Titanium - Surface Treatments and Cleaning

The fabrication of titanium product forms into complex shapes is routine for many fabricators. These shops recognized long ago that titanium is not an exotic material requiring exotic fabrication techniques. They quickly learned that titanium is handled much like other high performance engineering materials, provided titanium’s unique properties are taken into consideration.

Important differences between titanium and steel or nickel-base alloys need to be recognized. These are:

•         Titanium’s lower density

•         Titanium’s lower modulus of elasticity

•         Titanium’s higher melting point

•         Titanium’s lower ductility

•         Titanium’s propensity to gall

•         Titanium’s sensitivity toward contamination during welding

Compensation for these differences allows titanium to be fabricated, using techniques similar to those with stainless steel or nickel-base alloys. Post fabrication and in service titanium may need to have a surface treatment or cleaning process applied.  This article deals with common operations used in these areas. The information given is intended to be used as guidelines. It is by no means exhaustive.

Surface Treatments

Numerous surface treatments are applied to titanium for a variety of reasons. The prevention of galling and the improvement of corrosion, being perhaps, the most important reasons.

Prevention of Galling

Galling not only causes excessive wear on titanium but may also result in accelerated corrosion through fretting action. Simple lubrication, using graphite or molybdenum disulfide, is often sufficient to overcome galling. It is, therefore, possible to use titanium for moving parts or for parts in sliding contact with itself or other metals with light to moderate loads. Heavier loads, on the other hand, require hardened titanium surfaces. Commercially available case hardening techniques, such as plasma spraying, ion implantation, anodising or nitriding, or coating techniques such as hard chromium electroplating or flame spraying of tungsten carbide and other hard, wear-resistant materials, are used.

Such surface treatments possess the required qualities of good adherence plus wear and scuff resistance. However, careful consideration has to be given to the compatibility of the treated surface with the corrosive environment to which it will be exposed.

Cleaning Titanium Equipment

The efficiency of titanium surfaces can usually be maintained without elaborate cleaning procedures. There is generally no need to clean for corrosion protection as is sometimes required with stainless steel, nor does the thin oxide surface film in any way combine with cooling water to form heavy mineral deposits as sometimes occurs on copper based alloys.

Marine fouling of heat exchanger surfaces is sometimes controlled by chlorine injection. Titanium surfaces are totally unaffected by such treatments. Titanium surface condenser tubing is also kept clean in this way as well as by continuous cleaning systems utilizing rubber balls or nylon brushes, without deleterious effects.

Acid Cleaning

Acid cleaning of titanium surfaces to remove deposits is sometimes necessary. Conventional acid cleaning cycles can be used provided proper inhibitors are present. Organic inhibitors such as filming amines are not effective with titanium. Ferric ion as ferric chloride is very effective as an inhibitor for titanium in acid solutions. As little as 0.1 percent (by weight) ferric chloride will inhibit corrosion of titanium by hydrochloric acid, for instance. At ambient temperatures, as much as 25 percent (by weight) HCl inhibited with FeCl₃ can be safely used on titanium.

Nitric acid is an excellent passivating agent for titanium and may be used alone or with hydrochloric acid to clean titanium surfaces. See Table 1 for a more complete listing of recommended cleaning media, and proper inhibitor additions.

Table 1. Typical cleaning solutions for titanium equipment

Brush Cleaning

The use of carbon steel wire brushes to remove deposits from titanium is not recommended. Likewise, carbon steel pipe or tube should not be used to clean out plugged titanium tubes. Pickup of imbedded or smeared iron particles from steel can render titanium susceptible to corrosion when the unit is placed back in service. Stainless steel or titanium wire brushes and pipe are preferred. Careful utilization of titanium’s unique properties will provide many years of maintenance-free service for fabricated equipment. Misapplication of titanium, the use of improper cleaning procedures and other abuses can lead to failure. On the other hand, careful use of some preventive measures, particularly those concerned with corrosion and galling resistance, can significantly extend the useful life of titanium equipment.

Source: Timet

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