21.2.4 Etching during repairs
Etching and chemical stripping of coatings/platings has an especially high significance at overhauls respectively repairs (Ill.21.2.4-1) . In fact we deal with processes, which are also used for the production of new parts (volume 4, chapter 22.214.171.124) but they must act more intense, for example at hot parts. The reason is the operational formation of chemically extremely stable oxides. These must be removed, not only from the surface of the parts but also out of cracks, before processes like non destructive testing (penetrant inspection) or repair brazing (chapter 21.2.2). It's especially problematic, if oxidised and/or coated cavities like cooling air ducts/channels in turbine blades/vanes must be treated. Here as well a deterioration by an etching process (volume 4, Ill. 126.96.36.199-10) must be avoided as also, if necessary, certain to identify. Additionally it must be guaranteed, that no remains of etching media/fluid can act harmful during later operation. It is dangerous if a increased thermal insulating oxidation is triggered (volume 4, Ill. 188.8.131.52-9).
For accessible outer surfaces mostly abrasive blasting (`vapour blasting') is combined with additionally etching (volume 4, chapter 184.108.40.206). Additionally aggravating is, that operation caused changes of the material structure from the base material can promote deteriorating effects like grain boundary attack, e.g., by a sensitizing effect (volume 1, Ill. 220.127.116.11-9 and Ill. 18.104.22.168-10, Lit 21.2.4-2). Thereby it must be considered, that intense etching processes always represent the danger of a deterioration of the parts (Fig. "Problems by thr removal of coatings").
Figure "Etching can be dangerous damaging" (volume 4, chapter 22.214.171.124): During a repair for different causes, parts must be etched:
- Stripping of coatings/platings and hard facings (I, III, IV, VII).
- Opening of cracks before the penetrant inspection (V).
- Removing of oxides from cracks (IV),
- Removing of operation caused reaction layers and oxide layers (IV, V).
- Cleaning before a coating/plating/painting (V II).
- Adherence of bondings (VI).
- Activation of the surface, preparation for a coating process, brazing (IV).
- Visibility of the micro structure for test purposes (II).
- Markings/labels (volume 4, Ill. 17.4-3).
- Erosive/abrasive processes (ECM, drilling, volume 4, Ill. 126.96.36.199-7).
During the application of those processes attention must be payed at potential risks (Fig. "Problems by thr removal of coatings").
- Dangerous attack of the base material:Erosion/abrasion, crack formation, attack of structure elements, e.g., dissolving of carbides by glycolic acid (volume 4, Ill. 188.8.131.52-7).
- Development of dangerous reaction zones for the following processing steps and/or during operation (e.g., SCC at titanium alloy by Cl , volume 1, Ill. 184.108.40.206-4; volume 4, Ill. 220.127.116.11-8).
- Deterioration through remaining residues of etching media, e.g., in cooling air channels (volume 3, Ill. 18.104.22.168-2).
- No sufficient effectivity, e.g., if there are operation caused changes it the coating and/or the base material.
- Deterioration of other coatings/platings on the part (volume 4, Ill. 22.214.171.124-5 and Ill. 126.96.36.199-6).
- Deterioration of other component elements (thread inserets in light metal alloys (VII)).
- Hydrogen embrittlement of parts, made of high strength steels (volume 1, chapter 188.8.131.52; volume 4, chapter 184.108.40.206-14).
- Change of the operation properties at uncoated zones. Removal of thin hard surface zones and/or strain hardened zones and protecting zones with internal compression stresses. An example is the etching of shot peened surfaces.
- Aging of the etching baths which changes its behaviour like worse effectivity and selective attack (intercrystalline crack formation, volume 1, Ill. 220.127.116.11-9, volume 4, Ill. 18.104.22.168-10). Also unexpected attack of coatings/platings is possible (Fig. "Coating deteriorations by process baths")
- Contamination of other processes and process media (Fig. "Coating deteriorations by process baths").
- Influencing the environment, e.g., by exhaust air (volume 4, Ill. 22.214.171.124-2.2).Deterioration of stored parts, units and/or equipment in the surrounding by splashes and vapours (volume 4, Ill. 126.96.36.199-20).
- Health hazards. Because etch baths usually are highly aggressive acids, if a contact with the skin or the inhalation of vapours takes place, heavy injuries must be expected.
Figure "Problems by thr removal of coatings": Almost always before a recoating as repair the old coating must be removed. This processing step can be quite problematic. Applied are different processes, often also in combination:
- Abrasive blasting (e.g., vapour blasting, volume4, Ill. 188.8.131.52-18).
- Stripping with water jet.
- Chemical separating/dissolving: Paint, plastics.
- Machining: Turning, grinding.
Thereby process specific damages can occur at the part. In the following several are discussed.
Attack of the base material must be expected when etching processes (Fig. "Avoiding corrosion during etching process") and chemical dissolving are used.
Unfavourable conditions exist, if coating/layer and base material are very similar in composition.
Such a situatuion exists during the chemical removing of a, with Al powder filled paint/coating from an Al alloy base material. Unsuitable conditions are also present if,compared with the base material, a chemically more stable coating and/or a resistant operation caused layer is present. Unfortunately the same is especially true for the oxides on hot parts which must always be expected. This problem also exists for highly oxidation resistant coatings like MCrAlY thermal spray coatings. Also a seemingly harmless etching medium like glycolic acid can dissolve the typical carbides out of Ni alloys and form micronotches. This can lower the fatigue strength.
At casings from alloys of Al and Mg a chemical dissolving of paintings/coatings or an etching before an anodising can trigger corrosion. Therefore thread inserts of steel must be removed before.
Openes an etching process por0sity near the surface and/or solves infiltrated resin sealings, leakage will occur (volume 4, Ill. 15.2-9). This is especially dangerous if oil (gear casing, control unit, pumps) or fuel (pumps, control units) escapes.
Was the possibility of repair not already considered in the design phase, a relatively little removal after several repairs can lead to dimensional problems. Especially machining/chipping may also cut a bit the base material during a complete removal of the old coating. This situation must be expected when a (also only little distorted) casing ring must be turned on the inner side.
Abrasive blasting processes can close faults/flaws to the surface. This as well as an increase of the roughness (backgrounf fluorescence) can hinder the penetrant inspection.
Remaining blasting particles of an abrasive coating removal process can restrain the buildup of diffusion coatings (Fig. "Typical proglems with repair coatings").
Residues of etching media or chemical dissolving fluids, especially inside of components with cavities, disturb following processes (volume 4, Ill. 184.108.40.206-3) and trigger failures/deteriorations during operation. To these belongs oxidation in cooling air channels of hot parts. This leads to a worse heat flow with overtemperature.
Etching processes at parts of high strength martensitic steels (tempering steels) like bolts, nuts and gears, permit the absorption of hydrogen. With this the danger exists, that the material embrittles (hydrogen embrittlement, volume 1, chapter 220.127.116.11).
Against expectation the removal of coatings with a high pressure water jet can deteriorate/damage ('drop impact' , volume 1, Ill. 5.3.1-11.1). Is this process executed excessively, micro crack formation occurs with a drop of the fatigue strength.
Figure "Influencing NDT by abrsive blasting and etching": There are different effects of surface treatments which can affect the penetrant inspection (volume 4, chapter 17.3).
As well abrasive blasting as also shot peening can close cracks and porosity (detail middle). With this the penetrant fluid can no more intrude as necessary for the testing process. A further effect is the roughening of the surface. It can trigger a „background fluorescence“. This covers optically the crack indication. This effect develops, when the rough surface,if so, together with overlaps adheres penetrant fluid.
To open „smearings” etching is used after blasting/peening. If this causes an increased roughness this can mean also disturbing background fluorescence.
Etching is used to remove covering oxide layers and oxidation in cracks (detail above) and pores before a penetrant inspection. Tereby a further problem in a crack like, mostly intercrystalline attack emerges (detail below, Fig. "Avoiding corrosion during etching process" and Fig. "Problems by thr removal of coatings"). This damage, even in an already dangerous size, is not yet detectable with penetrant inspection.
Approaches for a risk minimization are contained in the frame at the left.
Figure "Coating deteriorations by process baths": Aged or contaminated process baths like etching baths and cleaning baths can dangerous damage materials, which they else don't attack. This danger of repairs is expecially high. The cause is, that used, possibly already at former times repaired parts, in contrary to new parts, can have unknown or undetected contaminations.
In the shown case in a short time period failures occurred during repair repeatedly at a multitude of parts (volume 4, Ill. 18.104.22.168-8). Concerned was a proven process bath in which TC/Co-wear protection coatings are normally not attacked. These coatings at parts of a tanium alloys from the compressor have been suddenly dissolved (?).
Investigations showed, that probably a contamination with copper was the cause for the aggressive behaviour of the bath.
To a repair respectively repair development belongs also the testing and the proof of suitable non destructive testings which guarantee the safety.
21.2.4-1 P.Adam, „Fertigungsverfahren von Turboflugtriebwerken“, Birkhäuser Verlag, 1998, ISBN 3-7643-5971-4, Seite 112-148, 130.
21.2.4-2 ASME, Metals Handbook Ninth Edition, Volume 11”Failure Analysis and Prevention“, Ausgabe 1986, Seite 428.