July 2009 Archives

(3) Pickling process with other acids

The following table shows various hydrogen embrittlement rates of a steel alloy ([SK5] [W1-8] [1.1525_C80W1]) in varying acid bath solutions.

The use of hydrochloric acid has been mentioned before, and other acid bath pickling also shows similar results. Pickling with sulfuric acid is as common as with hydrochloric acid. Sulfuric acid shows low hydrogen embrittlement rates at lower temperatures but has low rust removing capability, thus it is typically used at around 60 deg.C. At this temperature, sulfuric acid shows hydrogen embrittlement rate similar to that of hydrochloric acid. Phosphoric acid is also similar.
As seen in the table above, fluorine acids such as hydrofluoric acid, fluoroboric acid, and hydrochloric acid with commercial inhibitor shows low hydrogen embrittlement rates. And it can be recognized that de-smut bath used after pickling is also low in embrittlement rates.

(4)Electro rust removal and electro de-greasing, etc.

Rust removal, de-greasing, de-smutting, and activating by electrolysis are performed in acidic and alkaline solutions, after pickling processes. In these cases where products are processed by cathode electrolysis, hydrogen embrittlement occurs due to electrolysis of water. With anode electrolysis in alkali baths, very little embrittlement occurs. In acid bath, hydrogen embrittlement occurs during non-energized durations due to chemical dissolution.

Plating, in general, has required processes of [Pre-treatment > Surface finish > Post treatment]. Each process is shown in Table 1 as electro-plating as examples.

[Table 1] Plating processes

Many of these processes may result in hydrogen formation due to dissolution of steel and electrolysis of water. Care must be taken since the hydrogen may penetrate the steel under certain conditions.
The hydrogen penetration levels vary depending on the plating metals and solution bath compositions. However, the possibility of whether hydrogen embrittlement fracture may occur or not will largely depend on the host metal's susceptibility to hydrogen embrittlement.
Carbon steel's hydrogen embrittlement rate per process is shown next.

(1)Pickling

The pickling process is performed to remove scales and rust from the surface of steel objects to be plated. Normally, this process uses hydrochloric acid, sulfuric acid and the like, and prone to cause hydrogen embrittlement.
If this process is performed unconscientiously, the steel will certainly absorb hydrogen and embrittlement induced fracture will occur. For high tensile strength steel applications where safety is strongly desired, dry processes such as shot blasting and honing are therefore often specified.

(2)Pickling by hydrochloric acid

Pickling bath most typically used for steel alloys is at 10% solution strength, and the steel immersed in this bath in just a few minutes will reach 70% hydrogen embrittlement rate (see [Graph]). This is a significant problem for high tensile strength steel. Higher hydrochloric acid concentration will further raise the embrittlement rate. The immersion rate versus time will saturate beyond a few minutes.
In order to prevent this problem, commercially available inhibitors are usually added to the solution. The inhibitors will be mentioned later.

Hydrogen atom is the smallest of all atoms, and it's size is 1.06 angstrom = 1.06 x 10-10m. On the other hand, the lattice structure spacing of metal is 2~3 angstrom so the hydrogen atoms can easily penetrate into metals.
For example, sulfuric acid and hydrochloric acid used for pickling will dissolve the surfaces of steel alloys. The dissolution of the steel and hydrogen ion generation occur simultaneously, and the hydrogen ions will electrically discharge and become hydrogen atoms at the steel's surface. Much of the hydrogen atoms will be dissipated into air as hydrogen gas, but some will penetrate the steel.

As for the plating process, some hydrogen will also penetrate the steel if cathode current efficiency is low due to hydrogen generated by electrolysis of water. In some cases, hydrogen may penetrate the coating and increase the hardness of the coating, but this hydrogen may also penetrate the steel under certain conditions.
The hydrogen absorbed by steel exists in various places such as interstitial lattice, transitional region, grain boundary, and inclusions. The hydrogen in the interstitial structure maybe ignored as the cause of hydrogen embrittlement induced breakage since the existing amount is negligibly small. It is said that the hydrogen more closely related is "defective hydrogen".
With high strength steel alloys, hydrogen embrittlement induced breakage/crack occurs at grain boundaries. Hydrogen concentrates at grain boundaries and weakens the metal's atomic binding.
With low strength steel alloys, stress loadings cause transitions to gather around inclusions, and hydrogen is attracted towards the same area and result in destruction.

Hydrogen embrittlement induced destruction cases are organized as below.

Hydrogen formation during wet processes, including pre-processes, occurs in the following scenario.
During pickling, following chemical reaction occurs.

As seen above, hydrogen does not form when oxides only dissolve, but hydrogen ion is generated if the base steel alloy is dissolved also.
In zinc plating, redox of the zinc and electrolysis of water causes hydrogen ions, and hydrogen is formed.

Hydrogen ions generated as seen above immediately becomes atomic state hydrogen H, and the hydrogen penetrates the steel alloy's interstitial structure.

In this scenario, hydrogen will not be formed if all the electrical energy applied is spent on zinc metal deposition (redox). This condition is called Cathode Current Efficiency = 100%.
This cathode current efficiency varies depending on plating bath composition and cathode current density, etc. In general, the cathode current efficiency is good with mildly acidic single salt bath for nickel plating bath, but the efficiency tends to be low in zinc cyanide plating bath, and the hydrogen formation increases as the current density becomes higher.

Additionally, hydrogen overvoltage is another dominating factor for the formation of hydrogen during plating. Hydrogen overvoltage is an electrical potential difference that exists between atomic state hydrogen (H) becoming gaseous state hydrogen (H2) when the hydrogen ions electrically discharge on the surface of the host metal, and it varies with metal surface conditions. Generally, it is high for metals with low melting point, and small when the surfaces are rough. Hydrogen is more easily formed when hydrogen overvoltage is small.

Therefore, more hydrogen is generated during precious metal plating such as gold and silver, but less for zinc and tin plating. For instance, when zinc plating steel alloys, initial hydrogen generation is small since the hydrogen overvoltage is small with the steel alloy, but once the base metal is covered with zinc the hydrogen overvoltage increases thus the hydrogen generation is reduced.

The hydrogen generated here will penetrate the host metal as atomic state hydrogen. It is generally said that gaseous hydrogen cannot penetrate the host metal. Therefore, it can be assumed that hydrogen embrittlement will not occur if all the generated hydrogen is gasified and dispersed. A plating bath with chemical additives that promote such a reaction is highly anticipated.

It is frequently mentioned that electro-plated steel alloy objects can suffer breakage due to hydrogen embrittlement. What then is hydrogen embrittlement?
There are several definitions mentioned in JIS. Some are: "A phenomenon where plated object becomes brittle by absorption of hydrogen during plating and pre-treatment processes" (Electro-plating term), "Degradation of steel alloy's ductility or toughness caused by absorbed hydrogen into the steel alloy. This phenomena often occurs during pickling and electro-plating processes. The object may often break when subjected to tensile stress." (Steel alloy term). In all cases, it points to a phenomenon where materials become brittle due to hydrogen.

Occurrence of hydrogen embrittlement

Hydrogen embrittlement has been known for quite some time. It had become a problem when electroplating high carbon spring steel, notably. It has attracted technical attention in the late 1950s when high strength steel alloys came into wide usage by aerospace industry, and frequent failure of electro-plated aircraft parts produced from the high strength alloys occurred.

What type of surface finishing processes cause hydrogen embrittlement? General surface finishing process methods are classified in the following [Fig.].

Since the wet process methods use aqueous solutions, dangers of hydrogen embrittlement exits if hydrogen formation at base metal surface occurs.
On the other hand, the dry process methods which do not use aqueous solutions are not subject to hydrogen embrittlement, but pre-processes such as de-greasing and de-rusting processes are wet and they contain risks of hydrogen embrittlement.
As mentioned above, not just the actual plating process but all the associated processes must be evaluated when evaluating the risk of hydrogen embrittlement.