludge formation happens in three ways in the entire zinc phosphating process: 

1. Zinc dihydrogen phosphate- the zinc phosphate compound with which most zinc phosphating liquid baths are nearly at equilibrium. It is less soluble at higher temperatures than at lower temperatures. Hence some sludge is formed during the heating of the bath. 
2. The solubility of zinc dihydrogen phosphate is pH-dependent. As a result, some sludge formation happens during the neutralization of the bath, necessary to maintain the optimum free acid value during continued use of the bath. 
3. An unavoidable source of sludge stems from the reactions that produce the zinc phosphate conversion coating itself.

Question 5- What are the costs and effects of zinc phosphate sludge build-up?

Many pre-treatment systems are built with small zinc phosphating tanks to reduce operating costs—the smaller the tank, the lower the volume of bath that needs heating. Thus, smaller zinc phosphate tank sizes minimize energy consumption; however, it increases maintenance costs because the smaller the tank, the faster is the sludge build-up.  

The more sludge that accumulates, the less efficient the metal coating process becomes. The zinc phosphate sludge which settles at the bottom of the tank requires periodic removal. The amount of sludge is related to the number of components processed and the volume of chemicals in the bath, dependent on the bath tank size. 

Zinc phosphate sludge removal (without any system) involves the following steps- 

1. Stopping the coating cycle
2. Pumping out the entire bath along with sludge and discarding
3. Re-filling the bath with fresh water and chemicals
4. Raising the bath temperature to the desired level above ambient
5. Restarting the coating process

Costs involved in the phosphate tank cleaning process- 

1. Production loss during the tank cleaning process
2. Cost of properly discarding the used chemicals
3. Water cost of re-filling the bath
4. Chemical costs for the new bath
5. Fuel/electricity costs for re-heating the bath 

Question 6- How to remove zinc phosphate sludge effectively?

Innovation Filter System offers a highly effective way to keep the zinc phosphate bath fluid clean and sludge-free using the centrifuge separation system. This system consists of 2 main components- 

1. Intermediate tank
2. Centrifuge separator

The intermediate tank- 

We at Innovation Filter System design our systems with the primary goal: customers zinc phosphate bath should remain sludge-free. Hence, we connect the drain on the zinc phosphating tank to a separate intermediate tank. Thus, any sludge formed during the zinc phosphating process cannot settle in the process tank but flows by gravity to the intermediate tank. We customize The tank’s design and size as per the customer’s process tank used for zinc phosphating. Generally, this tank is divided into dirty and clean solution compartments, each having its level sensor. We also insulate this tank to avoid heat losses during the sludge removal process.

We provide overflow protection for this tank by using an actuated valve at the intermediate tank inlet. This valve closes if the fluid level inside the intermediate tank is high and opens when the liquid level reduces. Thus, ensuring complete safety and process automation. 

We use tank-mounted semi-open impeller pumps to pump out the dirty phosphating solution from the dirty compartment to the centrifuge. Using a semi-open impeller pump is imperative for this application since it deals with sludge, and any other pump may cease to operate if sludge builds up. We specifically prefer using tank-mounted, vertical pumps to avoid spillage of expensive chemicals on the shop floor because of seal failure. 

The centrifuge separator- Please click here for an animated video of the centrifuge

The zinc phosphate bath fluid continually circulates through the centrifuge system, where the centrifugal force is approximately 2,000 times the force of gravity. The centrifuge removes even the smallest micro-particles of zinc phosphate from the bath fluid. The resulting clean solution comes out of the centrifuge system by gravity, inside the clean solution compartment of the intermediate tank. From here, we pump the clean solution back to the process tank.

Innovation Filter Systems centrifuge is a separator and not a filter. Hence, users do not have to bear the replacement costs of filter changed. Instead, we work tirelessly to provide media-free solutions and hence have low operating and maintenance costs.

Question 7- What are the benefits of the Innovation Filter Systems- zinc phosphate sludge removal system–

– Eliminates tedious and expensive phosphate tank cleaning operations
– Improves heat transfer in the coating process
– Eliminates the need for filter media and its disposal costs
– The system needs very little monitoring
– Offers the lowest maintenance costs in the industry
– Maximizes the zinc phosphate fluid life
– Reduces waste stream / recyclable waste
– Constructed of 304 stainless steels (standard)
– Reduces pollution and environmental impact

By continually and adequately separating the zinc phosphate particles from the bath, the manufacturers ensure a smooth, efficient process. Perfect chemical balances and temperatures are maintained, leading to the elimination of downtime. 

While the zinc phosphate coating industry continues to become more competitive, manufacturers need to establish any competitive advantages available. The manufacturers implementing Innovation Filter Systems zinc phosphate bath fluid separation systems see increased production, decreased costs, and higher profits. Our ultra-efficient and effective zinc phosphate bath separation systems can dramatically and positively impact a manufacturer’s bottom line. We genuinely understand the zinc phosphate coating industry, and we are ready to work with your team to develop a centrifuge-based separation solution that not only works exceptionally well but one that requires virtually no maintenance.