Prevent Chemical Corrosion

The chemical processing industry is arguably the most damaging environment possible in terms of component corrosion. From strong acids to caustics, the very product itself can lay waste to conventional metals in short time. To ensure chemical process reliability, it is critical that plant designers prevent chemical corrosion in their systems.

FS Precision Tech (FSPT) has 50 years of experience providing the industry with the solutions needed to prevent chemical corrosion in nearly any environment. Our corrosion resistance solutions come in the form of precision titanium and zirconium near-net castings.

This cost-effective approach and meticulous adherence to industry standards produces cast components to exacting standards, ensuring optimal performance with minimum or zero corrosion in the most extreme chemical processing industry environments. FSPT has invested years to achieve a cost-effective casting process that delivers a product with mechanical strength properties similar to that of machined wrought while achieving exceptional corrosion resistance. Submit an inquiry and we will happily provide engineering support for your design team in order to develop an optimized solution approach for your project requirements.

This article will focus on the unique properties of titanium and zirconium that prevent chemical corrosion, as well as FSPT’s process to deliver those properties in a cost-effective manner. Particular attention will be paid to specific corrosive media in which FSPT alloys achieve minimal chemical degradation.

Types of Corrosion

The fact that components used in the chemical processing industry are exposed to highly corrosive environments should come as no surprise. One might ask more specifically “what are the precise types of corrosion that occur, and how can designers defend against them?”

Galvanic corrosion is a common form of corrosion that can be observed in both everyday household situations and in industrial scale operations. When two dissimilar materials are brought into close or near contact in the presence of an electrolyte – ions in solution – galvanic corrosion is likely to occur. This corrosion occurs due to the difference in electric potential between the two materials. The material with the higher potential – called the cathode – acts as an accelerant for an electrochemical attack on the material with the lower electric potential – or the anode. Electrons flow from the anode material to the cathode. This flow of electrons is the foundation of the reduction-oxidation – or redox – reaction that causes the anode to experience chemical corrosion.

In the chemical processing industry, galvanic corrosion is typically the greatest threat when different materials are used within the same system. For example, chemical transfer piping or tubing may be constructed from a different material than that of the manifolds and valves to which they are connected. In such cases, an electric potential may exist between the two materials that could lead to corrosion at the interface. This corrosion occurs even if both materials would otherwise resist direct attack from the chemical environment alone! Therefore, designers must take extreme care to research and anticipate possible electrical interaction between dissimilar materials that will be in close proximity to each other.

pitting corrosion
Extreme Pitting Corrosion

Pitting corrosion is another form of chemical corrosion that may cause damage to a chemical processing system. The effects of prolonged exposure to chemicals or direct physical damage may eventually weaken a particular site within the system. Once this site is compromised, highly localized corrosion will appear quickly to exploit the weakness. Consequently, pits form as the degradation migrates into the material, forming a small hole in the component. As the material degradation largely occurs within the pit beneath the material’s surface, there are generally few obvious signs that the component has been compromised.

Prevent Chemical Corrosion with Titanium & Zirconium

So, how does FS Precision Tech prevent chemical corrosion? Our titanium and zirconium castings have unique and specific properties that continually work to prevent chemical corrosion in a large majority of chemical processing applications. These unique properties take the form of corrosion resistant oxide layers which are created during the process of REDuction and OXidation – or RedOx Reaction. The RedOx process occurs due to a charge imbalance between the electrons within the metallic atoms and those in the surrounding chemical medium contacting the metallic surface.

This charge imbalance causes electrons to jump from their metallic hosts to find equilibrium within the contacting medium. The metallic atoms themselves also attract electrons from the atoms of the surrounding medium, to balance their respective electrochemical charges, as this electron exchange occurs. Redox occurs as electrons jump from one material to another; all the time seeking their lowest energy states. The material that loses electrons is said to be oxidized whereas the material receiving electrons is reduced.  Through a chain of reduction and oxidation reactions, electrochemical equilibrium may be ultimately achieved through the creation of an intermediary layer of stable material.

RedOx - Reduction Oxidation Reaction

In summary, this continuous process of the materials seeking electrical equilibrium leads to the precipitation of a thin, virtually inert oxide layer at the interface surfaces between the metals and the chemical environment in which they operate.  This oxide layer ultimately becomes the source of the metal’s corrosion resistance.

The precise crystalline structure and mechanical properties of this oxidized layer vary from material to material. In the case of iron, the surface layer formed through this process is typically Iron Hydroxide – i.e. “rust” – which is notoriously soft, loose, and powder like. Since this rust is so fragile, it is readily eroded from the parent iron, allowing for repeated exposure of the base iron material and continuous redox deterioration as described. This perpetual process may result in severe material loss and ultimate functional failure.

Not all oxide layers are as soft as rust, however. Both titanium and zirconium oxides exhibit remarkable mechanical integrity, and hence are able to maintain a relative resilience to the harmful effects of corrosion. To prevent chemical corrosion over the widest possible range of scenarios, FS Precision Tech produces titanium and zirconium through near-net casting to offer engineers the distinct strengths of both materials.

The Multiple Faces of Titanium Oxide

The oxides of titanium provide for an extreme range of resistance to corrosion, from virtually zero resistance to near impenetrable protection. Titanium can form a variety of oxide forms. In fact, titanium has three levels of oxidation, referred to as +2, +3, and +4, each for the number of electrons lost in the oxidation process.

The +2 level of oxidation yields Titanium Monoxide (TiO), which is a compound that is chemically unstable at normal operating temperatures. Even in the presence of water, TiO is relatively unstable.  At normal operating temperatures, TiO dissolves easily in most acids. At elevated temperatures, however, the chemically resistant, highly crystalline form of TiO may be formed. The crystalline form of this oxide is highly chemically resistant and is attacked by very few substances, including hot concentrated Sodium Hydroxide, Hydrochloric Acid, and Sulfuric Acid.

Dititanium Trioxide (Ti2O3) – formed in the +3 oxidation state – is slightly more stable than TiO at normal temperatures, but remains a poor defense against corrosion when compared to the +4 oxidation state, Titanium Dioxide.

Titanium Dioxide (TiO2) – representing titanium’s +4 oxidation state – is formed when a titanium atom reacts with dissolved oxygen as it gives up 4 electrons during oxidation. This is the most common and stable form of oxidation exhibited by titanium. TiO2 takes shape as a tenacious dull white film on every surface of titanium exposed to the surrounding environment. This film, while typically only about 0.002” thick, is both chemically inert and mechanically tenacious, and renders titanium virtually impervious to many of the corrosive solutions and conditions designers must expect in the chemical processing industry.

This protective TiO2 layer is entirely self-healing in the presence of oxygen. At any moment when a titanium component is damaged or scratched during operation, the Redox reaction described above will occur immediately and the freshly exposed titanium will again be oxidized, instantaneously repairing its protection against the effects of chemical corrosion.

While titanium – ultimately in the form of TiO2 – creates and extreme level of corrosion resistance in a variety of chemical environments, the formation of the protective TiO2 layer requires the presence of free oxygen, most often in the form of O2 or hydrous solutions. Not all substances used in the chemical processing industry contain readily free oxygen, however. For example, anhydrous solutions do not provide sufficient free Oxygen to enable formation of TiO2. Aqueous solutions of reducing acids – such as Hydroflouric Acid – also prevent the formation of TiO2 . In such environment, titanium would not be an appropriate solution to prevent chemical corrosion.

FS Precision offers alternative solutions for these environments: zirconium castings.

Zirconium for Chemical Corrosion

Zirconium mirrors the same oxidation formation as titanium to create zirconium dioxide (ZrO2). However, it is not limited to environments containing O2 or NO3 which are very common oxidizing agents. Instead, zirconium is one of the few materials that can form a stable crystallized oxide structure in solutions lacking oxidizing agents or containing inhibitory ions such as Cl- and S2– which generally disrupt the oxidation process.

As with titanium, zirconium’s oxide layer is highly inert and offers zirconium exceptional protection in order to prevent chemical corrosion. Additionally, ZrO2 is an excellent insulator, which allows zirconium to maintain high electric potentials in most applications. This insulating characteristic of ZrO2 enables zirconium to provide excellent protection from the previously discussed galvanic corrosion.

While both titanium and zirconium oxide layers offer exceptional levels of chemical corrosion resistance, both have unique strengths and applications which dictate their ideal deployment in the chemical processing field.

The Difference Between Titanium Oxides and Zirconium Oxides

The fundamental difference between zirconium and titanium’s oxide layers are the conditions under which they can form. While the formation of TiO2 is limited to environments with common oxidizing agents, such is not the case with zirconium. This difference corresponds directly to the type of chemicals in which these materials exhibit optimal corrosion resistance.

Specifically, titanium is best suited for highly oxidizing environments, whereas zirconium will ideally be used in reducing solutions, or those that lack oxidizers. Referring back to the discussion regarding RedOx reactions; the material that loses electrons is said to be oxidized while the material receiving electrons is reduced.  This information reveals that titanium’s protective TiO2 layer forms as titanium atoms give up electrons and react with the surrounding media. Zirconium’s oxide layer, conversely, forms as electrons are gained from the surrounding reducing agent.

Applying this information to the chemical processing industry gives engineers crucial information to ensure their systems perform reliably and prevent chemical corrosion. For example, processes involving halides – or halogen containing materials – are highly oxidizing and therefore facilitate rapid titanium dioxide growth. Thus, titanium is virtually immune from chemical corrosion and would be the material of choice in this situation. Hence, titanium is an ideal solution to prevent chemical corrosion against oxidizers.

Chemicals known as reducers have the opposite effect of oxidizers as they reduce – or direct electrons to – the materials they encounter. Reducing environments are an ideal operating environment for zirconium to prevent chemical corrosion. Oxides of zirconium can form with ease in reducing agents without any need for any oxidizing agent. In reducing environments, ZrO2 will quickly form a uniform protective layer that will continue to prevent chemical corrosion in otherwise highly destructive solutions such as Hydrogen Chloride (HCl).

The unique strengths of titanium and zirconium offer an ideal combination of solutions to enable reliable performance for the chemical processing industry. FS Precision Tech delivers these solutions as near-net castings, a cost-effective method to reduce wasteful machining costs and minimize material scrap while eliminating chemical corrosion in virtually all chemical environments.

FS Precision Tech Cost-Effective Near-Net Casting

FSPT delivers superior resistance to chemical corrosion through its near-net investment casting of titanium and zirconium alloys. Our highly sophisticated six sigma production processes ensure that our customers receive the ideal corrosion resistance solution while reducing the costs of waste and excessive machining that can otherwise drive up the costs associated with machined wrought applications.

Our cost-effective investment casting process begins with your design – in all of its complexity – and creates an exact wax copy through injection molding. This is called a pattern. From there, we coat the wax pattern in a ceramic shell, which is subsequently fired in what is essentially a kiln to remove the wax and strengthen the ceramic shell. This leaves behind a mold cavity in the shape of your complex component design.

With the precision mold in place, we then place the mold into an ultra-low vacuum environment, and fill it with molten titanium or zirconium. Once cooled, the ceramic mold is destroyed and removed from the component casting.  We then follow the casting process with a series of additional post-cast processes and inspections defined by AMS and/or ASTM specifications until your cast component is ready for its final quality inspection and certification prior to shipment.

When designing components for the rigors of chemical processing, two primary concerns that are likely in the forefront of the design engineers’ minds are 1) component quality and 2) tight tolerances. At FS Precision Tech, we address and resolve both concerns. Near-net casting is the only thing that we do, and we do it very well. We are AS9100 and NADCAP certified, and adhere to strict aerospace casting specifications of NORSOK, AMS 4991, AMS T-81915A, and several others.

After your castings are separated from their ceramic molds, they are typically put under extreme temperature and pressure – up to 15,000 psi – in a process known as hot isostatic pressing (HIP), to collapse any internal voids which may have formed during casting. Following HIP, we have the in-house capability to chemically mill your titanium components, where the hard alpha-case layer generated by the high temperature conditions of titanium casting is chemically removed.  Our chemical milling is performed by our in-house Lockheed Martin certified chemical milling system.

Up to this point, our casting process typically holds tolerances to 0.010” – 0.015”. This is why our process is referred to as near-net casting. The castings are typically very close to the final geometry, but FS Precision Tech also offers the option to perform finish machining in order to achieve even greater precision if required.

Throughout the entirety of your casting project, FS Precision Tech holds itself to extremely high quality standards maintained through AMS 2175 and similar inspection specifications.

Our process of near-net casting and machining ensures that no time or unnecessary expense is wasted machining a solid block of titanium or zirconium down to the final product.  For many of our casting projects, a fully machined version may otherwise have begun as a 100-pound billet, and then been machined down to a finished geometry of five pounds or less. That’s money spent on wasted titanium or zirconium and excessive machining operations! At FS Precision Tech, we like to say that “We put the air in your parts so that YOU don’t have to.

Complete Corrosion Protection

Titanium and Zirconium’s natural corrosion resistance paired with FS Precision Tech’s cost-effective casting method delivers optimum solutions for every facet of the chemical processing industry.

Titanium and its protective TiO2 oxide layer is delivered ready to take on even the most damaging of oxidizing agents. The regenerative oxide shield will stand fast against chemical exposure to ensure continuous reliable performance. With FSPT titanium present, the list of potentially damaging chemicals shrinks significantly. Amongst the chemicals that titanium scratches off the engineer’s list are:

  • Halides
  • Oxidizing Chlorides
  • Pure Terephthalic Acid (C6H4(CO2)H)2) – PTA
  • Hydrobromic Acid (HBr)
  • Sulfuric Acid (H2SO4)
  • Phosphoric Acid (H3PO4)

Matching each chemical to their respective product and process within the industry reveals the wide scope of titanium’s utility. Sulfuric acid, for one, is one of the most heavily produced chemicals in the world, and is used primarily for fertilizer production. Phosphoric Acid is also a primary chemical in the fertilizer industry as it is developed into synthetic phosphate. PTA is widely known for its role in producing Polyethylene terephthalate (PET), which is the primary resin used in common plastic manufacturing.

These substrates of the chemical processing industry are only a glimpse into the regions where FSPT cast titanium may offer complete corrosion protection.

Zirconium alloy offers designers a solution to prevent chemical corrosion in strong reducing agents. FSPT offers this solution as near-net cast zirconium through our cost-effective process. Recall that zirconium’s protective oxide film is not limited by dissolved oxygen and will protect your components from the harmful effects of strong acids and other damaging chemicals. Amongst the chemicals that zirconium is best suited for are:

  • Organic Compounds
  • Alkalines
  • Hydrogen Peroxide (H2O2)
  • Hydrogen Chloride (HCl)
  • Propylene (C3H6)
  • Ethylene (C2H4)

These chemicals reveal the breadth of zirconium’s applicability to several major subsidiaries within the global chemical processing industry. Propylene, for example, is a transitional chemical used in the production of synthetic textile fibers, glycols for the auto industry, and ABS plastics. Sodium hydroxide – a powerful alkaline – is a highly caustic chemical widely used in the papermaking industry, soap production, industrial cleaning applications, and nearly any application where a strong chemical base is required for pH adjustments.

Conclusion

FSPT understands the enormity of the task facing chemical processing engineers. From the moment chemical compounds enter a system, they constantly probe for even that slightest amount of vulnerabililty to exploit. In addition to the potential for extreme chemical attack, system components may even be causing each other to corrode through galvanic corrosion without any help from the environment. Any one of these corrosion mechanisms may lead to costly failures. But just as easily as they may cause damage, these damaging scenarios can also be avoided.

FS Precision Tech has developed the ideal cost-effective casting method for both zirconium and titanium with precisely these chemical processing challenges in mind. Contact us and we’ll work with your team from beginning to end to prevent chemical corrosion through our high-quality, precisely controlled, near-net castings.